[Sema] Mark explicit specialization declaration in a friend invalid

[ychen]

Summary

Down the path, if there is a implicit instantiation, this may trigger
the assertion "Member specialization must be an explicit specialization"
in clang::FunctionDecl::setFunctionTemplateSpecialization.

[ychen]

ping?

[ychen]

ping:-)

[ychen]

Hi @aaron.ballman, does this make sense to you? Thanks.

[aaron.ballman]

Thanks for the fix for this -- I think the changes look like they're correct, but the test coverage needs a bit of work.

clang/lib/Sema/SemaDecl.cpp

9198

Alternatively, we could set Invalid = true; here and let the code on 9207 do its thing. We do both in this function, and I believe either approach is equivalent for this specific code path.

clang/test/CXX/temp/temp.spec/temp.expl.spec/p20.cpp

16–28 ↗

(On Diff #384976)

I don't think you should coop the existing test for an unrelated bug; can you add a new test to either SemaCXX or SemaTemplate?

[ychen]

• Address Aaron's comments.

[ychen]

• format

[aaron.ballman]

LGTM! The failing precommit CI tests look to be unrelated to this patch.

[ychen]

@aaron.ballman Thanks for the review!

clang/lib/Sema/SemaDecl.cpp

if (TemplateParams) {
             InsertLoc = D.getName().getSourceRange().getEnd();
             InsertLoc = getLocForEndOfToken(InsertLoc);
           }
 
           Diag(D.getIdentifierLoc(), diag::err_template_spec_decl_friend)
             << Name << RemoveRange
             << FixItHint::CreateRemoval(RemoveRange)
             << FixItHint::CreateInsertion(InsertLoc, "<>");
+          Invalid = true;
         }
       }
     } else {
       // Check that we can declare a template here.
       if (!TemplateParamLists.empty() && isMemberSpecialization &&
           CheckTemplateDeclScope(S, TemplateParamLists.back()))
         NewFD->setInvalidDecl();
 

clang/test/CXX/temp/temp.spec/temp.expl.spec/p20-2.cpp

+// RUN: %clang_cc1 -fsyntax-only -verify %s
+template<typename T>
+void f(T);
+
+template<typename T>
+struct A {
+  // expected-error@+1{{cannot declare an explicit specialization in a friend}}
+  template <> friend void f<>(int) {}
+};
+
+// Makes sure implicit instantiation here does not trigger
+// the assertion "Member specialization must be an explicit specialization"
+void foo(void) {
+    A<int> a;
+}