[PATCH] c++/65579 - set readonly bit on static constexpr members of templates

[Martin Sebor]

While going through constexpr bugs looking for background
on one I'm currently working on I came across bug 65579 -
[C++11] gcc requires definition of a static constexpr member
even though it is not odr-used.

The bug points out that GCC (sometimes) emits references to
static constexpr data members of class templates even when
they aren't odr-used.  (A more detailed analysis of why this
happens is in my comment #1 on the bug.)

The attached rather trivial patch seems to fix the problem
and (somewhat to my surprise) pass regression tests on x86_64.

Martin

PR c++/65579 - [C++11] gcc requires definition of a static constexpr member
	even though it is not odr-used

gcc/testsuite/ChangeLog:
2016-03-09  Martin Sebor  <mse...@redhat.com>

	PR c++/65579
	* g++.dg/cpp0x/constexpr-static12.C: New test.

gcc/cp/ChangeLog:
2016-03-09  Martin Sebor  <mse...@redhat.com>

	PR c++/65579
	* typeck.c (cp_apply_type_quals_to_decl): Make sure constexpr
        data members of a template type are marked readonly.
diff --git a/gcc/cp/typeck.c b/gcc/cp/typeck.c
index 20f0afc..690eaa8 100644
--- a/gcc/cp/typeck.c
+++ b/gcc/cp/typeck.c
@@ -9266,7 +9266,9 @@ cp_apply_type_quals_to_decl (int type_quals, tree decl)
 
   /* If the type has (or might have) a mutable component, that component
      might be modified.  */
-  if (TYPE_HAS_MUTABLE_P (type) || !COMPLETE_TYPE_P (type))
+  if (TYPE_HAS_MUTABLE_P (type)
+      || (!COMPLETE_TYPE_P (type)
+	  && (!VAR_P (decl) || !DECL_DECLARED_CONSTEXPR_P (decl))))
     type_quals &= ~TYPE_QUAL_CONST;
 
   c_apply_type_quals_to_decl (type_quals, decl);
diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-static12.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-static12.C
new file mode 100644
index 0000000..abf1d66
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-static12.C
@@ -0,0 +1,23 @@
+// PR c++/65579 - [C++11] gcc requires definition of a static constexpr
+//   member even though it is not odr-used
+// { dg-do compile { target c++11 } }
+
+template <int>
+struct A
+{
+  int i;
+};
+
+struct B
+{
+  static constexpr A<0> constexpr_member = { 1 };
+};
+
+int foo ()
+{
+  return B::constexpr_member.i;
+}
+
+// Verify that the constexpr_member reference has been folded away
+// and isn't referenced in the assembly output.
+// { dg-final { scan-assembler-not "constexpr_member" } }