c++: Add -Wno-changes-meaning

In recent years this error has been coming up more because other compilers
don't diagnose it as consistently.  So let's add a flag for it, and be more
lenient about cases that aren't likely to cause bugs.

gcc/ChangeLog:

	* doc/invoke.texi: Document -Wno-changes-meaning.

gcc/c-family/ChangeLog:

	* c.opt: Add -Wno-changes-meaning.

gcc/cp/ChangeLog:

	* class.cc (note_name_declared_in_class): Change from permerror to
	-Wchanges-meaning pedwarn, forcing -pedantic-errors for most cases.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/changes-meaning2.C: New test.
	* g++.dg/warn/changes-meaning3.C: New test.

gcc/c-family/c.opt

@@ -494,6 +494,10 @@ Wcatch-value=
 C++ ObjC++ Var(warn_catch_value) Warning Joined RejectNegative UInteger LangEnabledBy(C++ ObjC++,Wall, 1, 0) IntegerRange(0, 3)
 Warn about catch handlers of non-reference type.
 
+Wchanges-meaning
+C++ ObjC++ Var(warn_changes_meaning) Warning Init(1)
+Complain about a name being declared as a class member after a previous use of the same name.
+
 Wchar-subscripts
 C ObjC C++ ObjC++ Var(warn_char_subscripts) Warning LangEnabledBy(C ObjC C++ ObjC++,Wall)
 Warn about subscripts whose type is \"char\".

gcc/cp/class.cc

@@ -9016,7 +9016,7 @@ note_name_declared_in_class (tree name, tree decl)
     return;
   /* The C language allows members to be declared with a type of the same
      name, and the C++ standard says this diagnostic is not required.  So
-     allow it in extern "C" blocks unless predantic is specified.
+     allow it in extern "C" blocks unless pedantic is specified.
      Allow it in all cases if -ms-extensions is specified.  */
   if ((!pedantic && current_lang_name == lang_name_c)
       || flag_ms_extensions)
@@ -9032,9 +9032,19 @@ note_name_declared_in_class (tree name, tree decl)
 	 A name N used in a class S shall refer to the same declaration
 	 in its context and when re-evaluated in the completed scope of
 	 S.  */
-      if (permerror (location_of (decl),
-		     "declaration of %q#D changes meaning of %qD",
-		     decl, OVL_NAME (decl)))
+      auto ov = make_temp_override (global_dc->pedantic_errors);
+      if (TREE_CODE (decl) == TYPE_DECL
+	  && TREE_CODE (olddecl) == TYPE_DECL
+	  && same_type_p (TREE_TYPE (decl), TREE_TYPE (olddecl)))
+	/* Different declaration, but same meaning; just warn.  */;
+      else if (flag_permissive)
+	/* Let -fpermissive make it a warning like past versions.  */;
+      else
+	/* Make it an error.  */
+	global_dc->pedantic_errors = 1;
+      if (pedwarn (location_of (decl), OPT_Wchanges_meaning,
+		   "declaration of %q#D changes meaning of %qD",
+		   decl, OVL_NAME (decl)))
 	{
 	  inform (loc, "used here to mean %q#D", olddecl);
 	  inform (location_of (olddecl), "declared here" );

gcc/doc/invoke.texi

@@ -6287,6 +6287,23 @@ union U @{
 
 @end itemize
 
+@item -Wno-changes-meaning @r{(C++ and Objective-C++ only)}
+C++ requires that unqualified uses of a name within a class have the
+same meaning in the complete scope of the class, so declaring the name
+after using it is ill-formed:
+@smallexample
+struct A;
+struct B1 @{ A a; typedef A A; @}; // warning, 'A' changes meaning
+struct B2 @{ A a; struct A @{ @}; @}; // error, 'A' changes meaning
+@end smallexample
+By default, the B1 case is only a warning because the two declarations
+have the same type, while the B2 case is an error.  Both diagnostics
+can be disabled with @option{-Wno-changes-meaning}.  Alternately, the
+error case can be reduced to a warning with
+@option{-Wno-error=changes-meaning} or @option{-fpermissive}.
+
+Both diagnostics are also suppressed by @option{-fms-extensions}.
+
 @item -Wchar-subscripts
 @opindex Wchar-subscripts
 @opindex Wno-char-subscripts

gcc/testsuite/g++.dg/warn/changes-meaning2.C

@@ -0,0 +1,16 @@
+// It's an error to redeclare a name after using it in the class, but be
+// lenient if it has the same meaning.
+
+// { dg-options "" }
+
+struct Lock { };
+struct Traits
+{
+  Lock lock;
+  typedef ::Lock Lock;		// { dg-warning -Wchanges-meaning }
+};
+struct Traits2
+{
+  Lock lock;
+  typedef int Lock;		// { dg-error -Wchanges-meaning }
+};

gcc/testsuite/g++.dg/warn/changes-meaning3.C

@@ -0,0 +1,13 @@
+// { dg-additional-options "-Wno-changes-meaning" }
+
+struct Lock { };
+struct Traits
+{
+  Lock lock;
+  typedef ::Lock Lock;
+};
+struct Traits2
+{
+  Lock lock;
+  typedef int Lock;
+};