c++: Fix crash in gimplifier with paren init of aggregates [PR94155]

Here we crash in the gimplifier because gimplify_init_ctor_eval doesn't
expect null indexes for a constructor:

      /* ??? Here's to hoping the front end fills in all of the indices,
         so we don't have to figure out what's missing ourselves.  */
      gcc_assert (purpose);

The indexes weren't filled because we never called reshape_init: for
a constructor that represents parenthesized initialization of an
aggregate we don't allow brace elision or designated initializers.

	PR c++/94155 - crash in gimplifier with paren init of aggregates.
	* init.c (build_vec_init): Fill in indexes.

	* g++.dg/cpp2a/paren-init22.C: New test.

gcc/cp/ChangeLog

+2020-04-04  Marek Polacek  <polacek@redhat.com>
+	    Jason Merrill  <jason@redhat.com>
+
+	PR c++/94155 - crash in gimplifier with paren init of aggregates.
+	* init.c (build_vec_init): Fill in indexes.
+
 2020-04-04  Jason Merrill  <jason@redhat.com>
 
 	PR c++/91377

gcc/cp/init.c

@@ -4438,6 +4438,8 @@ build_vec_init (tree base, tree maxindex, tree init,
 	    errors = true;
 	  if (try_const)
 	    {
+	      if (!field)
+		field = size_int (idx);
 	      tree e = maybe_constant_init (one_init);
 	      if (reduced_constant_expression_p (e))
 		{

gcc/testsuite/ChangeLog

+2020-04-04  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/94155 - crash in gimplifier with paren init of aggregates.
+	* g++.dg/cpp2a/paren-init22.C: New test.
+
 2020-04-05  Iain Sandoe  <iain@sandoe.co.uk>
 
 	* g++.dg/coroutines/torture/co-await-14-template-traits.C: Rename...

gcc/testsuite/g++.dg/cpp2a/paren-init22.C

+// PR c++/94155 - crash in gimplifier with paren init of aggregates.
+// { dg-do compile { target c++2a } }
+
+struct S { int i, j; };
+
+struct A {
+  S s;
+  constexpr A(S e) : s(e) {}
+};
+
+void
+f()
+{
+  A g[1]({{1, 1}});
+}