[Clang] [Parser] Improve diagnostic for friend concept (#105121)

Diagnose this early after parsing declaration specifiers; this allows us
to issue a better diagnostic. This also checks for `concept friend` and
concept declarations w/o a template-head because it’s easiest to do that
at the same time.

Fixes #45182.

clang/docs/ReleaseNotes.rst

@@ -241,6 +241,8 @@ Improvements to Clang's diagnostics
 
 - Don't emit duplicated dangling diagnostics. (#GH93386).
 
+- Improved diagnostic when trying to befriend a concept. (#GH45182).
+
 Improvements to Clang's time-trace
 ----------------------------------
 

clang/include/clang/Basic/DiagnosticParseKinds.td

@@ -974,6 +974,9 @@ def warn_cxx23_variadic_friends : Warning<
   "variadic 'friend' declarations are incompatible with C++ standards before C++2c">,
   DefaultIgnore, InGroup<CXXPre26Compat>;
 
+def err_friend_concept : Error<
+  "friend declaration cannot be a concept">;
+
 // C++11 default member initialization
 def ext_nonstatic_member_init : ExtWarn<
   "default member initializer for non-static data member is a C++11 "

clang/lib/Parse/ParseDeclCXX.cpp

@@ -3139,6 +3139,19 @@ Parser::DeclGroupPtrTy Parser::ParseCXXClassMemberDeclaration(
     return Actions.BuildDeclaratorGroup(Decls);
   }
 
+  // Befriending a concept is invalid and would already fail if
+  // we did nothing here, but this allows us to issue a more
+  // helpful diagnostic.
+  if (Tok.is(tok::kw_concept)) {
+    Diag(Tok.getLocation(),
+         DS.isFriendSpecified() || NextToken().is(tok::kw_friend)
+             ? diag::err_friend_concept
+             : diag::
+                   err_concept_decls_may_only_appear_in_global_namespace_scope);
+    SkipUntil(tok::semi, tok::r_brace, StopBeforeMatch);
+    return nullptr;
+  }
+
   ParsingDeclarator DeclaratorInfo(*this, DS, DeclAttrs,
                                    DeclaratorContext::Member);
   if (TemplateInfo.TemplateParams)

clang/test/Parser/friend-concept.cpp

@@ -0,0 +1,13 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++20 %s
+
+template<class>
+concept fooable = true;
+
+struct S {
+  template<class> friend concept x = requires { requires true; }; // expected-error {{friend declaration cannot be a concept}}
+  template<class> friend concept fooable; // expected-error {{friend declaration cannot be a concept}}
+  template<class> concept friend fooable; // expected-error {{expected unqualified-id}}
+  friend concept fooable; // expected-error {{friend declaration cannot be a concept}}
+  concept friend fooable; // expected-error {{friend declaration cannot be a concept}}
+  concept fooable; // expected-error {{concept declarations may only appear in global or namespace scope}}
+};