diff --git a/toolchain/check/function.cpp b/toolchain/check/function.cpp
index c7ce61cc3c0b1..a6cdb14506a1f 100644
--- a/toolchain/check/function.cpp
+++ b/toolchain/check/function.cpp
@@ -191,60 +191,93 @@ auto CheckFunctionTypeMatches(Context& context,
                      context.functions().Get(prev_function_id), substitutions);
 }
 
+// Checks to see if a structurally valid redeclaration is allowed in context.
+// These all still merge.
+static auto CheckIsAllowedRedecl(Context& context, Parse::NodeId node_id,
+                                 SemIR::Function& new_function,
+                                 bool new_is_definition,
+                                 SemIR::Function& prev_function,
+                                 bool prev_is_import) -> void {
+  CARBON_DIAGNOSTIC(FunctionPreviousDecl, Note, "Previously declared here.");
+  if (prev_is_import) {
+    // TODO: Allow non-extern declarations in the same library.
+    if (!new_function.is_extern && !prev_function.is_extern) {
+      CARBON_DIAGNOSTIC(
+          FunctionNonExternRedecl, Error,
+          "Only one library can declare function {0} without `extern`.",
+          SemIR::NameId);
+      context.emitter()
+          .Build(node_id, FunctionNonExternRedecl, prev_function.name_id)
+          .Note(prev_function.decl_id, FunctionPreviousDecl)
+          .Emit();
+      return;
+    }
+  } else {
+    if (!new_is_definition) {
+      CARBON_DIAGNOSTIC(FunctionRedecl, Error,
+                        "Redundant redeclaration of function {0}.",
+                        SemIR::NameId);
+      context.emitter()
+          .Build(node_id, FunctionRedecl, prev_function.name_id)
+          .Note(prev_function.decl_id, FunctionPreviousDecl)
+          .Emit();
+      return;
+    }
+    if (prev_function.definition_id.is_valid()) {
+      CARBON_DIAGNOSTIC(FunctionRedefinition, Error,
+                        "Redefinition of function {0}.", SemIR::NameId);
+      CARBON_DIAGNOSTIC(FunctionPreviousDefinition, Note,
+                        "Previously defined here.");
+      context.emitter()
+          .Build(node_id, FunctionRedefinition, prev_function.name_id)
+          .Note(prev_function.definition_id, FunctionPreviousDefinition)
+          .Emit();
+      return;
+    }
+    // `extern` definitions are prevented in handle_function.cpp; this is only
+    // checking for a non-`extern` definition after an `extern` declaration.
+    if (prev_function.is_extern) {
+      CARBON_DIAGNOSTIC(FunctionDefiningExtern, Error,
+                        "Redeclaring `extern` function `{0}` as non-`extern`.",
+                        SemIR::NameId);
+      CARBON_DIAGNOSTIC(FunctionPreviousExternDecl, Note,
+                        "Previously declared `extern` here.");
+      context.emitter()
+          .Build(node_id, FunctionDefiningExtern, prev_function.name_id)
+          .Note(prev_function.decl_id, FunctionPreviousExternDecl)
+          .Emit();
+      return;
+    }
+  }
+}
+
 // TODO: Detect conflicting cross-file declarations, as well as uses of imported
 // declarations followed by a redeclaration.
 auto MergeFunctionRedecl(Context& context, Parse::NodeId node_id,
-                         SemIR::Function& new_function,
-                         SemIR::FunctionId prev_function_id, bool is_definition)
-    -> bool {
+                         SemIR::Function& new_function, bool new_is_definition,
+                         SemIR::FunctionId prev_function_id,
+                         bool prev_is_import) -> bool {
   auto& prev_function = context.functions().Get(prev_function_id);
 
   if (!CheckRedecl(context, new_function, prev_function, {})) {
     return false;
   }
 
-  if (!is_definition) {
-    CARBON_DIAGNOSTIC(FunctionRedecl, Error,
-                      "Redundant redeclaration of function {0}.",
-                      SemIR::NameId);
-    CARBON_DIAGNOSTIC(FunctionPreviousDecl, Note, "Previously declared here.");
-    context.emitter()
-        .Build(node_id, FunctionRedecl, prev_function.name_id)
-        .Note(prev_function.decl_id, FunctionPreviousDecl)
-        .Emit();
-    // The diagnostic doesn't prevent a merge.
-    return true;
-  } else if (prev_function.definition_id.is_valid()) {
-    CARBON_DIAGNOSTIC(FunctionRedefinition, Error,
-                      "Redefinition of function {0}.", SemIR::NameId);
-    CARBON_DIAGNOSTIC(FunctionPreviousDefinition, Note,
-                      "Previously defined here.");
-    context.emitter()
-        .Build(node_id, FunctionRedefinition, prev_function.name_id)
-        .Note(prev_function.definition_id, FunctionPreviousDefinition)
-        .Emit();
-    // The second definition will be unused as a consequence of the error.
-    return true;
-  } else if (prev_function.is_extern) {
-    CARBON_DIAGNOSTIC(FunctionDefiningExtern, Error,
-                      "Cannot define `extern` function `{0}`.", SemIR::NameId);
-    CARBON_DIAGNOSTIC(FunctionPreviousExternDecl, Note,
-                      "Previously declared `extern` here.");
-    context.emitter()
-        .Build(node_id, FunctionDefiningExtern, prev_function.name_id)
-        .Note(prev_function.decl_id, FunctionPreviousExternDecl)
-        .Emit();
-    // The diagnostic doesn't prevent a merge.
-    return true;
-  }
+  CheckIsAllowedRedecl(context, node_id, new_function, new_is_definition,
+                       prev_function, prev_is_import);
 
-  // Track the signature from the definition, so that IDs in the body
-  // match IDs in the signature.
-  prev_function.definition_id = new_function.definition_id;
-  prev_function.implicit_param_refs_id = new_function.implicit_param_refs_id;
-  prev_function.param_refs_id = new_function.param_refs_id;
-  prev_function.return_type_id = new_function.return_type_id;
-  prev_function.return_slot_id = new_function.return_slot_id;
+  if (new_is_definition) {
+    // Track the signature from the definition, so that IDs in the body
+    // match IDs in the signature.
+    prev_function.definition_id = new_function.definition_id;
+    prev_function.implicit_param_refs_id = new_function.implicit_param_refs_id;
+    prev_function.param_refs_id = new_function.param_refs_id;
+    prev_function.return_type_id = new_function.return_type_id;
+    prev_function.return_slot_id = new_function.return_slot_id;
+  }
+  if (!new_function.is_extern) {
+    prev_function.is_extern = false;
+  }
   return true;
 }
 
diff --git a/toolchain/check/function.h b/toolchain/check/function.h
index f6add8cd21208..69f2cb0ea9ea1 100644
--- a/toolchain/check/function.h
+++ b/toolchain/check/function.h
@@ -28,9 +28,9 @@ auto CheckFunctionTypeMatches(Context& context,
 // If merging is successful, updates the FunctionId on new_function and returns
 // true. Otherwise, returns false. Prints a diagnostic when appropriate.
 auto MergeFunctionRedecl(Context& context, Parse::NodeId node_id,
-                         SemIR::Function& new_function,
-                         SemIR::FunctionId prev_function_id, bool is_definition)
-    -> bool;
+                         SemIR::Function& new_function, bool new_is_definition,
+                         SemIR::FunctionId prev_function_id,
+                         bool prev_is_imported) -> bool;
 
 }  // namespace Carbon::Check
 
diff --git a/toolchain/check/handle_function.cpp b/toolchain/check/handle_function.cpp
index 309b72c1dbfac..01027804244a4 100644
--- a/toolchain/check/handle_function.cpp
+++ b/toolchain/check/handle_function.cpp
@@ -165,14 +165,22 @@ static auto BuildFunctionDecl(Context& context,
   }
 
   // Check whether this is a redeclaration.
-  auto existing_id =
+  auto prev_id =
       context.decl_name_stack().LookupOrAddName(name_context, lookup_result_id);
-  if (existing_id.is_valid()) {
-    if (auto existing_function_decl =
-            context.insts().Get(existing_id).TryAs<SemIR::FunctionDecl>()) {
-      if (MergeFunctionRedecl(context, node_id, function_info,
+  if (prev_id.is_valid()) {
+    auto prev_inst = context.insts().Get(prev_id);
+    bool prev_is_import = false;
+
+    if (prev_inst.Is<SemIR::ImportRefUsed>()) {
+      prev_inst =
+          context.insts().Get(context.constant_values().Get(prev_id).inst_id());
+      prev_is_import = true;
+    }
+
+    if (auto existing_function_decl = prev_inst.TryAs<SemIR::FunctionDecl>()) {
+      if (MergeFunctionRedecl(context, node_id, function_info, is_definition,
                               existing_function_decl->function_id,
-                              is_definition)) {
+                              prev_is_import)) {
         // When merging, use the existing function rather than adding a new one.
         function_decl.function_id = existing_function_decl->function_id;
       }
@@ -180,7 +188,7 @@ static auto BuildFunctionDecl(Context& context,
       // This is a redeclaration of something other than a function. This
       // includes the case where an associated function redeclares another
       // associated function.
-      context.DiagnoseDuplicateName(function_info.decl_id, existing_id);
+      context.DiagnoseDuplicateName(function_info.decl_id, prev_id);
     }
   }
 
diff --git a/toolchain/check/import_ref.cpp b/toolchain/check/import_ref.cpp
index 492bdff7f5262..5377b37c5e1c6 100644
--- a/toolchain/check/import_ref.cpp
+++ b/toolchain/check/import_ref.cpp
@@ -707,8 +707,11 @@ class ImportRefResolver {
              GetLocalParamRefsId(function.param_refs_id, param_const_ids),
          .return_type_id = new_return_type_id,
          .return_slot_id = new_return_slot,
+         .is_extern = function.is_extern,
          .builtin_kind = function.builtin_kind});
     // Write the function ID into the FunctionDecl.
+    // TODO: The Invalid NodeId means it's hard to associate diagnostics. We
+    // should replace this.
     context_.ReplaceInstBeforeConstantUse(
         function_decl_id, {Parse::NodeId::Invalid, function_decl});
     return {context_.constant_values().Get(function_decl_id)};
diff --git a/toolchain/check/testdata/function/builtin/fail_redefined.carbon b/toolchain/check/testdata/function/builtin/fail_redefined.carbon
index 7dc8e380f36cc..4a5bfd73f09e5 100644
--- a/toolchain/check/testdata/function/builtin/fail_redefined.carbon
+++ b/toolchain/check/testdata/function/builtin/fail_redefined.carbon
@@ -43,57 +43,57 @@ fn C(n: i32, m: i32) -> i32 = "int.add";
 // CHECK:STDOUT:   }
 // CHECK:STDOUT:   %A.loc7: <function> = fn_decl @A [template] {
 // CHECK:STDOUT:     %n.loc7_6.1: i32 = param n
-// CHECK:STDOUT:     @A.%n: i32 = bind_name n, %n.loc7_6.1
+// CHECK:STDOUT:     %n.loc7_6.2: i32 = bind_name n, %n.loc7_6.1
 // CHECK:STDOUT:     %m.loc7_14.1: i32 = param m
-// CHECK:STDOUT:     @A.%m: i32 = bind_name m, %m.loc7_14.1
+// CHECK:STDOUT:     %m.loc7_14.2: i32 = bind_name m, %m.loc7_14.1
 // CHECK:STDOUT:     %return.var.loc7: ref i32 = var <return slot>
 // CHECK:STDOUT:   }
 // CHECK:STDOUT:   %A.loc15: <function> = fn_decl @A [template] {
 // CHECK:STDOUT:     %n.loc15_6.1: i32 = param n
-// CHECK:STDOUT:     %n.loc15_6.2: i32 = bind_name n, %n.loc15_6.1
+// CHECK:STDOUT:     @A.%n: i32 = bind_name n, %n.loc15_6.1
 // CHECK:STDOUT:     %m.loc15_14.1: i32 = param m
-// CHECK:STDOUT:     %m.loc15_14.2: i32 = bind_name m, %m.loc15_14.1
+// CHECK:STDOUT:     @A.%m: i32 = bind_name m, %m.loc15_14.1
 // CHECK:STDOUT:     %return.var.loc15: ref i32 = var <return slot>
 // CHECK:STDOUT:   }
 // CHECK:STDOUT:   %B.loc17: <function> = fn_decl @B [template] {
 // CHECK:STDOUT:     %n.loc17_6.1: i32 = param n
-// CHECK:STDOUT:     @B.%n: i32 = bind_name n, %n.loc17_6.1
+// CHECK:STDOUT:     %n.loc17_6.2: i32 = bind_name n, %n.loc17_6.1
 // CHECK:STDOUT:     %m.loc17_14.1: i32 = param m
-// CHECK:STDOUT:     @B.%m: i32 = bind_name m, %m.loc17_14.1
+// CHECK:STDOUT:     %m.loc17_14.2: i32 = bind_name m, %m.loc17_14.1
 // CHECK:STDOUT:     %return.var.loc17: ref i32 = var <return slot>
 // CHECK:STDOUT:   }
 // CHECK:STDOUT:   %B.loc25: <function> = fn_decl @B [template] {
 // CHECK:STDOUT:     %n.loc25_6.1: i32 = param n
-// CHECK:STDOUT:     %n.loc25_6.2: i32 = bind_name n, %n.loc25_6.1
+// CHECK:STDOUT:     @B.%n: i32 = bind_name n, %n.loc25_6.1
 // CHECK:STDOUT:     %m.loc25_14.1: i32 = param m
-// CHECK:STDOUT:     %m.loc25_14.2: i32 = bind_name m, %m.loc25_14.1
+// CHECK:STDOUT:     @B.%m: i32 = bind_name m, %m.loc25_14.1
 // CHECK:STDOUT:     %return.var.loc25: ref i32 = var <return slot>
 // CHECK:STDOUT:   }
 // CHECK:STDOUT:   %C.loc27: <function> = fn_decl @C [template] {
 // CHECK:STDOUT:     %n.loc27_6.1: i32 = param n
-// CHECK:STDOUT:     @C.%n: i32 = bind_name n, %n.loc27_6.1
+// CHECK:STDOUT:     %n.loc27_6.2: i32 = bind_name n, %n.loc27_6.1
 // CHECK:STDOUT:     %m.loc27_14.1: i32 = param m
-// CHECK:STDOUT:     @C.%m: i32 = bind_name m, %m.loc27_14.1
+// CHECK:STDOUT:     %m.loc27_14.2: i32 = bind_name m, %m.loc27_14.1
 // CHECK:STDOUT:     %return.var.loc27: ref i32 = var <return slot>
 // CHECK:STDOUT:   }
 // CHECK:STDOUT:   %C.loc34: <function> = fn_decl @C [template] {
 // CHECK:STDOUT:     %n.loc34_6.1: i32 = param n
-// CHECK:STDOUT:     %n.loc34_6.2: i32 = bind_name n, %n.loc34_6.1
+// CHECK:STDOUT:     @C.%n: i32 = bind_name n, %n.loc34_6.1
 // CHECK:STDOUT:     %m.loc34_14.1: i32 = param m
-// CHECK:STDOUT:     %m.loc34_14.2: i32 = bind_name m, %m.loc34_14.1
+// CHECK:STDOUT:     @C.%m: i32 = bind_name m, %m.loc34_14.1
 // CHECK:STDOUT:     %return.var.loc34: ref i32 = var <return slot>
 // CHECK:STDOUT:   }
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: fn @A(%n: i32, %m: i32) -> i32 = "int.add" {
 // CHECK:STDOUT: !entry:
-// CHECK:STDOUT:   %n.ref: i32 = name_ref n, file.%n.loc15_6.2
+// CHECK:STDOUT:   %n.ref: i32 = name_ref n, %n
 // CHECK:STDOUT:   return %n.ref
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: fn @B(%n: i32, %m: i32) -> i32 = "int.add" {
 // CHECK:STDOUT: !entry:
-// CHECK:STDOUT:   %n.ref: i32 = name_ref n, %n
+// CHECK:STDOUT:   %n.ref: i32 = name_ref n, file.%n.loc17_6.2
 // CHECK:STDOUT:   return %n.ref
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
diff --git a/toolchain/check/testdata/function/declaration/import.carbon b/toolchain/check/testdata/function/declaration/import.carbon
index 233daf6e9569f..fbf60fed5bf12 100644
--- a/toolchain/check/testdata/function/declaration/import.carbon
+++ b/toolchain/check/testdata/function/declaration/import.carbon
@@ -17,6 +17,15 @@ fn B(b: i32) -> i32;
 fn C(c: (i32,)) -> {.c: i32};
 extern fn D();
 
+// --- extern_api.carbon
+
+library "extern_api" api;
+
+extern fn A();
+extern fn B(b: i32) -> i32;
+extern fn C(c: (i32,)) -> {.c: i32};
+extern fn D();
+
 // ============================================================================
 // Test files
 // ============================================================================
@@ -32,15 +41,165 @@ var b: i32 = B(1);
 var c: {.c: i32} = C((1,));
 var d: () = D();
 
-// --- redecl_extern.carbon
+// --- redecl_api.carbon
 
-library "redecl_extern" api;
+library "redecl_api" api;
+
+import library "api";
 
 extern fn A();
 extern fn B(b: i32) -> i32;
 extern fn C(c: (i32,)) -> {.c: i32};
 extern fn D();
 
+var a: () = A();
+var b: i32 = B(1);
+var c: {.c: i32} = C((1,));
+var d: () = D();
+
+// --- redecl_extern_api.carbon
+
+library "redecl_extern_api" api;
+
+import library "extern_api";
+
+extern fn A();
+extern fn B(b: i32) -> i32;
+extern fn C(c: (i32,)) -> {.c: i32};
+extern fn D();
+
+var a: () = A();
+var b: i32 = B(1);
+var c: {.c: i32} = C((1,));
+var d: () = D();
+
+// --- fail_todo_merge.carbon
+
+library "merge" api;
+
+import library "api";
+// CHECK:STDERR: fail_todo_merge.carbon:[[@LINE+52]]:1: In import.
+// CHECK:STDERR: import library "extern_api";
+// CHECK:STDERR: ^~~~~~
+// CHECK:STDERR: extern_api.carbon:4:1: ERROR: Duplicate name being declared in the same scope.
+// CHECK:STDERR: extern fn A();
+// CHECK:STDERR: ^~~~~~~~~~~~~~
+// CHECK:STDERR: fail_todo_merge.carbon:[[@LINE-7]]:1: In import.
+// CHECK:STDERR: import library "api";
+// CHECK:STDERR: ^~~~~~
+// CHECK:STDERR: api.carbon:4:1: Name is previously declared here.
+// CHECK:STDERR: fn A();
+// CHECK:STDERR: ^~~~~~~
+// CHECK:STDERR:
+// CHECK:STDERR: fail_todo_merge.carbon:[[@LINE+39]]:1: In import.
+// CHECK:STDERR: import library "extern_api";
+// CHECK:STDERR: ^~~~~~
+// CHECK:STDERR: extern_api.carbon:5:1: ERROR: Duplicate name being declared in the same scope.
+// CHECK:STDERR: extern fn B(b: i32) -> i32;
+// CHECK:STDERR: ^~~~~~~~~~~~~~~~~~~~~~~~~~~
+// CHECK:STDERR: fail_todo_merge.carbon:[[@LINE-20]]:1: In import.
+// CHECK:STDERR: import library "api";
+// CHECK:STDERR: ^~~~~~
+// CHECK:STDERR: api.carbon:5:1: Name is previously declared here.
+// CHECK:STDERR: fn B(b: i32) -> i32;
+// CHECK:STDERR: ^~~~~~~~~~~~~~~~~~~~
+// CHECK:STDERR:
+// CHECK:STDERR: fail_todo_merge.carbon:[[@LINE+26]]:1: In import.
+// CHECK:STDERR: import library "extern_api";
+// CHECK:STDERR: ^~~~~~
+// CHECK:STDERR: extern_api.carbon:6:1: ERROR: Duplicate name being declared in the same scope.
+// CHECK:STDERR: extern fn C(c: (i32,)) -> {.c: i32};
+// CHECK:STDERR: ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+// CHECK:STDERR: fail_todo_merge.carbon:[[@LINE-33]]:1: In import.
+// CHECK:STDERR: import library "api";
+// CHECK:STDERR: ^~~~~~
+// CHECK:STDERR: api.carbon:6:1: Name is previously declared here.
+// CHECK:STDERR: fn C(c: (i32,)) -> {.c: i32};
+// CHECK:STDERR: ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+// CHECK:STDERR:
+// CHECK:STDERR: fail_todo_merge.carbon:[[@LINE+13]]:1: In import.
+// CHECK:STDERR: import library "extern_api";
+// CHECK:STDERR: ^~~~~~
+// CHECK:STDERR: extern_api.carbon:7:1: ERROR: Duplicate name being declared in the same scope.
+// CHECK:STDERR: extern fn D();
+// CHECK:STDERR: ^~~~~~~~~~~~~~
+// CHECK:STDERR: fail_todo_merge.carbon:[[@LINE-46]]:1: In import.
+// CHECK:STDERR: import library "api";
+// CHECK:STDERR: ^~~~~~
+// CHECK:STDERR: api.carbon:7:1: Name is previously declared here.
+// CHECK:STDERR: extern fn D();
+// CHECK:STDERR: ^~~~~~~~~~~~~~
+// CHECK:STDERR:
+import library "extern_api";
+
+var a: () = A();
+var b: i32 = B(1);
+var c: {.c: i32} = C((1,));
+var d: () = D();
+
+// --- fail_todo_merge_reverse.carbon
+
+library "merge_reverse" api;
+
+import library "extern_api";
+// CHECK:STDERR: fail_todo_merge_reverse.carbon:[[@LINE+51]]:1: In import.
+// CHECK:STDERR: import library "api";
+// CHECK:STDERR: ^~~~~~
+// CHECK:STDERR: api.carbon:4:1: ERROR: Duplicate name being declared in the same scope.
+// CHECK:STDERR: fn A();
+// CHECK:STDERR: ^~~~~~~
+// CHECK:STDERR: fail_todo_merge_reverse.carbon:[[@LINE-7]]:1: In import.
+// CHECK:STDERR: import library "extern_api";
+// CHECK:STDERR: ^~~~~~
+// CHECK:STDERR: extern_api.carbon:4:1: Name is previously declared here.
+// CHECK:STDERR: extern fn A();
+// CHECK:STDERR: ^~~~~~~~~~~~~~
+// CHECK:STDERR:
+// CHECK:STDERR: fail_todo_merge_reverse.carbon:[[@LINE+38]]:1: In import.
+// CHECK:STDERR: import library "api";
+// CHECK:STDERR: ^~~~~~
+// CHECK:STDERR: api.carbon:5:1: ERROR: Duplicate name being declared in the same scope.
+// CHECK:STDERR: fn B(b: i32) -> i32;
+// CHECK:STDERR: ^~~~~~~~~~~~~~~~~~~~
+// CHECK:STDERR: fail_todo_merge_reverse.carbon:[[@LINE-20]]:1: In import.
+// CHECK:STDERR: import library "extern_api";
+// CHECK:STDERR: ^~~~~~
+// CHECK:STDERR: extern_api.carbon:5:1: Name is previously declared here.
+// CHECK:STDERR: extern fn B(b: i32) -> i32;
+// CHECK:STDERR: ^~~~~~~~~~~~~~~~~~~~~~~~~~~
+// CHECK:STDERR:
+// CHECK:STDERR: fail_todo_merge_reverse.carbon:[[@LINE+25]]:1: In import.
+// CHECK:STDERR: import library "api";
+// CHECK:STDERR: ^~~~~~
+// CHECK:STDERR: api.carbon:6:1: ERROR: Duplicate name being declared in the same scope.
+// CHECK:STDERR: fn C(c: (i32,)) -> {.c: i32};
+// CHECK:STDERR: ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+// CHECK:STDERR: fail_todo_merge_reverse.carbon:[[@LINE-33]]:1: In import.
+// CHECK:STDERR: import library "extern_api";
+// CHECK:STDERR: ^~~~~~
+// CHECK:STDERR: extern_api.carbon:6:1: Name is previously declared here.
+// CHECK:STDERR: extern fn C(c: (i32,)) -> {.c: i32};
+// CHECK:STDERR: ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+// CHECK:STDERR:
+// CHECK:STDERR: fail_todo_merge_reverse.carbon:[[@LINE+12]]:1: In import.
+// CHECK:STDERR: import library "api";
+// CHECK:STDERR: ^~~~~~
+// CHECK:STDERR: api.carbon:7:1: ERROR: Duplicate name being declared in the same scope.
+// CHECK:STDERR: extern fn D();
+// CHECK:STDERR: ^~~~~~~~~~~~~~
+// CHECK:STDERR: fail_todo_merge_reverse.carbon:[[@LINE-46]]:1: In import.
+// CHECK:STDERR: import library "extern_api";
+// CHECK:STDERR: ^~~~~~
+// CHECK:STDERR: extern_api.carbon:7:1: Name is previously declared here.
+// CHECK:STDERR: extern fn D();
+// CHECK:STDERR: ^~~~~~~~~~~~~~
+import library "api";
+
+var a: () = A();
+var b: i32 = B(1);
+var c: {.c: i32} = C((1,));
+var d: () = D();
+
 // CHECK:STDOUT: --- api.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
@@ -81,6 +240,46 @@ extern fn D();
 // CHECK:STDOUT:
 // CHECK:STDOUT: fn @D();
 // CHECK:STDOUT:
+// CHECK:STDOUT: --- extern_api.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %.1: type = tuple_type (type) [template]
+// CHECK:STDOUT:   %.2: type = tuple_type (i32) [template]
+// CHECK:STDOUT:   %.3: type = struct_type {.c: i32} [template]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace [template] {
+// CHECK:STDOUT:     .A = %A
+// CHECK:STDOUT:     .B = %B
+// CHECK:STDOUT:     .C = %C
+// CHECK:STDOUT:     .D = %D
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %A: <function> = fn_decl @A [template] {}
+// CHECK:STDOUT:   %B: <function> = fn_decl @B [template] {
+// CHECK:STDOUT:     %b.loc5_13.1: i32 = param b
+// CHECK:STDOUT:     @B.%b: i32 = bind_name b, %b.loc5_13.1
+// CHECK:STDOUT:     %return.var.loc5: ref i32 = var <return slot>
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %C: <function> = fn_decl @C [template] {
+// CHECK:STDOUT:     %.loc6_21.1: (type,) = tuple_literal (i32)
+// CHECK:STDOUT:     %.loc6_21.2: type = converted %.loc6_21.1, constants.%.2 [template = constants.%.2]
+// CHECK:STDOUT:     %c.loc6_13.1: (i32,) = param c
+// CHECK:STDOUT:     @C.%c: (i32,) = bind_name c, %c.loc6_13.1
+// CHECK:STDOUT:     %.loc6_35: type = struct_type {.c: i32} [template = constants.%.3]
+// CHECK:STDOUT:     %return.var.loc6: ref {.c: i32} = var <return slot>
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %D: <function> = fn_decl @D [template] {}
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @A();
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @B(%b: i32) -> i32;
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @C(%c: (i32,)) -> {.c: i32};
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @D();
+// CHECK:STDOUT:
 // CHECK:STDOUT: --- basics.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
@@ -151,36 +350,60 @@ extern fn D();
 // CHECK:STDOUT:   return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: --- redecl_extern.carbon
+// CHECK:STDOUT: --- redecl_api.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %.1: type = tuple_type (type) [template]
 // CHECK:STDOUT:   %.2: type = tuple_type (i32) [template]
 // CHECK:STDOUT:   %.3: type = struct_type {.c: i32} [template]
+// CHECK:STDOUT:   %.4: type = tuple_type () [template]
+// CHECK:STDOUT:   %.5: i32 = int_literal 1 [template]
+// CHECK:STDOUT:   %.6: (i32,) = tuple_value (%.5) [template]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: file {
 // CHECK:STDOUT:   package: <namespace> = namespace [template] {
-// CHECK:STDOUT:     .A = %A
-// CHECK:STDOUT:     .B = %B
-// CHECK:STDOUT:     .C = %C
-// CHECK:STDOUT:     .D = %D
+// CHECK:STDOUT:     .A = %import_ref.1
+// CHECK:STDOUT:     .B = %import_ref.2
+// CHECK:STDOUT:     .C = %import_ref.3
+// CHECK:STDOUT:     .D = %import_ref.4
+// CHECK:STDOUT:     .a = %a
+// CHECK:STDOUT:     .b = %b.loc12
+// CHECK:STDOUT:     .c = %c.loc13
+// CHECK:STDOUT:     .d = %d
 // CHECK:STDOUT:   }
+// CHECK:STDOUT:   %import_ref.1: <function> = import_ref ir1, inst+1, used [template = imports.%A]
+// CHECK:STDOUT:   %import_ref.2: <function> = import_ref ir1, inst+5, used [template = imports.%B]
+// CHECK:STDOUT:   %import_ref.3: <function> = import_ref ir1, inst+17, used [template = imports.%C]
+// CHECK:STDOUT:   %import_ref.4: <function> = import_ref ir1, inst+18, used [template = imports.%D]
 // CHECK:STDOUT:   %A: <function> = fn_decl @A [template] {}
 // CHECK:STDOUT:   %B: <function> = fn_decl @B [template] {
-// CHECK:STDOUT:     %b.loc5_13.1: i32 = param b
-// CHECK:STDOUT:     @B.%b: i32 = bind_name b, %b.loc5_13.1
-// CHECK:STDOUT:     %return.var.loc5: ref i32 = var <return slot>
+// CHECK:STDOUT:     %b.loc7_13.1: i32 = param b
+// CHECK:STDOUT:     %b.loc7_13.2: i32 = bind_name b, %b.loc7_13.1
+// CHECK:STDOUT:     %return.var.loc7: ref i32 = var <return slot>
 // CHECK:STDOUT:   }
 // CHECK:STDOUT:   %C: <function> = fn_decl @C [template] {
-// CHECK:STDOUT:     %.loc6_21.1: (type,) = tuple_literal (i32)
-// CHECK:STDOUT:     %.loc6_21.2: type = converted %.loc6_21.1, constants.%.2 [template = constants.%.2]
-// CHECK:STDOUT:     %c.loc6_13.1: (i32,) = param c
-// CHECK:STDOUT:     @C.%c: (i32,) = bind_name c, %c.loc6_13.1
-// CHECK:STDOUT:     %.loc6_35: type = struct_type {.c: i32} [template = constants.%.3]
-// CHECK:STDOUT:     %return.var.loc6: ref {.c: i32} = var <return slot>
+// CHECK:STDOUT:     %.loc8_21.1: (type,) = tuple_literal (i32)
+// CHECK:STDOUT:     %.loc8_21.2: type = converted %.loc8_21.1, constants.%.2 [template = constants.%.2]
+// CHECK:STDOUT:     %c.loc8_13.1: (i32,) = param c
+// CHECK:STDOUT:     %c.loc8_13.2: (i32,) = bind_name c, %c.loc8_13.1
+// CHECK:STDOUT:     %.loc8_35: type = struct_type {.c: i32} [template = constants.%.3]
+// CHECK:STDOUT:     %return.var.loc8: ref {.c: i32} = var <return slot>
 // CHECK:STDOUT:   }
 // CHECK:STDOUT:   %D: <function> = fn_decl @D [template] {}
+// CHECK:STDOUT:   %.loc11_9.1: () = tuple_literal ()
+// CHECK:STDOUT:   %.loc11_9.2: type = converted %.loc11_9.1, constants.%.4 [template = constants.%.4]
+// CHECK:STDOUT:   %a.var: ref () = var a
+// CHECK:STDOUT:   %a: ref () = bind_name a, %a.var
+// CHECK:STDOUT:   %b.var: ref i32 = var b
+// CHECK:STDOUT:   %b.loc12: ref i32 = bind_name b, %b.var
+// CHECK:STDOUT:   %.loc13: type = struct_type {.c: i32} [template = constants.%.3]
+// CHECK:STDOUT:   %c.var: ref {.c: i32} = var c
+// CHECK:STDOUT:   %c.loc13: ref {.c: i32} = bind_name c, %c.var
+// CHECK:STDOUT:   %.loc14_9.1: () = tuple_literal ()
+// CHECK:STDOUT:   %.loc14_9.2: type = converted %.loc14_9.1, constants.%.4 [template = constants.%.4]
+// CHECK:STDOUT:   %d.var: ref () = var d
+// CHECK:STDOUT:   %d: ref () = bind_name d, %d.var
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: fn @A();
@@ -191,3 +414,259 @@ extern fn D();
 // CHECK:STDOUT:
 // CHECK:STDOUT: fn @D();
 // CHECK:STDOUT:
+// CHECK:STDOUT: fn @__global_init() {
+// CHECK:STDOUT: !entry:
+// CHECK:STDOUT:   %A.ref: <function> = name_ref A, file.%import_ref.1 [template = imports.%A]
+// CHECK:STDOUT:   %.loc11: init () = call %A.ref()
+// CHECK:STDOUT:   assign file.%a.var, %.loc11
+// CHECK:STDOUT:   %B.ref: <function> = name_ref B, file.%import_ref.2 [template = imports.%B]
+// CHECK:STDOUT:   %.loc12_16: i32 = int_literal 1 [template = constants.%.5]
+// CHECK:STDOUT:   %.loc12_15: init i32 = call %B.ref(%.loc12_16)
+// CHECK:STDOUT:   assign file.%b.var, %.loc12_15
+// CHECK:STDOUT:   %C.ref: <function> = name_ref C, file.%import_ref.3 [template = imports.%C]
+// CHECK:STDOUT:   %.loc13_23: i32 = int_literal 1 [template = constants.%.5]
+// CHECK:STDOUT:   %.loc13_25.1: (i32,) = tuple_literal (%.loc13_23)
+// CHECK:STDOUT:   %.loc13_25.2: (i32,) = tuple_value (%.loc13_23) [template = constants.%.6]
+// CHECK:STDOUT:   %.loc13_25.3: (i32,) = converted %.loc13_25.1, %.loc13_25.2 [template = constants.%.6]
+// CHECK:STDOUT:   %.loc13_21: init {.c: i32} = call %C.ref(%.loc13_25.3)
+// CHECK:STDOUT:   assign file.%c.var, %.loc13_21
+// CHECK:STDOUT:   %D.ref: <function> = name_ref D, file.%import_ref.4 [template = imports.%D]
+// CHECK:STDOUT:   %.loc14: init () = call %D.ref()
+// CHECK:STDOUT:   assign file.%d.var, %.loc14
+// CHECK:STDOUT:   return
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: --- redecl_extern_api.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %.1: type = tuple_type (type) [template]
+// CHECK:STDOUT:   %.2: type = tuple_type (i32) [template]
+// CHECK:STDOUT:   %.3: type = struct_type {.c: i32} [template]
+// CHECK:STDOUT:   %.4: type = tuple_type () [template]
+// CHECK:STDOUT:   %.5: i32 = int_literal 1 [template]
+// CHECK:STDOUT:   %.6: (i32,) = tuple_value (%.5) [template]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace [template] {
+// CHECK:STDOUT:     .A = %import_ref.1
+// CHECK:STDOUT:     .B = %import_ref.2
+// CHECK:STDOUT:     .C = %import_ref.3
+// CHECK:STDOUT:     .D = %import_ref.4
+// CHECK:STDOUT:     .a = %a
+// CHECK:STDOUT:     .b = %b.loc12
+// CHECK:STDOUT:     .c = %c.loc13
+// CHECK:STDOUT:     .d = %d
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %import_ref.1: <function> = import_ref ir1, inst+1, used [template = imports.%A]
+// CHECK:STDOUT:   %import_ref.2: <function> = import_ref ir1, inst+5, used [template = imports.%B]
+// CHECK:STDOUT:   %import_ref.3: <function> = import_ref ir1, inst+17, used [template = imports.%C]
+// CHECK:STDOUT:   %import_ref.4: <function> = import_ref ir1, inst+18, used [template = imports.%D]
+// CHECK:STDOUT:   %A: <function> = fn_decl @A [template] {}
+// CHECK:STDOUT:   %B: <function> = fn_decl @B [template] {
+// CHECK:STDOUT:     %b.loc7_13.1: i32 = param b
+// CHECK:STDOUT:     %b.loc7_13.2: i32 = bind_name b, %b.loc7_13.1
+// CHECK:STDOUT:     %return.var.loc7: ref i32 = var <return slot>
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %C: <function> = fn_decl @C [template] {
+// CHECK:STDOUT:     %.loc8_21.1: (type,) = tuple_literal (i32)
+// CHECK:STDOUT:     %.loc8_21.2: type = converted %.loc8_21.1, constants.%.2 [template = constants.%.2]
+// CHECK:STDOUT:     %c.loc8_13.1: (i32,) = param c
+// CHECK:STDOUT:     %c.loc8_13.2: (i32,) = bind_name c, %c.loc8_13.1
+// CHECK:STDOUT:     %.loc8_35: type = struct_type {.c: i32} [template = constants.%.3]
+// CHECK:STDOUT:     %return.var.loc8: ref {.c: i32} = var <return slot>
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %D: <function> = fn_decl @D [template] {}
+// CHECK:STDOUT:   %.loc11_9.1: () = tuple_literal ()
+// CHECK:STDOUT:   %.loc11_9.2: type = converted %.loc11_9.1, constants.%.4 [template = constants.%.4]
+// CHECK:STDOUT:   %a.var: ref () = var a
+// CHECK:STDOUT:   %a: ref () = bind_name a, %a.var
+// CHECK:STDOUT:   %b.var: ref i32 = var b
+// CHECK:STDOUT:   %b.loc12: ref i32 = bind_name b, %b.var
+// CHECK:STDOUT:   %.loc13: type = struct_type {.c: i32} [template = constants.%.3]
+// CHECK:STDOUT:   %c.var: ref {.c: i32} = var c
+// CHECK:STDOUT:   %c.loc13: ref {.c: i32} = bind_name c, %c.var
+// CHECK:STDOUT:   %.loc14_9.1: () = tuple_literal ()
+// CHECK:STDOUT:   %.loc14_9.2: type = converted %.loc14_9.1, constants.%.4 [template = constants.%.4]
+// CHECK:STDOUT:   %d.var: ref () = var d
+// CHECK:STDOUT:   %d: ref () = bind_name d, %d.var
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @A();
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @B(%b: i32) -> i32;
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @C(%c: (i32,)) -> {.c: i32};
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @D();
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @__global_init() {
+// CHECK:STDOUT: !entry:
+// CHECK:STDOUT:   %A.ref: <function> = name_ref A, file.%import_ref.1 [template = imports.%A]
+// CHECK:STDOUT:   %.loc11: init () = call %A.ref()
+// CHECK:STDOUT:   assign file.%a.var, %.loc11
+// CHECK:STDOUT:   %B.ref: <function> = name_ref B, file.%import_ref.2 [template = imports.%B]
+// CHECK:STDOUT:   %.loc12_16: i32 = int_literal 1 [template = constants.%.5]
+// CHECK:STDOUT:   %.loc12_15: init i32 = call %B.ref(%.loc12_16)
+// CHECK:STDOUT:   assign file.%b.var, %.loc12_15
+// CHECK:STDOUT:   %C.ref: <function> = name_ref C, file.%import_ref.3 [template = imports.%C]
+// CHECK:STDOUT:   %.loc13_23: i32 = int_literal 1 [template = constants.%.5]
+// CHECK:STDOUT:   %.loc13_25.1: (i32,) = tuple_literal (%.loc13_23)
+// CHECK:STDOUT:   %.loc13_25.2: (i32,) = tuple_value (%.loc13_23) [template = constants.%.6]
+// CHECK:STDOUT:   %.loc13_25.3: (i32,) = converted %.loc13_25.1, %.loc13_25.2 [template = constants.%.6]
+// CHECK:STDOUT:   %.loc13_21: init {.c: i32} = call %C.ref(%.loc13_25.3)
+// CHECK:STDOUT:   assign file.%c.var, %.loc13_21
+// CHECK:STDOUT:   %D.ref: <function> = name_ref D, file.%import_ref.4 [template = imports.%D]
+// CHECK:STDOUT:   %.loc14: init () = call %D.ref()
+// CHECK:STDOUT:   assign file.%d.var, %.loc14
+// CHECK:STDOUT:   return
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: --- fail_todo_merge.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %.1: type = tuple_type () [template]
+// CHECK:STDOUT:   %.2: i32 = int_literal 1 [template]
+// CHECK:STDOUT:   %.3: type = struct_type {.c: i32} [template]
+// CHECK:STDOUT:   %.4: type = tuple_type (i32) [template]
+// CHECK:STDOUT:   %.5: (i32,) = tuple_value (%.2) [template]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace [template] {
+// CHECK:STDOUT:     .A = %import_ref.1
+// CHECK:STDOUT:     .B = %import_ref.2
+// CHECK:STDOUT:     .C = %import_ref.3
+// CHECK:STDOUT:     .D = %import_ref.4
+// CHECK:STDOUT:     .a = %a
+// CHECK:STDOUT:     .b = %b
+// CHECK:STDOUT:     .c = %c
+// CHECK:STDOUT:     .d = %d
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %import_ref.1: <function> = import_ref ir1, inst+1, used [template = imports.%A]
+// CHECK:STDOUT:   %import_ref.2: <function> = import_ref ir1, inst+5, used [template = imports.%B]
+// CHECK:STDOUT:   %import_ref.3: <function> = import_ref ir1, inst+17, used [template = imports.%C]
+// CHECK:STDOUT:   %import_ref.4: <function> = import_ref ir1, inst+18, used [template = imports.%D]
+// CHECK:STDOUT:   %import_ref.5 = import_ref ir2, inst+1, unused
+// CHECK:STDOUT:   %import_ref.6 = import_ref ir2, inst+5, unused
+// CHECK:STDOUT:   %import_ref.7 = import_ref ir2, inst+17, unused
+// CHECK:STDOUT:   %import_ref.8 = import_ref ir2, inst+18, unused
+// CHECK:STDOUT:   %.loc59_9.1: () = tuple_literal ()
+// CHECK:STDOUT:   %.loc59_9.2: type = converted %.loc59_9.1, constants.%.1 [template = constants.%.1]
+// CHECK:STDOUT:   %a.var: ref () = var a
+// CHECK:STDOUT:   %a: ref () = bind_name a, %a.var
+// CHECK:STDOUT:   %b.var: ref i32 = var b
+// CHECK:STDOUT:   %b: ref i32 = bind_name b, %b.var
+// CHECK:STDOUT:   %.loc61: type = struct_type {.c: i32} [template = constants.%.3]
+// CHECK:STDOUT:   %c.var: ref {.c: i32} = var c
+// CHECK:STDOUT:   %c: ref {.c: i32} = bind_name c, %c.var
+// CHECK:STDOUT:   %.loc62_9.1: () = tuple_literal ()
+// CHECK:STDOUT:   %.loc62_9.2: type = converted %.loc62_9.1, constants.%.1 [template = constants.%.1]
+// CHECK:STDOUT:   %d.var: ref () = var d
+// CHECK:STDOUT:   %d: ref () = bind_name d, %d.var
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @A();
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @B(%b: i32) -> i32;
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @C(%c: (i32,)) -> {.c: i32};
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @D();
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @__global_init() {
+// CHECK:STDOUT: !entry:
+// CHECK:STDOUT:   %A.ref: <function> = name_ref A, file.%import_ref.1 [template = imports.%A]
+// CHECK:STDOUT:   %.loc59: init () = call %A.ref()
+// CHECK:STDOUT:   assign file.%a.var, %.loc59
+// CHECK:STDOUT:   %B.ref: <function> = name_ref B, file.%import_ref.2 [template = imports.%B]
+// CHECK:STDOUT:   %.loc60_16: i32 = int_literal 1 [template = constants.%.2]
+// CHECK:STDOUT:   %.loc60_15: init i32 = call %B.ref(%.loc60_16)
+// CHECK:STDOUT:   assign file.%b.var, %.loc60_15
+// CHECK:STDOUT:   %C.ref: <function> = name_ref C, file.%import_ref.3 [template = imports.%C]
+// CHECK:STDOUT:   %.loc61_23: i32 = int_literal 1 [template = constants.%.2]
+// CHECK:STDOUT:   %.loc61_25.1: (i32,) = tuple_literal (%.loc61_23)
+// CHECK:STDOUT:   %.loc61_25.2: (i32,) = tuple_value (%.loc61_23) [template = constants.%.5]
+// CHECK:STDOUT:   %.loc61_25.3: (i32,) = converted %.loc61_25.1, %.loc61_25.2 [template = constants.%.5]
+// CHECK:STDOUT:   %.loc61_21: init {.c: i32} = call %C.ref(%.loc61_25.3)
+// CHECK:STDOUT:   assign file.%c.var, %.loc61_21
+// CHECK:STDOUT:   %D.ref: <function> = name_ref D, file.%import_ref.4 [template = imports.%D]
+// CHECK:STDOUT:   %.loc62: init () = call %D.ref()
+// CHECK:STDOUT:   assign file.%d.var, %.loc62
+// CHECK:STDOUT:   return
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: --- fail_todo_merge_reverse.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %.1: type = tuple_type () [template]
+// CHECK:STDOUT:   %.2: i32 = int_literal 1 [template]
+// CHECK:STDOUT:   %.3: type = struct_type {.c: i32} [template]
+// CHECK:STDOUT:   %.4: type = tuple_type (i32) [template]
+// CHECK:STDOUT:   %.5: (i32,) = tuple_value (%.2) [template]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace [template] {
+// CHECK:STDOUT:     .A = %import_ref.1
+// CHECK:STDOUT:     .B = %import_ref.2
+// CHECK:STDOUT:     .C = %import_ref.3
+// CHECK:STDOUT:     .D = %import_ref.4
+// CHECK:STDOUT:     .a = %a
+// CHECK:STDOUT:     .b = %b
+// CHECK:STDOUT:     .c = %c
+// CHECK:STDOUT:     .d = %d
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %import_ref.1: <function> = import_ref ir1, inst+1, used [template = imports.%A]
+// CHECK:STDOUT:   %import_ref.2: <function> = import_ref ir1, inst+5, used [template = imports.%B]
+// CHECK:STDOUT:   %import_ref.3: <function> = import_ref ir1, inst+17, used [template = imports.%C]
+// CHECK:STDOUT:   %import_ref.4: <function> = import_ref ir1, inst+18, used [template = imports.%D]
+// CHECK:STDOUT:   %import_ref.5 = import_ref ir2, inst+1, unused
+// CHECK:STDOUT:   %import_ref.6 = import_ref ir2, inst+5, unused
+// CHECK:STDOUT:   %import_ref.7 = import_ref ir2, inst+17, unused
+// CHECK:STDOUT:   %import_ref.8 = import_ref ir2, inst+18, unused
+// CHECK:STDOUT:   %.loc58_9.1: () = tuple_literal ()
+// CHECK:STDOUT:   %.loc58_9.2: type = converted %.loc58_9.1, constants.%.1 [template = constants.%.1]
+// CHECK:STDOUT:   %a.var: ref () = var a
+// CHECK:STDOUT:   %a: ref () = bind_name a, %a.var
+// CHECK:STDOUT:   %b.var: ref i32 = var b
+// CHECK:STDOUT:   %b: ref i32 = bind_name b, %b.var
+// CHECK:STDOUT:   %.loc60: type = struct_type {.c: i32} [template = constants.%.3]
+// CHECK:STDOUT:   %c.var: ref {.c: i32} = var c
+// CHECK:STDOUT:   %c: ref {.c: i32} = bind_name c, %c.var
+// CHECK:STDOUT:   %.loc61_9.1: () = tuple_literal ()
+// CHECK:STDOUT:   %.loc61_9.2: type = converted %.loc61_9.1, constants.%.1 [template = constants.%.1]
+// CHECK:STDOUT:   %d.var: ref () = var d
+// CHECK:STDOUT:   %d: ref () = bind_name d, %d.var
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @A();
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @B(%b: i32) -> i32;
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @C(%c: (i32,)) -> {.c: i32};
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @D();
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @__global_init() {
+// CHECK:STDOUT: !entry:
+// CHECK:STDOUT:   %A.ref: <function> = name_ref A, file.%import_ref.1 [template = imports.%A]
+// CHECK:STDOUT:   %.loc58: init () = call %A.ref()
+// CHECK:STDOUT:   assign file.%a.var, %.loc58
+// CHECK:STDOUT:   %B.ref: <function> = name_ref B, file.%import_ref.2 [template = imports.%B]
+// CHECK:STDOUT:   %.loc59_16: i32 = int_literal 1 [template = constants.%.2]
+// CHECK:STDOUT:   %.loc59_15: init i32 = call %B.ref(%.loc59_16)
+// CHECK:STDOUT:   assign file.%b.var, %.loc59_15
+// CHECK:STDOUT:   %C.ref: <function> = name_ref C, file.%import_ref.3 [template = imports.%C]
+// CHECK:STDOUT:   %.loc60_23: i32 = int_literal 1 [template = constants.%.2]
+// CHECK:STDOUT:   %.loc60_25.1: (i32,) = tuple_literal (%.loc60_23)
+// CHECK:STDOUT:   %.loc60_25.2: (i32,) = tuple_value (%.loc60_23) [template = constants.%.5]
+// CHECK:STDOUT:   %.loc60_25.3: (i32,) = converted %.loc60_25.1, %.loc60_25.2 [template = constants.%.5]
+// CHECK:STDOUT:   %.loc60_21: init {.c: i32} = call %C.ref(%.loc60_25.3)
+// CHECK:STDOUT:   assign file.%c.var, %.loc60_21
+// CHECK:STDOUT:   %D.ref: <function> = name_ref D, file.%import_ref.4 [template = imports.%D]
+// CHECK:STDOUT:   %.loc61: init () = call %D.ref()
+// CHECK:STDOUT:   assign file.%d.var, %.loc61
+// CHECK:STDOUT:   return
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
diff --git a/toolchain/check/testdata/function/definition/extern.carbon b/toolchain/check/testdata/function/definition/extern.carbon
index 421d9d1790daf..4f740c6bf8fa9 100644
--- a/toolchain/check/testdata/function/definition/extern.carbon
+++ b/toolchain/check/testdata/function/definition/extern.carbon
@@ -19,7 +19,7 @@ extern fn F() {}
 library "def_for_extern_decl" api;
 
 extern fn F();
-// CHECK:STDERR: fail_def_for_extern_decl.carbon:[[@LINE+7]]:1: ERROR: Cannot define `extern` function `F`.
+// CHECK:STDERR: fail_def_for_extern_decl.carbon:[[@LINE+7]]:1: ERROR: Redeclaring `extern` function `F` as non-`extern`.
 // CHECK:STDERR: fn F() {}
 // CHECK:STDERR: ^~~~~~~~
 // CHECK:STDERR: fail_def_for_extern_decl.carbon:[[@LINE-4]]:1: Previously declared `extern` here.
@@ -28,6 +28,21 @@ extern fn F();
 // CHECK:STDERR:
 fn F() {}
 
+// --- fail_extern_diag_suppressed.carbon
+
+library "extern_diag_suppressed" api;
+
+extern fn F();
+// CHECK:STDERR: fail_extern_diag_suppressed.carbon:[[@LINE+7]]:1: ERROR: Redundant redeclaration of function F.
+// CHECK:STDERR: fn F();
+// CHECK:STDERR: ^~~~~~~
+// CHECK:STDERR: fail_extern_diag_suppressed.carbon:[[@LINE-4]]:1: Previously declared here.
+// CHECK:STDERR: extern fn F();
+// CHECK:STDERR: ^~~~~~~~~~~~~~
+// CHECK:STDERR:
+fn F();
+fn F() {}
+
 // --- fail_extern_decl_after_def.carbon
 
 library "extern_decl_after_def" api;
@@ -70,6 +85,22 @@ extern fn F();
 // CHECK:STDOUT:   return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
+// CHECK:STDOUT: --- fail_extern_diag_suppressed.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace [template] {
+// CHECK:STDOUT:     .F = %F.loc4
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %F.loc4: <function> = fn_decl @F [template] {}
+// CHECK:STDOUT:   %F.loc12: <function> = fn_decl @F [template] {}
+// CHECK:STDOUT:   %F.loc13: <function> = fn_decl @F [template] {}
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @F() {
+// CHECK:STDOUT: !entry:
+// CHECK:STDOUT:   return
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
 // CHECK:STDOUT: --- fail_extern_decl_after_def.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: file {
diff --git a/toolchain/check/testdata/function/definition/fail_todo_import_forward_decl.carbon b/toolchain/check/testdata/function/definition/fail_todo_import_forward_decl.carbon
deleted file mode 100644
index 06008179cc449..0000000000000
--- a/toolchain/check/testdata/function/definition/fail_todo_import_forward_decl.carbon
+++ /dev/null
@@ -1,59 +0,0 @@
-// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
-// Exceptions. See /LICENSE for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-// AUTOUPDATE
-
-// --- a.carbon
-
-library "a" api;
-
-fn Foo();
-
-// --- fail_b.carbon
-
-library "b" api;
-
-import library "a";
-
-// TODO: This should be an error until we can be clear whether `a` or `b` should
-// own the definition, as there might be a separate definition in `a`'s impl.
-// CHECK:STDERR: fail_b.carbon:[[@LINE+9]]:1: ERROR: Duplicate name being declared in the same scope.
-// CHECK:STDERR: fn Foo() {}
-// CHECK:STDERR: ^~~~~~~~~~
-// CHECK:STDERR: fail_b.carbon:[[@LINE-7]]:1: In import.
-// CHECK:STDERR: import library "a";
-// CHECK:STDERR: ^~~~~~
-// CHECK:STDERR: a.carbon:4:1: Name is previously declared here.
-// CHECK:STDERR: fn Foo();
-// CHECK:STDERR: ^~~~~~~~~
-fn Foo() {}
-
-// CHECK:STDOUT: --- a.carbon
-// CHECK:STDOUT:
-// CHECK:STDOUT: file {
-// CHECK:STDOUT:   package: <namespace> = namespace [template] {
-// CHECK:STDOUT:     .Foo = %Foo
-// CHECK:STDOUT:   }
-// CHECK:STDOUT:   %Foo: <function> = fn_decl @Foo [template] {}
-// CHECK:STDOUT: }
-// CHECK:STDOUT:
-// CHECK:STDOUT: fn @Foo();
-// CHECK:STDOUT:
-// CHECK:STDOUT: --- fail_b.carbon
-// CHECK:STDOUT:
-// CHECK:STDOUT: file {
-// CHECK:STDOUT:   package: <namespace> = namespace [template] {
-// CHECK:STDOUT:     .Foo = %import_ref
-// CHECK:STDOUT:   }
-// CHECK:STDOUT:   %import_ref: <function> = import_ref ir1, inst+1, used [template = imports.%Foo]
-// CHECK:STDOUT:   %.loc17: <function> = fn_decl @.1 [template] {}
-// CHECK:STDOUT: }
-// CHECK:STDOUT:
-// CHECK:STDOUT: fn @Foo();
-// CHECK:STDOUT:
-// CHECK:STDOUT: fn @.1() {
-// CHECK:STDOUT: !entry:
-// CHECK:STDOUT:   return
-// CHECK:STDOUT: }
-// CHECK:STDOUT:
diff --git a/toolchain/check/testdata/function/definition/import.carbon b/toolchain/check/testdata/function/definition/import.carbon
index cb0c7606750f1..28f4261d9c694 100644
--- a/toolchain/check/testdata/function/definition/import.carbon
+++ b/toolchain/check/testdata/function/definition/import.carbon
@@ -4,6 +4,10 @@
 //
 // AUTOUPDATE
 
+// ============================================================================
+// Setup files
+// ============================================================================
+
 // --- a.carbon
 
 library "a" api;
@@ -11,10 +15,15 @@ library "a" api;
 fn A() {}
 fn B(b: i32) -> i32 { return b; }
 fn C(c: (i32,)) -> {.c: i32} { return {.c = c[0]}; }
+fn D();
+
+// ============================================================================
+// Test files
+// ============================================================================
 
-// --- b.carbon
+// --- basics.carbon
 
-library "b" api;
+library "basics" api;
 
 import library "a";
 
@@ -22,6 +31,38 @@ var a: () = A();
 var b: i32 = B(1);
 var c: {.c: i32} = C((1,));
 
+// --- fail_def_ownership.carbon
+
+library "def_ownership" api;
+
+import library "a";
+
+// CHECK:STDERR: fail_def_ownership.carbon:[[@LINE+5]]:1: ERROR: Only one library can declare function A without `extern`.
+// CHECK:STDERR: fn A() {};
+// CHECK:STDERR: ^~~~~~~~
+// CHECK:STDERR: fail_def_ownership.carbon: Previously declared here.
+// CHECK:STDERR:
+fn A() {};
+// CHECK:STDERR: fail_def_ownership.carbon:[[@LINE+5]]:1: ERROR: Only one library can declare function B without `extern`.
+// CHECK:STDERR: fn B(b: i32) -> i32;
+// CHECK:STDERR: ^~~~~~~~~~~~~~~~~~~~
+// CHECK:STDERR: fail_def_ownership.carbon: Previously declared here.
+// CHECK:STDERR:
+fn B(b: i32) -> i32;
+
+// --- fail_mix_extern_decl.carbon
+
+library "mix_extern_decl" api;
+
+import library "a";
+
+extern fn D();
+// CHECK:STDERR: fail_mix_extern_decl.carbon:[[@LINE+4]]:1: ERROR: Only one library can declare function D without `extern`.
+// CHECK:STDERR: fn D() {}
+// CHECK:STDERR: ^~~~~~~~
+// CHECK:STDERR: fail_mix_extern_decl.carbon: Previously declared here.
+fn D() {}
+
 // CHECK:STDOUT: --- a.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
@@ -36,6 +77,7 @@ var c: {.c: i32} = C((1,));
 // CHECK:STDOUT:     .A = %A
 // CHECK:STDOUT:     .B = %B
 // CHECK:STDOUT:     .C = %C
+// CHECK:STDOUT:     .D = %D
 // CHECK:STDOUT:   }
 // CHECK:STDOUT:   %A: <function> = fn_decl @A [template] {}
 // CHECK:STDOUT:   %B: <function> = fn_decl @B [template] {
@@ -51,6 +93,7 @@ var c: {.c: i32} = C((1,));
 // CHECK:STDOUT:     %.loc6_28: type = struct_type {.c: i32} [template = constants.%.3]
 // CHECK:STDOUT:     %return.var.loc6: ref {.c: i32} = var <return slot>
 // CHECK:STDOUT:   }
+// CHECK:STDOUT:   %D: <function> = fn_decl @D [template] {}
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: fn @A() {
@@ -75,7 +118,9 @@ var c: {.c: i32} = C((1,));
 // CHECK:STDOUT:   return %.loc6_49.3
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: --- b.carbon
+// CHECK:STDOUT: fn @D();
+// CHECK:STDOUT:
+// CHECK:STDOUT: --- basics.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %.1: type = tuple_type () [template]
@@ -90,6 +135,7 @@ var c: {.c: i32} = C((1,));
 // CHECK:STDOUT:     .A = %import_ref.1
 // CHECK:STDOUT:     .B = %import_ref.2
 // CHECK:STDOUT:     .C = %import_ref.3
+// CHECK:STDOUT:     .D = %import_ref.4
 // CHECK:STDOUT:     .a = %a
 // CHECK:STDOUT:     .b = %b
 // CHECK:STDOUT:     .c = %c
@@ -97,6 +143,7 @@ var c: {.c: i32} = C((1,));
 // CHECK:STDOUT:   %import_ref.1: <function> = import_ref ir1, inst+1, used [template = imports.%A]
 // CHECK:STDOUT:   %import_ref.2: <function> = import_ref ir1, inst+6, used [template = imports.%B]
 // CHECK:STDOUT:   %import_ref.3: <function> = import_ref ir1, inst+20, used [template = imports.%C]
+// CHECK:STDOUT:   %import_ref.4 = import_ref ir1, inst+30, unused
 // CHECK:STDOUT:   %.loc6_9.1: () = tuple_literal ()
 // CHECK:STDOUT:   %.loc6_9.2: type = converted %.loc6_9.1, constants.%.1 [template = constants.%.1]
 // CHECK:STDOUT:   %a.var: ref () = var a
@@ -133,3 +180,53 @@ var c: {.c: i32} = C((1,));
 // CHECK:STDOUT:   return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
+// CHECK:STDOUT: --- fail_def_ownership.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace [template] {
+// CHECK:STDOUT:     .A = %import_ref.1
+// CHECK:STDOUT:     .B = %import_ref.2
+// CHECK:STDOUT:     .C = %import_ref.3
+// CHECK:STDOUT:     .D = %import_ref.4
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %import_ref.1: <function> = import_ref ir1, inst+1, used [template = imports.%A]
+// CHECK:STDOUT:   %import_ref.2: <function> = import_ref ir1, inst+6, used [template = imports.%B]
+// CHECK:STDOUT:   %import_ref.3 = import_ref ir1, inst+20, unused
+// CHECK:STDOUT:   %import_ref.4 = import_ref ir1, inst+30, unused
+// CHECK:STDOUT:   %A: <function> = fn_decl @A [template] {}
+// CHECK:STDOUT:   %B: <function> = fn_decl @B [template] {
+// CHECK:STDOUT:     %b.loc17_6.1: i32 = param b
+// CHECK:STDOUT:     %b.loc17_6.2: i32 = bind_name b, %b.loc17_6.1
+// CHECK:STDOUT:     %return.var: ref i32 = var <return slot>
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @A() {
+// CHECK:STDOUT: !entry:
+// CHECK:STDOUT:   return
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @B(%b: i32) -> i32;
+// CHECK:STDOUT:
+// CHECK:STDOUT: --- fail_mix_extern_decl.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace [template] {
+// CHECK:STDOUT:     .A = %import_ref.1
+// CHECK:STDOUT:     .B = %import_ref.2
+// CHECK:STDOUT:     .C = %import_ref.3
+// CHECK:STDOUT:     .D = %import_ref.4
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %import_ref.1 = import_ref ir1, inst+1, unused
+// CHECK:STDOUT:   %import_ref.2 = import_ref ir1, inst+6, unused
+// CHECK:STDOUT:   %import_ref.3 = import_ref ir1, inst+20, unused
+// CHECK:STDOUT:   %import_ref.4: <function> = import_ref ir1, inst+30, used [template = imports.%D]
+// CHECK:STDOUT:   %D.loc6: <function> = fn_decl @D [template] {}
+// CHECK:STDOUT:   %D.loc11: <function> = fn_decl @D [template] {}
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @D() {
+// CHECK:STDOUT: !entry:
+// CHECK:STDOUT:   return
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
diff --git a/toolchain/check/testdata/packages/cross_package_import.carbon b/toolchain/check/testdata/packages/cross_package_import.carbon
index 1d9e28be561a9..f0ff127d0f7c0 100644
--- a/toolchain/check/testdata/packages/cross_package_import.carbon
+++ b/toolchain/check/testdata/packages/cross_package_import.carbon
@@ -18,8 +18,7 @@ fn F() {}
 
 package Other library "fn_extern" api;
 
-// TODO: Mark extern
-fn F();
+extern fn F();
 
 // --- other_fn_conflict.carbon
 
@@ -63,9 +62,9 @@ import Other library "fn";
 // CHECK:STDERR: fail_main_use_other_extern.carbon:[[@LINE+12]]:1: In import.
 // CHECK:STDERR: import Other library "fn_extern";
 // CHECK:STDERR: ^~~~~~
-// CHECK:STDERR: other_fn_extern.carbon:5:1: ERROR: Duplicate name being declared in the same scope.
-// CHECK:STDERR: fn F();
-// CHECK:STDERR: ^~~~~~~
+// CHECK:STDERR: other_fn_extern.carbon:4:1: ERROR: Duplicate name being declared in the same scope.
+// CHECK:STDERR: extern fn F();
+// CHECK:STDERR: ^~~~~~~~~~~~~~
 // CHECK:STDERR: fail_main_use_other_extern.carbon:[[@LINE-7]]:1: In import.
 // CHECK:STDERR: import Other library "fn";
 // CHECK:STDERR: ^~~~~~
@@ -133,15 +132,10 @@ import library "other_ns";
 // CHECK:STDERR:
 import Other library "fn";
 
-// CHECK:STDERR: fail_main_namespace_conflict.carbon:[[@LINE+10]]:1: ERROR: Duplicate name being declared in the same scope.
+// CHECK:STDERR: fail_main_namespace_conflict.carbon:[[@LINE+5]]:1: ERROR: Only one library can declare function F without `extern`.
 // CHECK:STDERR: fn Other.F() {}
 // CHECK:STDERR: ^~~~~~~~~~~~~~
-// CHECK:STDERR: fail_main_namespace_conflict.carbon:[[@LINE-5]]:1: In import.
-// CHECK:STDERR: import Other library "fn";
-// CHECK:STDERR: ^~~~~~
-// CHECK:STDERR: other_fn.carbon:4:1: Name is previously declared here.
-// CHECK:STDERR: fn F() {}
-// CHECK:STDERR: ^~~~~~~~
+// CHECK:STDERR: fail_main_namespace_conflict.carbon: Previously declared here.
 // CHECK:STDERR:
 fn Other.F() {}
 
@@ -350,12 +344,10 @@ fn Other.G() {}
 // CHECK:STDOUT:   %import_ref.1: <namespace> = import_ref ir1, inst+1, used
 // CHECK:STDOUT:   %Other: <namespace> = namespace %import_ref.1, [template] {}
 // CHECK:STDOUT:   %import_ref.2: <function> = import_ref ir2, inst+1, used [template = imports.%F]
-// CHECK:STDOUT:   %.loc27: <function> = fn_decl @.1 [template] {}
+// CHECK:STDOUT:   %F: <function> = fn_decl @F [template] {}
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: fn @F();
-// CHECK:STDOUT:
-// CHECK:STDOUT: fn @.1() {
+// CHECK:STDOUT: fn @F() {
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   return
 // CHECK:STDOUT: }
diff --git a/toolchain/diagnostics/diagnostic_kind.def b/toolchain/diagnostics/diagnostic_kind.def
index 728a77d7f8d4a..8b5fa9d1fca5c 100644
--- a/toolchain/diagnostics/diagnostic_kind.def
+++ b/toolchain/diagnostics/diagnostic_kind.def
@@ -160,6 +160,7 @@ CARBON_DIAGNOSTIC_KIND(MissingObjectInMethodCall)
 // Function declaration checking.
 CARBON_DIAGNOSTIC_KIND(FunctionPreviousDecl)
 CARBON_DIAGNOSTIC_KIND(FunctionRedecl)
+CARBON_DIAGNOSTIC_KIND(FunctionNonExternRedecl)
 CARBON_DIAGNOSTIC_KIND(FunctionPreviousDefinition)
 CARBON_DIAGNOSTIC_KIND(FunctionRedefinition)
 CARBON_DIAGNOSTIC_KIND(FunctionDefiningExtern)