[PR55641] drop spurious const_type from reference_type variables

Although C++ reference types, denoted by DW_TAG_reference_type in
DWARFv2+ debug info, are unchangeable, we output names of reference type
with DW_TAG_const_type, because internally we mark such variables as
TREE_READONLY.  That's an internal implementation detail that shouldn't
leak to debug information.  This patch fixes this.

for  gcc/ChangeLog

	PR debug/55641
	* dwarf2out.c (decl_quals): Don't map TREE_READONLY to
	TYPE_QUAL_CONST in reference-typed decls.

for  gcc/testsuite/ChangeLog

	PR debug/55641
	* g++.dg/debug/dwarf2/ref-1.C: New.

From-SVN: r239402

gcc/ChangeLog

 2016-08-12  Alexandre Oliva <aoliva@redhat.com>
 
+	PR debug/55641
+	* dwarf2out.c (decl_quals): Don't map TREE_READONLY to
+	TYPE_QUAL_CONST in reference-typed decls.
+
 	PR debug/49366
 	* dwarf2out.c (loc_list_from_tree_1): Expand some CONSTRUCTORs
 	in DW_OP_pieces, just enough to handle pointers to member

gcc/dwarf2out.c

@@ -11126,6 +11126,10 @@ static int
 decl_quals (const_tree decl)
 {
   return ((TREE_READONLY (decl)
+	   /* The C++ front-end correctly marks reference-typed
+	      variables as readonly, but from a language (and debug
+	      info) standpoint they are not const-qualified.  */
+	   && TREE_CODE (TREE_TYPE (decl)) != REFERENCE_TYPE
 	   ? TYPE_QUAL_CONST : TYPE_UNQUALIFIED)
 	  | (TREE_THIS_VOLATILE (decl)
 	     ? TYPE_QUAL_VOLATILE : TYPE_UNQUALIFIED));

gcc/testsuite/ChangeLog

 2016-08-12  Alexandre Oliva <aoliva@redhat.com>
 
+	PR debug/55641
+	* g++.dg/debug/dwarf2/ref-1.C: New.
+
 	PR debug/49366
 	* g++.dg/debug/dwarf2/template-params-12.H: New.
 	* g++.dg/debug/dwarf2/template-params-12f.C: New.

gcc/testsuite/g++.dg/debug/dwarf2/ref-1.C

+// { dg-do compile }
+// { dg-options "-O -g -dA -gno-strict-dwarf" }
+// { dg-final { scan-assembler-not "DW_TAG_const_type" { xfail { powerpc-ibm-aix* } } } }
+
+int x;
+int &y = x;
+
+typedef int &z_t;
+z_t z = x;
+
+void f(int &p) {}
+
+struct foo {
+  int &bar;
+  typedef int &bart;
+  bart fool;
+};
+
+void f3(struct foo &p) {}