c++: Accept elaborated-enum-base with pedwarn

macOS SDK headers using the CF_ENUM macro can expand to invalid C++ code
of the form:

typedef enum T : BaseType T;

i.e. an elaborated-type-specifier with an additional enum-base.
Upstream LLVM can be made to accept the above construct with
-Wno-error=elaborated-enum-base.

This patch adds the -Welaborated-enum-base warning to GCC and adjusts
the C++ parser to emit this warning instead of rejecting this code
outright.

The macro expansion in the macOS headers occurs in the case that the
compiler declares support for enums with underlying type using
__has_feature, see
https://gcc.gnu.org/pipermail/gcc-patches/2023-May/618450.html

GCC rejecting this construct outright means that GCC fails to bootstrap
on Darwin in the case that it (correctly) implements __has_feature and
declares support for C++ enums with underlying type.

With this patch, GCC can bootstrap on Darwin in combination with the
(WIP) __has_feature patch posted at:
https://gcc.gnu.org/pipermail/gcc-patches/2023-May/617878.html

gcc/c-family/ChangeLog:

	* c.opt (Welaborated-enum-base): New.

gcc/ChangeLog:

	* doc/invoke.texi: Document -Welaborated-enum-base.

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_enum_specifier): Don't reject
	elaborated-type-specifier with enum-base, instead emit new
	Welaborated-enum-base warning.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/enum40.C: Adjust expected diagnostics.
	* g++.dg/cpp0x/forw_enum6.C: Likewise.
	* g++.dg/cpp0x/elab-enum-base.C: New test.

gcc/c-family/c.opt

@@ -1488,6 +1488,10 @@ Wsubobject-linkage
 C++ ObjC++ Var(warn_subobject_linkage) Warning Init(1)
 Warn if a class type has a base or a field whose type uses the anonymous namespace or depends on a type with no linkage.
 
+Welaborated-enum-base
+C++ ObjC++ Var(warn_elaborated_enum_base) Warning Init(1)
+Warn if an additional enum-base is used in an elaborated-type-specifier.
+
 Wduplicate-decl-specifier
 C ObjC Var(warn_duplicate_decl_specifier) Warning LangEnabledBy(C ObjC,Wall)
 Warn when a declaration has duplicate const, volatile, restrict or _Atomic specifier.

gcc/cp/parser.cc

@@ -21028,11 +21028,13 @@ cp_parser_enum_specifier (cp_parser* parser)
 
   /* Check for the `:' that denotes a specified underlying type in C++0x.
      Note that a ':' could also indicate a bitfield width, however.  */
+  location_t colon_loc = UNKNOWN_LOCATION;
   if (cp_lexer_next_token_is (parser->lexer, CPP_COLON))
     {
       cp_decl_specifier_seq type_specifiers;
 
       /* Consume the `:'.  */
+      colon_loc = cp_lexer_peek_token (parser->lexer)->location;
       cp_lexer_consume_token (parser->lexer);
 
       auto tdf
@@ -21081,10 +21083,13 @@ cp_parser_enum_specifier (cp_parser* parser)
 	  && cp_lexer_next_token_is_not (parser->lexer, CPP_SEMICOLON))
 	{
 	  if (has_underlying_type)
-	    cp_parser_commit_to_tentative_parse (parser);
-	  cp_parser_error (parser, "expected %<;%> or %<{%>");
-	  if (has_underlying_type)
-	    return error_mark_node;
+	    pedwarn (colon_loc,
+		     OPT_Welaborated_enum_base,
+		     "declaration of enumeration with "
+		     "fixed underlying type and no enumerator list is "
+		     "only permitted as a standalone declaration");
+	  else
+	    cp_parser_error (parser, "expected %<;%> or %<{%>");
 	}
     }
 

gcc/doc/invoke.texi

@@ -254,7 +254,8 @@ in the following sections.
 -Wdelete-non-virtual-dtor  -Wno-deprecated-array-compare
 -Wdeprecated-copy -Wdeprecated-copy-dtor
 -Wno-deprecated-enum-enum-conversion -Wno-deprecated-enum-float-conversion
--Weffc++  -Wno-exceptions -Wextra-semi  -Wno-inaccessible-base
+-Weffc++ -Wno-elaborated-enum-base
+-Wno-exceptions -Wextra-semi  -Wno-inaccessible-base
 -Wno-inherited-variadic-ctor  -Wno-init-list-lifetime
 -Winvalid-constexpr -Winvalid-imported-macros
 -Wno-invalid-offsetof  -Wno-literal-suffix
@@ -3845,6 +3846,15 @@ bool b = e <= 3.7;
 @option{-std=c++20}.  In pre-C++20 dialects, this warning can be enabled
 by @option{-Wenum-conversion}.
 
+@opindex Welaborated-enum-base
+@opindex Wno-elaborated-enum-base
+@item -Wno-elaborated-enum-base
+For C++11 and above, warn if an (invalid) additional enum-base is used
+in an elaborated-type-specifier.  That is, if an enum with given
+underlying type and no enumerator list is used in a declaration other
+than just a standalone declaration of the enum.  Enabled by default.  This
+warning is upgraded to an error with -pedantic-errors.
+
 @opindex Winit-list-lifetime
 @opindex Wno-init-list-lifetime
 @item -Wno-init-list-lifetime @r{(C++ and Objective-C++ only)}
@@ -6054,6 +6064,7 @@ errors by @option{-pedantic-errors}.  For instance:
 -Wchanges-meaning @r{(C++)}
 -Wcomma-subscript @r{(C++23 or later)}
 -Wdeclaration-after-statement @r{(C90 or earlier)}
+-Welaborated-enum-base @r{(C++11 or later)}
 -Wimplicit-int @r{(C99 or later)}
 -Wimplicit-function-declaration @r{(C99 or later)}
 -Wincompatible-pointer-types

gcc/testsuite/g++.dg/cpp0x/elab-enum-base.C

@@ -0,0 +1,7 @@
+// { dg-do compile { target c++11 } }
+// { dg-options "" }
+// Empty dg-options to override -pedantic-errors.
+
+typedef long CFIndex;
+typedef enum CFComparisonResult : CFIndex CFComparisonResult;
+// { dg-warning "declaration of enumeration with fixed underlying type" "" { target *-*-* } .-1 }

gcc/testsuite/g++.dg/cpp0x/enum40.C

@@ -4,23 +4,25 @@
 void
 foo ()
 {
-  enum : int a alignas;		// { dg-error "expected" }
+  enum : int a alignas;		// { dg-error "declaration of enum" }
+  // { dg-error {expected '\(' before ';'} "" { target *-*-* } .-1 }
 }
 
 void
 bar ()
 {
-  enum : int a;			// { dg-error "expected" }
+  enum : int a;			// { dg-error "declaration of enum" }
 }
 
 void
 baz ()
 {
-  enum class a : int b alignas;	// { dg-error "expected" }
+  enum class a : int b alignas;	// { dg-error "declaration of enum" }
+  // { dg-error {expected '\(' before ';'} "" { target *-*-* } .-1 }
 }
 
 void
 qux ()
 {
-  enum class a : int b;		// { dg-error "expected" }
+  enum class a : int b;		// { dg-error "declaration of enum" }
 }

gcc/testsuite/g++.dg/cpp0x/forw_enum6.C

@@ -23,7 +23,7 @@ enum class E7 : int; //ok
 
 enum class E3 e3; // { dg-error "scoped enum must not use" }
 enum struct E3 e4; // { dg-error "scoped enum must not use" }
-enum E5 : int e5; // { dg-error "expected|invalid type" }
+enum E5 : int e5; // { dg-error "declaration of enumeration with fixed underlying type|invalid type" }
 
 enum E6 : int { a, b, c }; // { dg-message "previous definition" }
 enum E6 : int { a, b, c }; // { dg-error "multiple definition" }