https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66159
--- Comment #4 from Eric Gallager <egallager at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #3)
> Patch for stage 1:
>
> diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
> index 4fa546a086c..4dc4e751b28 100644
> --- a/gcc/cp/parser.c
> +++ b/gcc/cp/parser.c
> @@ -17818,9 +17818,11 @@ cp_parser_elaborated_type_specifier (cp_parser*
> parser,
> caught elsewhere in parsing. Those that are pointless arrive
> here. */
>
> - if (cp_lexer_next_token_is (parser->lexer, CPP_SEMICOLON)
> + if (warn_redundant_decls
> + && cp_lexer_next_token_is (parser->lexer, CPP_SEMICOLON)
> && !is_friend && !processing_explicit_instantiation)
> - warning (0, "declaration %qD does not declare anything", decl);
> + warning (OPT_Wredundant_decls,
> + "declaration %qD does not declare anything", decl);
>
> type = TREE_TYPE (decl);
> }
> diff --git a/gcc/testsuite/g++.dg/warn/forward-inner.C
> b/gcc/testsuite/g++.dg/warn/forward-inner.C
> index 5336d4ed946..f720a6cdae3 100644
> --- a/gcc/testsuite/g++.dg/warn/forward-inner.C
> +++ b/gcc/testsuite/g++.dg/warn/forward-inner.C
> @@ -1,5 +1,6 @@
> // Check that the compiler warns about inner-style forward declarations in
> // contexts where they're not actually illegal, but merely useless.
> +// { dg-options "-Wredundant-decls" }
>
> // Verify warnings for and within classes, and by extension, struct and
> union.
> class C1;
> @@ -70,7 +71,7 @@ template class TC6<int>::TC7; // Valid explicit
> instantiation, no warning
>
>
> // Verify that friend declarations, also easy to confuse with forward
> -// declrations, are similarly not warned about.
> +// declarations, are similarly not warned about.
> class C8 {
> public:
> class C9 { };
> @@ -79,3 +80,10 @@ class C10 {
> public:
> friend class C8::C9; // Valid friend declaration, no warning
> };
> +
> +#if __cplusplus >= 201103L
> +// Verify that alias-declarations using an elaborated-type-specifier and
> +// nested-name-specifier are not warned about (PR c++/66159).
> +struct C11;
> +using A1 = struct ::C11;
> +#endif
> diff --git a/gcc/testsuite/g++.dg/warn/pr36999.C
> b/gcc/testsuite/g++.dg/warn/pr36999.C
> index ce2286efcf4..6f69e192d02 100644
> --- a/gcc/testsuite/g++.dg/warn/pr36999.C
> +++ b/gcc/testsuite/g++.dg/warn/pr36999.C
> @@ -1,5 +1,6 @@
> /* PR36999: Erroneous "does not declare anything" warnings. */
> /* { dg-do compile } */
> +/* { dg-options "-Wredundant-decls" } */
>
> class C1 {
> public: class C2 { };
It's stage 1 now.
