Clang produces unhelpful diagnostics for a concept declared in a function head

[cjdb]

Given void f(concept int);, Clang will emit the following:

error: variable has incomplete type 'void'
void f(concept int);
     ^
error: expected expression
void f(concept int);
       ^

Neither of these diagnostics describe the two problems with the source. Better diagnostics would look like:

error: concepts can only be declared a global entity, but it's declared as a function parameter here
void f(concept int);
       ^
error: expected unqualified-id
void f(concept int);
               ^

I'd personally prefer for the second one to be expected an unqualified identifier, got keyword 'int', but that's beyond the scope of this commit due to how much it impacts.

[11happy]

Hello @cjdb, I have written a local fix but not sure if this is alsong the expected lines of work, here the diagnostic can be changed accordingly I have added the following code to ExprResult Parser::ParseCastExpression in the clang/lib/Parse/ParseExpr.cpp

case tok::kw_concept:{
    if(getLangOpts().CPlusPlus20 || getLangOpts().C23)
      Diag(Tok.getLocation(),diag::err_concept_decls_may_only_appear_in_global_namespace_scope);
  }

working :

/home/happy/c++/n1.cpp:9:9: error: concept declarations may only appear in global or namespace scope
    9 |  void f(concept int);
      |         ^