c++: Fix non-constant TARGET_EXPR constexpr handing [PR93998]

We ICE on the following testcase since I've added the SAVE_EXPR-like
constexpr handling where the TARGET_EXPR initializer (and cleanup) is
evaluated only once (because it might have side-effects like new or delete
expressions in it).
The problem is if the TARGET_EXPR (but I guess in theory SAVE_EXPR too)
initializer is *non_constant_p.  We still remember the result, but already
not that it is *non_constant_p.  Normally that wouldn't be a big problem,
if something is *non_constant_p, we only or into it and so the whole
expression will be non-constant too.  Except in the builtins handling,
we try to evaluate the arguments with non_constant_p pointing into a dummy1
bool which we ignore.  This is because some builtins might fold into a
constant even if they don't have a constexpr argument.  Unfortunately if
we evaluate the TARGET_EXPR first in the argument of such a builtin and then
once again, we don't set *non_constant_p.

So, either we don't remember the TARGET_EXPR/SAVE_EXPR result if it wasn't
constant, like the following patch does, or we could remember it, but in
some way that would make it clear that it is non-constant (e.g. by
pushing into the global->values SAVE_EXPR, SAVE_EXPR entry and perhaps
for TARGET_EXPR don't remember it on TARGET_EXPR_SLOT, but the TARGET_EXPR
itself and similarly push TARGET_EXPR, TARGET_EXPR and if we see those
after the lookup, diagnose + set *non_constant_p.  Or we could perhaps
during the builtin argument evaluation push expressions into a different
save_expr vec and undo them afterwards.

2020-03-03  Jakub Jelinek  <jakub@redhat.com>

	PR c++/93998
	* constexpr.c (cxx_eval_constant_expression)
	<case TARGET_EXPR, case SAVE_EXPR>: Don't record anything if
	*non_constant_p is true.

	* g++.dg/ext/pr93998.C: New test.

gcc/cp/ChangeLog

-2020-03-03  Jun Ma <JunMa@linux.alibaba.com>
+2020-03-03  Jakub Jelinek  <jakub@redhat.com>
+
+	PR c++/93998
+	* constexpr.c (cxx_eval_constant_expression)
+	<case TARGET_EXPR, case SAVE_EXPR>: Don't record anything if
+	*non_constant_p is true.
+
+2020-03-03  Jun Ma  <JunMa@linux.alibaba.com>
 
 	* coroutines.cc (finish_co_await_expr): Build co_await_expr
 	with unknown_type_node.

gcc/cp/constexpr.c

@@ -5474,9 +5474,10 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
       r = cxx_eval_constant_expression (ctx, TREE_OPERAND (t, 1),
 					false,
 					non_constant_p, overflow_p);
-      if (!*non_constant_p)
-	/* Adjust the type of the result to the type of the temporary.  */
-	r = adjust_temp_type (TREE_TYPE (t), r);
+      if (*non_constant_p)
+	break;
+      /* Adjust the type of the result to the type of the temporary.  */
+      r = adjust_temp_type (TREE_TYPE (t), r);
       if (TARGET_EXPR_CLEANUP (t) && !CLEANUP_EH_ONLY (t))
 	ctx->global->cleanups->safe_push (TARGET_EXPR_CLEANUP (t));
       r = unshare_constructor (r);
@@ -5528,6 +5529,8 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
 	{
 	  r = cxx_eval_constant_expression (ctx, TREE_OPERAND (t, 0), false,
 					    non_constant_p, overflow_p);
+	  if (*non_constant_p)
+	    break;
 	  ctx->global->values.put (t, r);
 	  if (ctx->save_exprs)
 	    ctx->save_exprs->safe_push (t);

gcc/testsuite/ChangeLog

+2020-03-03  Jakub Jelinek  <jakub@redhat.com>
+
+	PR c++/93998
+	* g++.dg/ext/pr93998.C: New test.
+
 2020-03-03  Dennis Zhang  <dennis.zhang@arm.com>
 
 	* gcc.target/arm/simd/bf16_cvt_1.c: New test.

gcc/testsuite/g++.dg/ext/pr93998.C

+// PR c++/93998
+// { dg-do compile { target c++11 } }
+
+struct C
+{
+  constexpr bool operator== (C x) const noexcept { return v == x.v; }
+  int v;
+};
+
+int
+foo (const C a, const C b, bool c)
+{
+  return __builtin_expect (!!(a == b || c), 1) ? 0 : 1;
+}