c++: constexpr static data member instantiation [PR94523]

Here due to my recent change to store_init_value we were expanding the
initializer of aw knowing that we were initializing aw.  When
cxx_eval_call_expression finished the constructor, it wanted to look up the
value of aw to set TREE_READONLY on it, but we haven't set DECL_INITIAL yet,
so decl_constant_value tried to instantiate the initializer again.  And
infinite recursion.  Stopped by optimizing the case of asking for the value
of ctx->object, which is ctx->value.  It also would have worked to look in
the values hash table, so let's move that up before decl_constant_value as
well.

gcc/cp/ChangeLog
2020-04-09  Jason Merrill  <jason@redhat.com>

	PR c++/94523
	* constexpr.c (cxx_eval_constant_expression) [VAR_DECL]: Look at
	ctx->object and ctx->global->values first.

gcc/cp/ChangeLog

+2020-04-09  Jason Merrill  <jason@redhat.com>
+
+	PR c++/94523
+	* constexpr.c (cxx_eval_constant_expression) [VAR_DECL]: Look at
+	ctx->object and ctx->global->values first.
+
 2020-04-09  Marek Polacek  <polacek@redhat.com>
 
 	PR c++/93790

gcc/cp/constexpr.c

@@ -5485,6 +5485,15 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
 	 CONST_DECL for aggregate constants.  */
       if (lval)
 	return t;
+      else if (t == ctx->object)
+	return ctx->ctor;
+      if (VAR_P (t))
+	if (tree *p = ctx->global->values.get (t))
+	  if (*p != NULL_TREE)
+	    {
+	      r = *p;
+	      break;
+	    }
       if (COMPLETE_TYPE_P (TREE_TYPE (t))
 	  && is_really_empty_class (TREE_TYPE (t), /*ignore_vptr*/false))
 	{
@@ -5499,10 +5508,6 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
       if (TREE_CODE (r) == TARGET_EXPR
 	  && TREE_CODE (TARGET_EXPR_INITIAL (r)) == CONSTRUCTOR)
 	r = TARGET_EXPR_INITIAL (r);
-      if (VAR_P (r))
-	if (tree *p = ctx->global->values.get (r))
-	  if (*p != NULL_TREE)
-	    r = *p;
       if (DECL_P (r))
 	{
 	  if (!ctx->quiet)

gcc/testsuite/g++.dg/cpp1y/constexpr-static1.C

+// PR c++/94523
+// { dg-do compile { target c++14 } }
+
+template <bool, typename a> using b = a;
+struct d {
+  char ao;
+  template <typename ap> constexpr d(ap) : ao{} {}
+};
+template <int... au> struct e { static constexpr auto aw = d(au...); };
+template <int c> b<c, d> ax(e<1>::aw);