c++: ICE with -fno-elide-constructors and trivial fn [PR101073]

In constexpr-nsdmi3.C, with -fno-elide-constructors, we don't elide
the Y::Y(const Y&) call used to initialize o.c.  So store_init_value
-> cxx_constant_init must constexpr-evaluate the call to Y::Y(const Y&)
in cxx_eval_call_expression.  It's a trivial function, so we do the
"Shortcut trivial constructor/op=" code and rather than evaluating
the function, we just create an assignment

  o.c = *(const struct Y &) (const struct Y *) &(&<PLACEHOLDER_EXPR struct X>)->b

which is a MODIFY_EXPR, so the preeval code in cxx_eval_store_expression
clears .ctor and .object, therefore we can't replace the PLACEHOLDER_EXPR
whereupon we crash at

      /* A placeholder without a referent.  We can get here when
         checking whether NSDMIs are noexcept, or in massage_init_elt;
         just say it's non-constant for now.  */
      gcc_assert (ctx->quiet);

The PLACEHOLDER_EXPR can also be on the LHS as in constexpr-nsdmi10.C.
I don't think we can do much here, but I noticed that the whole
trivial_fn_p (fun) block is only entered when -fno-elide-constructors.
This is true since GCC 9; it wasn't easy to bisect what changes made it
so, but r240845 is probably one of them.  -fno-elide-constructors is an
option for experiments only so it's not clear to me why we'd still want
to shortcut trivial constructor/op=.  I propose to remove the code and
add a checking assert to make sure we're not getting a trivial_fn_p
unless -fno-elide-constructors.

	PR c++/101073

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_eval_call_expression): Replace shortcutting trivial
	constructor/op= with a checking assert.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/constexpr-nsdmi3.C: New test.
	* g++.dg/cpp1y/constexpr-nsdmi10.C: New test.

1 files changed, 3 insertions, 22 deletions

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index aa2c1435..b4d3e95 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -2866,28 +2866,9 @@ cxx_eval_call_expression (const constexpr_ctx *ctx, tree t,
       ctx = &new_ctx;
     }
 
-  /* Shortcut trivial constructor/op=.  */
-  if (trivial_fn_p (fun))
-    {
-      tree init = NULL_TREE;
-      if (call_expr_nargs (t) == 2)
-	init = convert_from_reference (get_nth_callarg (t, 1));
-      else if (TREE_CODE (t) == AGGR_INIT_EXPR
-	       && AGGR_INIT_ZERO_FIRST (t))
-	init = build_zero_init (DECL_CONTEXT (fun), NULL_TREE, false);
-      if (init)
-	{
-	  tree op = get_nth_callarg (t, 0);
-	  if (is_dummy_object (op))
-	    op = ctx->object;
-	  else
-	    op = build1 (INDIRECT_REF, TREE_TYPE (TREE_TYPE (op)), op);
-	  tree set = build2 (MODIFY_EXPR, TREE_TYPE (op), op, init);
-	  new_ctx.call = &new_call;
-	  return cxx_eval_constant_expression (&new_ctx, set, lval,
-					       non_constant_p, overflow_p);
-	}
-    }
+  /* We used to shortcut trivial constructor/op= here, but nowadays
+     we can only get a trivial function here with -fno-elide-constructors.  */
+  gcc_checking_assert (!trivial_fn_p (fun) || !flag_elide_constructors);
 
   bool non_constant_args = false;
   new_call.bindings