c++: Fix ICE with ill-formed array list-initialization [PR93712]

My P0388R4 patch changed build_array_conv to create an identity
conversion at the start of the conversion chain and now we crash
in convert_like_real:

 7457     case ck_identity:
 7458       if (BRACE_ENCLOSED_INITIALIZER_P (expr))
 7459         {
 7460           int nelts = CONSTRUCTOR_NELTS (expr);
 7461           if (nelts == 0)
 7462             expr = build_value_init (totype, complain);
 7463           else if (nelts == 1)
 7464             expr = CONSTRUCTOR_ELT (expr, 0)->value;
 7465           else
 7466             gcc_unreachable ();  // HERE
 7467         }

in a test like this

  int f (int const (&)[2])
  { return f({1, "M"}); }

Instead of creating a ck_identity at the start of the conversion chain,
so that conv_get_original_expr can be used with a ck_aggr, let's set
u.expr for a ck_aggr, and adjust next_conversion not to try to see
what's next in the chain if it gets a ck_aggr.

2020-02-24  Marek Polacek  <polacek@redhat.com>

	PR c++/93712 - ICE with ill-formed array list-initialization.
	* call.c (next_conversion): Return NULL for ck_aggr.
	(build_aggr_conv): Set u.expr instead of u.next.
	(build_array_conv): Likewise.
	(build_complex_conv): Likewise.
	(conv_get_original_expr): Handle ck_aggr.

	* g++.dg/cpp0x/initlist-array11.C: New test.

gcc/cp/ChangeLog

+2020-02-24  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/93712 - ICE with ill-formed array list-initialization.
+	* call.c (next_conversion): Return NULL for ck_aggr.
+	(build_aggr_conv): Set u.expr instead of u.next.
+	(build_array_conv): Likewise.
+	(build_complex_conv): Likewise.
+	(conv_get_original_expr): Handle ck_aggr.
+
 2020-02-24  Jakub Jelinek  <jakub@redhat.com>
 
 	P1937R2 - Fixing inconsistencies between const{expr,eval} functions

gcc/cp/call.c

@@ -117,13 +117,13 @@ struct conversion {
     /* The next conversion in the chain.  Since the conversions are
        arranged from outermost to innermost, the NEXT conversion will
        actually be performed before this conversion.  This variant is
-       used only when KIND is neither ck_identity, ck_ambig nor
+       used only when KIND is neither ck_identity, ck_aggr, ck_ambig nor
        ck_list.  Please use the next_conversion function instead
        of using this field directly.  */
     conversion *next;
     /* The expression at the beginning of the conversion chain.  This
-       variant is used only if KIND is ck_identity or ck_ambig.  You can
-       use conv_get_original_expr to get this expression.  */
+       variant is used only if KIND is ck_identity, ck_aggr, or ck_ambig.
+       You can use conv_get_original_expr to get this expression.  */
     tree expr;
     /* The array of conversions for an initializer_list, so this
        variant is used only when KIN D is ck_list.  */
@@ -861,7 +861,8 @@ next_conversion (conversion *conv)
   if (conv == NULL
       || conv->kind == ck_identity
       || conv->kind == ck_ambig
-      || conv->kind == ck_list)
+      || conv->kind == ck_list
+      || conv->kind == ck_aggr)
     return NULL;
   return conv->u.next;
 }
@@ -1030,7 +1031,7 @@ build_aggr_conv (tree type, tree ctor, int flags, tsubst_flags_t complain)
   c->rank = cr_exact;
   c->user_conv_p = true;
   c->check_narrowing = true;
-  c->u.next = NULL;
+  c->u.expr = ctor;
   return c;
 }
 
@@ -1083,7 +1084,7 @@ build_array_conv (tree type, tree ctor, int flags, tsubst_flags_t complain)
   c->rank = rank;
   c->user_conv_p = user;
   c->bad_p = bad;
-  c->u.next = build_identity_conv (TREE_TYPE (ctor), ctor);
+  c->u.expr = ctor;
   return c;
 }
 
@@ -1129,7 +1130,7 @@ build_complex_conv (tree type, tree ctor, int flags,
   c->rank = rank;
   c->user_conv_p = user;
   c->bad_p = bad;
-  c->u.next = NULL;
+  c->u.expr = ctor;
   return c;
 }
 
@@ -10500,7 +10501,7 @@ static tree
 conv_get_original_expr (conversion *c)
 {
   for (; c; c = next_conversion (c))
-    if (c->kind == ck_identity || c->kind == ck_ambig)
+    if (c->kind == ck_identity || c->kind == ck_ambig || c->kind == ck_aggr)
       return c->u.expr;
   return NULL_TREE;
 }

gcc/testsuite/ChangeLog

+2020-02-24  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/93712 - ICE with ill-formed array list-initialization.
+	* g++.dg/cpp0x/initlist-array11.C: New test.
+
 2020-02-20  Mark Eggleston  <mark.eggleston@codethink.com>
 
 	PR fortran/93604

gcc/testsuite/g++.dg/cpp0x/initlist-array11.C

+// PR c++/93712 - ICE with ill-formed array list-initialization.
+// { dg-do compile { target c++11 } }
+
+int f (const int (&)[2]);
+
+int g ()
+{
+  const int (&r)[2] = {1, "foo"}; // { dg-error "conversion" }
+  return f({1, "foo"}); // { dg-error "conversion" }
+}