[C++ Patch] PR 64644 (""warning: anonymous union with no members" should be an error with -pedantic-errors")

[Paolo Carlini <paolo.carlini@oracle.com>]

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Hi,

submitter and Manuel analyzed this a while ago and came to the 
conclusion - which I think is still valid vs the current working draft - 
that strictly speaking this kind of code violates [dcl.dcl], thus a 
pedwarn seems more suited than a plain warning. The below one-liner, 
suggested by Manuel at the time, passes testing on x86_64-linux together 
with my testsuite changes.

Thanks,

Paolo.

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/cp
2017-06-16  Manuel Lopez-Ibanez  <manu@gcc.gnu.org>
	    Paolo Carlini  <paolo.carlini@oracle.com>

	PR c++/64644
	* decl2.c (finish_anon_union): Complain about "anonymous union with
	no members" with a pedwarn.

/testsuite
2017-06-16  Manuel Lopez-Ibanez  <manu@gcc.gnu.org>
	    Paolo Carlini  <paolo.carlini@oracle.com>

	PR c++/64644
	* g++.dg/other/anon-union4.C: New.
	* g++.old-deja/g++.law/union4.C: Adjust.
	* g++.old-deja/g++.other/anon1.C: Likewise.

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Index: cp/decl2.c
===================================================================
--- cp/decl2.c	(revision 249233)
+++ cp/decl2.c	(working copy)
@@ -1559,7 +1559,7 @@ finish_anon_union (tree anon_union_decl)
     return;
   if (main_decl == NULL_TREE)
     {
-      warning (0, "anonymous union with no members");
+      pedwarn (input_location, 0, "anonymous union with no members");
       return;
     }
 
Index: testsuite/g++.dg/other/anon-union4.C
===================================================================
--- testsuite/g++.dg/other/anon-union4.C	(revision 0)
+++ testsuite/g++.dg/other/anon-union4.C	(working copy)
@@ -0,0 +1,3 @@
+// PR c++/64644
+
+static union { };  // { dg-error "anonymous union with no members" }
Index: testsuite/g++.old-deja/g++.law/union4.C
===================================================================
--- testsuite/g++.old-deja/g++.law/union4.C	(revision 249233)
+++ testsuite/g++.old-deja/g++.law/union4.C	(working copy)
@@ -10,4 +10,4 @@ static union {
         struct SS {
                 int ss;
         };
-};// { dg-warning "no members" }
+};// { dg-error "no members" }
Index: testsuite/g++.old-deja/g++.other/anon1.C
===================================================================
--- testsuite/g++.old-deja/g++.other/anon1.C	(revision 249233)
+++ testsuite/g++.old-deja/g++.other/anon1.C	(working copy)
@@ -3,4 +3,4 @@
 static union {
   union {
   };
-}; // { dg-warning "" } anonymous union with no members
+}; // { dg-error "" } anonymous union with no members