c++: ICE initializing array of aggrs [PR117985]

This crash started with my r12-7803 but I believe the problem lies
elsewhere.

build_vec_init has cleanup_flags whose purpose is -- if I grok this
correctly -- to avoid destructing an object multiple times.  Let's
say we are initializing an array of A.  Then we might end up in
a scenario similar to initlist-eh1.C:

  try
    {
      call A::A in a loop
      // #0
      try
        {
	  call a fn using the array
	}
      finally
	{
	  // #1
	  call A::~A in a loop
	}
    }
  catch
    {
      // #2
      call A::~A in a loop
    }

cleanup_flags makes us emit a statement like

  D.3048 = 2;

at #0 to disable performing the cleanup at #2, since #1 will take
care of the destruction of the array.

But if we are not emitting the loop because we can use a constant
initializer (and use a single { a, b, ...}), we shouldn't generate
the statement resetting the iterator to its initial value.  Otherwise
we crash in gimplify_var_or_parm_decl because it gets the stray decl
D.3048.

	PR c++/117985

gcc/cp/ChangeLog:

	* init.cc (build_vec_init): Pop CLEANUP_FLAGS if we're not
	generating the loop.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/initlist-array23.C: New test.
	* g++.dg/cpp0x/initlist-array24.C: New test.

gcc/cp/init.cc

@@ -5109,6 +5109,15 @@ build_vec_init (tree base, tree maxindex, tree init,
     {
       if (!saw_non_const)
 	{
+	  /* If we're not generating the loop, we don't need to reset the
+	     iterator.  */
+	  if (cleanup_flags
+	      && !vec_safe_is_empty (*cleanup_flags))
+	    {
+	      auto l = (*cleanup_flags)->last ();
+	      gcc_assert (TREE_PURPOSE (l) == iterator);
+	      (*cleanup_flags)->pop ();
+	    }
 	  tree const_init = build_constructor (atype, const_vec);
 	  return build2 (INIT_EXPR, atype, obase, const_init);
 	}

gcc/testsuite/g++.dg/cpp0x/initlist-array23.C

@@ -0,0 +1,28 @@
+// PR c++/117985
+// { dg-do compile { target c++11 } }
+
+struct _Vector_impl {
+  constexpr
+    _Vector_impl() {}
+};
+struct _Vector_base {
+  ~_Vector_base();
+  _Vector_impl _M_impl;
+};
+struct vector : private _Vector_base {};
+struct string {
+  string();
+};
+struct VEC {
+  vector pane{};
+};
+struct FOO {
+  VEC screen[1]{};
+  string debug_name;
+};
+
+int
+main ()
+{
+  FOO{};
+}

gcc/testsuite/g++.dg/cpp0x/initlist-array24.C

@@ -0,0 +1,27 @@
+// PR c++/117985
+// { dg-do compile { target c++20 } }
+
+struct _Vector_impl {
+  constexpr _Vector_impl() {}
+};
+struct _Vector_base {
+  constexpr ~_Vector_base() {}
+  _Vector_impl _M_impl;
+};
+struct vector : private _Vector_base {};
+struct string {
+  string();
+};
+struct VEC {
+  vector pane{};
+};
+struct FOO {
+  VEC screen[1]{};
+  string debug_name;
+};
+
+int
+main ()
+{
+  FOO{};
+}