Support builtin conversions of adaptor classes

When doing a builtin conversion of an adaptor class:
- if there's no conversion happening on the adaptor class itself,
- if the conversion is from and to the same adaptor class type,

Then we attempt the builtin conversion step on the foundation type as
well.

To do this, we must convert the value from its adaptor to its
foundation type. Then perform the conversion. And convert the converted
(foundation) value back to the adaptor class type.

This might sound funny since they are the same type, so there can be no
conversion happening. But in particular, this recursion allows copy
construction of the elements of a tuple or struct with more than one
element that is the foundation type of a class.

A struct or tuple with more than one element is no longer passed by
copy; its ValueRepr becomes a pointer. So it requires a builtin
conversion to perform the copy on its interior elements. This already
happens for these types in PerformBuiltinConversion, but now will also
be done for adaptors of these types.

toolchain/check/convert.cpp

@@ -707,7 +707,7 @@ static auto CanUseValueOfInitializer(const SemIR::File& sem_ir,
 }
 
 // Returns the non-adapter type that is compatible with the specified type.
-static auto GetCompatibleBaseType(Context& context, SemIR::TypeId type_id)
+static auto GetCompatibleFoundationType(Context& context, SemIR::TypeId type_id)
     -> SemIR::TypeId {
   // If the type is an adapter, its object representation type is its compatible
   // non-adapter type.
@@ -782,9 +782,11 @@ static auto PerformBuiltinConversion(Context& context, SemIR::LocId loc_id,
   // T explicitly converts to U if T is compatible with U.
   if (target.kind == ConversionTarget::Kind::ExplicitAs &&
       target.type_id != value_type_id) {
-    auto target_base_id = GetCompatibleBaseType(context, target.type_id);
-    auto value_base_id = GetCompatibleBaseType(context, value_type_id);
-    if (target_base_id == value_base_id) {
+    auto target_foundation_id =
+        GetCompatibleFoundationType(context, target.type_id);
+    auto value_foundation_id =
+        GetCompatibleFoundationType(context, value_type_id);
+    if (target_foundation_id == value_foundation_id) {
       // For a struct or tuple literal, perform a category conversion if
       // necessary.
       if (SemIR::GetExprCategory(context.sem_ir(), value_id) ==
@@ -905,6 +907,43 @@ static auto PerformBuiltinConversion(Context& context, SemIR::LocId loc_id,
     }
   }
 
+  // When converting between adaptor classes of the same type (we don't
+  // implicitly convert between different adaptors even if they adapt the same
+  // foundation type), and when no other conversion was available on the adaptor
+  // class itself, we do the builtin conversion step on their foundation type.
+  // To do so, we have to insert a conversion up to the foundation and back
+  // down.
+  if (value_type_id == target.type_id) {
+    auto foundation_value_type_id =
+        GetCompatibleFoundationType(context, value_type_id);
+    if (foundation_value_type_id != value_type_id) {
+      auto foundation_target_type_id =
+          GetCompatibleFoundationType(context, target.type_id);
+      CARBON_CHECK(foundation_value_type_id == foundation_target_type_id,
+                   "Same type ids have different foundation type ids");
+
+      auto foundation_value_id = context.AddInst<SemIR::AsCompatible>(
+          loc_id, {.type_id = foundation_value_type_id, .source_id = value_id});
+      // We recurse here, but at most one time, since we have walked the full
+      // adapt chain to the root foundation type before recursing.
+      auto converted_value_id =
+          PerformBuiltinConversion(context, loc_id, foundation_value_id,
+                                   {
+                                       .kind = target.kind,
+                                       .type_id = foundation_target_type_id,
+                                       .init_id = target.init_id,
+                                       .init_block = target.init_block,
+                                   });
+      auto target_value_id = context.AddInst<SemIR::AsCompatible>(
+          loc_id, {.type_id = target.type_id, .source_id = converted_value_id});
+      // If no builtin conversion actually happened, we orphan the two
+      // AsCompatible instructions and don't refer to them. If a conversion did
+      // happen, then we return the result of the second AsCompatible.
+      return converted_value_id == foundation_value_id ? value_id
+                                                       : target_value_id;
+    }
+  }
+
   // No builtin conversion applies.
   return value_id;
 }

toolchain/check/testdata/as/adapter_conversion.carbon

@@ -565,6 +565,8 @@ var b: B = {.x = 1} as B;
 // CHECK:STDOUT:   %B.ref.loc13: type = name_ref B, file.%B.decl [template = constants.%B]
 // CHECK:STDOUT:   %.loc13_42.1: ref %B = as_compatible %.loc13_36
 // CHECK:STDOUT:   %.loc13_42.2: ref %B = converted %.loc13_36, %.loc13_42.1
+// CHECK:STDOUT:   %.loc13_46.1: ref %A = as_compatible %.loc13_42.2
+// CHECK:STDOUT:   %.loc13_46.2: ref %B = as_compatible %.loc13_46.1
 // CHECK:STDOUT:   %.loc13_42.3: %B = bind_value %.loc13_42.2
 // CHECK:STDOUT:   %b_value: %B = bind_name b_value, %.loc13_42.3
 // CHECK:STDOUT:   %int_1.loc24: Core.IntLiteral = int_value 1 [template = constants.%int_1.1]
@@ -592,6 +594,8 @@ var b: B = {.x = 1} as B;
 // CHECK:STDOUT:   %B.ref.loc24: type = name_ref B, file.%B.decl [template = constants.%B]
 // CHECK:STDOUT:   %.loc24_41.1: ref %B = as_compatible %.loc24_35
 // CHECK:STDOUT:   %.loc24_41.2: ref %B = converted %.loc24_35, %.loc24_41.1
+// CHECK:STDOUT:   %.loc24_45.1: ref %A = as_compatible %.loc24_41.2
+// CHECK:STDOUT:   %.loc24_45.2: ref %B = as_compatible %.loc24_45.1
 // CHECK:STDOUT:   %.loc24_41.3: %B = bind_value %.loc24_41.2
 // CHECK:STDOUT:   assign file.%b_init.var, <error>
 // CHECK:STDOUT:   return
@@ -667,6 +671,8 @@ var b: B = {.x = 1} as B;
 // CHECK:STDOUT:   %.loc21_19: %struct_type.x.2 = struct_literal (%int_1)
 // CHECK:STDOUT:   %B.ref: type = name_ref B, file.%B.decl [template = constants.%B]
 // CHECK:STDOUT:   %.loc21_21: %B = converted %.loc21_19, <error> [template = <error>]
+// CHECK:STDOUT:   %.loc21_25.1: %A = as_compatible %.loc21_21 [template = <error>]
+// CHECK:STDOUT:   %.loc21_25.2: %B = as_compatible %.loc21_25.1 [template = <error>]
 // CHECK:STDOUT:   assign file.%b.var, <error>
 // CHECK:STDOUT:   return
 // CHECK:STDOUT: }