c++: 'mutable' member within constexpr [PR92505]

This patch permits accessing 'mutable' members of local objects during
constexpr evaluation, while continuing to reject it for global objects
(as in the last line of cpp0x/constexpr-mutable1.C).  To distinguish
between the two cases, it looks like it suffices to just check
CONSTRUCTOR_MUTABLE_POSION in cxx_eval_component_reference before
deciding to reject a DECL_MUTABLE_P member access.

	PR c++/92505

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_eval_component_reference): Check non_constant_p
	sooner.  In C++14 or later, reject a DECL_MUTABLE_P member access
	only if CONSTRUCTOR_MUTABLE_POISION is also set.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/constexpr-mutable3.C: New test.
	* g++.dg/cpp1y/constexpr-mutable1.C: New test.

3 files changed, 32 insertions, 4 deletions

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index 57283ea..1063987 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -4088,6 +4088,8 @@ cxx_eval_component_reference (const constexpr_ctx *ctx, tree t,
   tree whole = cxx_eval_constant_expression (ctx, orig_whole,
 					     lval,
 					     non_constant_p, overflow_p);
+  if (*non_constant_p)
+    return t;
   if (INDIRECT_REF_P (whole)
       && integer_zerop (TREE_OPERAND (whole, 0)))
     {
@@ -4108,20 +4110,21 @@ cxx_eval_component_reference (const constexpr_ctx *ctx, tree t,
 			whole, part, NULL_TREE);
   /* Don't VERIFY_CONSTANT here; we only want to check that we got a
      CONSTRUCTOR.  */
-  if (!*non_constant_p && TREE_CODE (whole) != CONSTRUCTOR)
+  if (TREE_CODE (whole) != CONSTRUCTOR)
     {
       if (!ctx->quiet)
 	error ("%qE is not a constant expression", orig_whole);
       *non_constant_p = true;
+      return t;
     }
-  if (DECL_MUTABLE_P (part))
+  if ((cxx_dialect < cxx14 || CONSTRUCTOR_MUTABLE_POISON (whole))
+      && DECL_MUTABLE_P (part))
     {
       if (!ctx->quiet)
 	error ("mutable %qD is not usable in a constant expression", part);
       *non_constant_p = true;
+      return t;
     }
-  if (*non_constant_p)
-    return t;
   bool pmf = TYPE_PTRMEMFUNC_P (TREE_TYPE (whole));
   FOR_EACH_CONSTRUCTOR_ELT (CONSTRUCTOR_ELTS (whole), i, field, value)
     {
diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-mutable3.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-mutable3.C
new file mode 100644
index 0000000..51499fa
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-mutable3.C
@@ -0,0 +1,9 @@
+// PR c++/92505
+// { dg-do compile { target c++11 } }
+
+struct A { mutable int m; };
+
+constexpr int f(A a) { return a.m; }
+
+static_assert(f({42}) == 42, "");
+// { dg-error "non-constant|mutable" "" { target c++11_only } .-1 }
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-mutable1.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-mutable1.C
new file mode 100644
index 0000000..6c47988
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-mutable1.C
@@ -0,0 +1,16 @@
+// PR c++/92505
+// { dg-do compile { target c++14 } }
+
+struct S { mutable int m; };
+
+static_assert(S{42}.m == 42, "");
+
+constexpr int f() {
+  S s = {40};
+  s.m++;
+  const auto& cs = s;
+  ++cs.m;
+  return cs.m;
+}
+
+static_assert(f() == 42, "");