[Bug c++/66159] bogus warning for alias-declaration using elaborated-type-specifier

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66159

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
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 { };