Basic name poisoning support

toolchain/check/context.cpp

@@ -216,12 +216,13 @@ auto Context::DiagnoseDuplicateName(SemIRLoc dup_def, SemIRLoc prev_def)
       .Emit();
 }
 
-auto Context::DiagnosePoisonedName(SemIRLoc dup_def) -> void {
+auto Context::DiagnosePoisonedName(SemIRLoc loc) -> void {
+  // TODO: Improve the diagnostic to point to where the name was poisoned.
   CARBON_DIAGNOSTIC(
-      NameDeclPoisoned, Error,
-      "cannot declare this name in this scope since is was already used "
-      "without qualification in the context of this scope");
-  emitter_->Build(dup_def, NameDeclPoisoned).Emit();
+      NameDeclUsedUnqualifiedBefore, Error,
+      "name previously used by unqualified name lookup and not found; "
+      "cannot later be declared");
+  emitter_->Build(loc, NameDeclUsedUnqualifiedBefore).Emit();
 }
 
 auto Context::DiagnoseNameNotFound(SemIRLoc loc, SemIR::NameId name_id)
@@ -335,25 +336,26 @@ auto Context::LookupUnqualifiedName(Parse::NodeId node_id,
 
   // Walk the non-lexical scopes and perform lookups into each of them.
   // Collect scopes to poison this name when it's found.
-  std::vector<LookupScope> scopes_to_poison;
+  llvm::SmallVector<LookupScope> scopes_to_poison;
   for (auto [index, lookup_scope_id, specific_id] :
        llvm::reverse(non_lexical_scopes)) {
     if (auto non_lexical_result =
             LookupQualifiedName(node_id, name_id,
                                 LookupScope{.name_scope_id = lookup_scope_id,
                                             .specific_id = specific_id},
                                 /*required=*/false);
-        non_lexical_result.inst_id.is_valid() &&
         !non_lexical_result.inst_id.is_poisoned()) {
-      // Poison the scopes for this name.
-      for (const auto [scope_id, specific_id] : scopes_to_poison) {
-        name_scopes().Get(scope_id).AddPoison(name_id);
-      }
+      if (non_lexical_result.inst_id.is_valid()) {
+        // Poison the scopes for this name.
+        for (const auto [scope_id, specific_id] : scopes_to_poison) {
+          name_scopes().Get(scope_id).AddPoison(name_id);
+        }
 
-      return non_lexical_result;
+        return non_lexical_result;
+      }
+      scopes_to_poison.push_back(
+          {.name_scope_id = lookup_scope_id, .specific_id = specific_id});
     }
-    scopes_to_poison.push_back(
-        {.name_scope_id = lookup_scope_id, .specific_id = specific_id});
   }
 
   if (lexical_result.is_valid()) {

toolchain/check/context.h

@@ -246,7 +246,7 @@ class Context {
   auto DiagnoseDuplicateName(SemIRLoc dup_def, SemIRLoc prev_def) -> void;
 
   // Prints a diagnostic for a poisoned name.
-  auto DiagnosePoisonedName(SemIRLoc dup_def) -> void;
+  auto DiagnosePoisonedName(SemIRLoc loc) -> void;
 
   // Prints a diagnostic for a missing name.
   auto DiagnoseNameNotFound(SemIRLoc loc, SemIR::NameId name_id) -> void;

toolchain/check/decl_name_stack.cpp

@@ -292,8 +292,8 @@ static auto CheckQualifierIsResolved(
       }
       return true;
 
-    case DeclNameStack::NameContext::State::Unresolved:
     case DeclNameStack::NameContext::State::Poisoned:
+    case DeclNameStack::NameContext::State::Unresolved:
       // Because more qualifiers were found, we diagnose that the earlier
       // qualifier failed to resolve.
       context.DiagnoseNameNotFound(name_context.loc_id,

toolchain/check/handle_class.cpp

@@ -114,7 +114,7 @@ static auto MergeOrAddName(Context& context, Parse::AnyClassDeclId node_id,
   }
 
   if (prev_id.is_poisoned()) {
-    // This is declaration of a poisoned name.
+    // This is a declaration of a poisoned name.
     context.DiagnosePoisonedName(class_decl_id);
     return;
   }

toolchain/check/handle_function.cpp

@@ -123,6 +123,11 @@ static auto TryMergeRedecl(Context& context, Parse::AnyFunctionDeclId node_id,
     return;
   }
 
+  if (prev_id.is_poisoned()) {
+    context.DiagnosePoisonedName(function_info.latest_decl_id());
+    return;
+  }
+
   auto prev_function_id = SemIR::FunctionId::Invalid;
   auto prev_import_ir_id = SemIR::ImportIRId::Invalid;
   CARBON_KIND_SWITCH(context.insts().Get(prev_id)) {

toolchain/check/handle_interface.cpp

@@ -66,8 +66,13 @@ static auto BuildInterfaceDecl(Context& context,
   auto existing_id = context.decl_name_stack().LookupOrAddName(
       name_context, interface_decl_id, introducer.modifier_set.GetAccessKind());
   if (existing_id.is_valid()) {
-    if (auto existing_interface_decl =
-            context.insts().Get(existing_id).TryAs<SemIR::InterfaceDecl>()) {
+    if (existing_id.is_poisoned()) {
+      // This is a declaration of a poisoned name.
+      context.DiagnosePoisonedName(interface_decl_id);
+    } else if (auto existing_interface_decl =
+                   context.insts()
+                       .Get(existing_id)
+                       .TryAs<SemIR::InterfaceDecl>()) {
       auto existing_interface =
           context.interfaces().Get(existing_interface_decl->interface_id);
       if (CheckRedeclParamsMatch(

toolchain/check/handle_namespace.cpp

@@ -43,10 +43,13 @@ auto HandleParseNode(Context& context, Parse::NamespaceId node_id) -> bool {
   auto existing_inst_id = context.decl_name_stack().LookupOrAddName(
       name_context, namespace_id, SemIR::AccessKind::Public);
   if (existing_inst_id.is_valid()) {
-    // If there's a name conflict with a namespace, "merge" by using the
-    // previous declaration. Otherwise, diagnose the issue.
-    if (auto existing =
-            context.insts().TryGetAs<SemIR::Namespace>(existing_inst_id)) {
+    if (existing_inst_id.is_poisoned()) {
+      context.DiagnosePoisonedName(namespace_id);
+    } else if (auto existing = context.insts().TryGetAs<SemIR::Namespace>(
+                   existing_inst_id)) {
+      // If there's a name conflict with a namespace, "merge" by using the
+      // previous declaration. Otherwise, diagnose the issue.
+
       // Point at the other namespace.
       namespace_inst.name_scope_id = existing->name_scope_id;
 

toolchain/check/import.cpp

@@ -334,6 +334,9 @@ static auto ImportScopeFromApiFile(Context& context,
   auto& impl_scope = context.name_scopes().Get(impl_scope_id);
 
   for (const auto& api_entry : api_scope.entries()) {
+    if (api_entry.inst_id.is_poisoned()) {
+      continue;
+    }
     auto impl_name_id =
         CopyNameFromImportIR(context, api_sem_ir, api_entry.name_id);
     if (auto ns =