Disallow base virtual in adapter

toolchain/check/handle_class.cpp

@@ -596,6 +596,25 @@ static auto CheckCompleteAdapterClassType(Context& context,
     return SemIR::InstId::BuiltinError;
   }
 
+  for (auto inst_id : context.inst_block_stack().PeekCurrentBlockContents()) {
+    if (auto function_decl =
+            context.insts().TryGetAs<SemIR::FunctionDecl>(inst_id)) {
+      auto& function = context.functions().Get(function_decl->function_id);
+      if (function.virtual_modifier ==
+          SemIR::Function::VirtualModifier::Virtual) {
+        CARBON_DIAGNOSTIC(AdaptWithVirtual, Error,
+                          "adapter with virtual function");
+        CARBON_DIAGNOSTIC(AdaptWithVirtualHere, Note,
+                          "first virtual function declaration is here");
+        context.emitter()
+            .Build(class_info.adapt_id, AdaptWithVirtual)
+            .Note(inst_id, AdaptWithVirtualHere)
+            .Emit();
+        return SemIR::InstId::BuiltinError;
+      }
+    }
+  }
+
   // The object representation of the adapter is the object representation
   // of the adapted type. This is the adapted type itself unless it's a class
   // type.

toolchain/check/testdata/class/fail_adapt_with_base.carbon

@@ -0,0 +1,77 @@
+// 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
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/class/fail_adapt_with_base.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/class/fail_adapt_with_base.carbon
+class Simple {};
+base class AdaptWithVirtual {
+  virtual fn F();
+  // CHECK:STDERR: fail_adapt_with_base.carbon:[[@LINE+6]]:3: error: adapter with virtual function
+  // CHECK:STDERR:   adapt Simple;
+  // CHECK:STDERR:   ^~~~~~~~~~~~~
+  // CHECK:STDERR: fail_adapt_with_base.carbon:[[@LINE-4]]:3: note: first virtual function declaration is here
+  // CHECK:STDERR:   virtual fn F();
+  // CHECK:STDERR:   ^~~~~~~~~~~~~~~
+  adapt Simple;
+}
+
+// CHECK:STDOUT: --- fail_adapt_with_base.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %Simple: type = class_type @Simple [template]
+// CHECK:STDOUT:   %.1: type = struct_type {} [template]
+// CHECK:STDOUT:   %.2: <witness> = complete_type_witness %.1 [template]
+// CHECK:STDOUT:   %AdaptWithVirtual: type = class_type @AdaptWithVirtual [template]
+// CHECK:STDOUT:   %F.type: type = fn_type @F [template]
+// CHECK:STDOUT:   %.3: type = tuple_type () [template]
+// CHECK:STDOUT:   %F: %F.type = struct_value () [template]
+// CHECK:STDOUT:   %.4: type = ptr_type %.1 [template]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: imports {
+// CHECK:STDOUT:   %Core: <namespace> = namespace file.%Core.import, [template] {
+// CHECK:STDOUT:     import Core//prelude
+// CHECK:STDOUT:     import Core//prelude/operators
+// CHECK:STDOUT:     import Core//prelude/types
+// CHECK:STDOUT:     import Core//prelude/operators/arithmetic
+// CHECK:STDOUT:     import Core//prelude/operators/as
+// CHECK:STDOUT:     import Core//prelude/operators/bitwise
+// CHECK:STDOUT:     import Core//prelude/operators/comparison
+// CHECK:STDOUT:     import Core//prelude/types/bool
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace [template] {
+// CHECK:STDOUT:     .Core = imports.%Core
+// CHECK:STDOUT:     .Simple = %Simple.decl
+// CHECK:STDOUT:     .AdaptWithVirtual = %AdaptWithVirtual.decl
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %Core.import = import Core
+// CHECK:STDOUT:   %Simple.decl: type = class_decl @Simple [template = constants.%Simple] {} {}
+// CHECK:STDOUT:   %AdaptWithVirtual.decl: type = class_decl @AdaptWithVirtual [template = constants.%AdaptWithVirtual] {} {}
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: class @Simple {
+// CHECK:STDOUT:   %.loc10: <witness> = complete_type_witness %.1 [template = constants.%.2]
+// CHECK:STDOUT:
+// CHECK:STDOUT: !members:
+// CHECK:STDOUT:   .Self = constants.%Simple
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: class @AdaptWithVirtual {
+// CHECK:STDOUT:   %F.decl: %F.type = fn_decl @F [template = constants.%F] {} {}
+// CHECK:STDOUT:   %Simple.ref: type = name_ref Simple, file.%Simple.decl [template = constants.%Simple]
+// CHECK:STDOUT:   adapt_decl %Simple
+// CHECK:STDOUT:
+// CHECK:STDOUT: !members:
+// CHECK:STDOUT:   .Self = constants.%AdaptWithVirtual
+// CHECK:STDOUT:   .F = %F.decl
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: virtual fn @F();
+// CHECK:STDOUT:

toolchain/diagnostics/diagnostic_kind.def

@@ -205,8 +205,10 @@ CARBON_DIAGNOSTIC_KIND(InvalidBuiltinSignature)
 CARBON_DIAGNOSTIC_KIND(AdaptDeclRepeated)
 CARBON_DIAGNOSTIC_KIND(AdaptWithBase)
 CARBON_DIAGNOSTIC_KIND(AdaptWithFields)
+CARBON_DIAGNOSTIC_KIND(AdaptWithVirtual)
 CARBON_DIAGNOSTIC_KIND(AdaptWithBaseHere)
 CARBON_DIAGNOSTIC_KIND(AdaptWithFieldHere)
+CARBON_DIAGNOSTIC_KIND(AdaptWithVirtualHere)
 CARBON_DIAGNOSTIC_KIND(BaseDeclRepeated)
 CARBON_DIAGNOSTIC_KIND(BaseIsFinal)
 CARBON_DIAGNOSTIC_KIND(BaseMissingExtend)