c++: build initializer_list<string> in a loop [PR105838]

I previously applied this change in r13-4565 but reverted it due to
PR108071.  That PR was then fixed by r13-4712, but I didn't re-apply this
change then because we weren't making the array static; since r14-1500 for
PR110070 we now make the initializer array static, so let's bring this back.

In situations where the maybe_init_list_as_range optimization isn't viable,
we can build an initializer_list<string> with a loop over a constant array
of string literals.

This is represented using a VEC_INIT_EXPR, which required adjusting a couple
of places that expected the initializer array to have the same type as the
target array and fixing build_vec_init not to undo our efforts.

	PR c++/105838

gcc/cp/ChangeLog:

	* call.cc (convert_like_internal) [ck_list]: Use
	maybe_init_list_as_array.
	* constexpr.cc (cxx_eval_vec_init_1): Init might have
	a different type.
	* tree.cc (build_vec_init_elt): Likewise.
	* init.cc (build_vec_init): Handle from_array from a
	TARGET_EXPR.  Retain TARGET_EXPR of a different type.

gcc/testsuite/ChangeLog:

	* g++.dg/tree-ssa/initlist-opt5.C: New test.

1 files changed, 3 insertions, 3 deletions

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index 9122a5e..432b3a2 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -5243,12 +5243,12 @@ cxx_eval_vec_init_1 (const constexpr_ctx *ctx, tree atype, tree init,
       else
 	{
 	  /* Copying an element.  */
-	  gcc_assert (same_type_ignoring_top_level_qualifiers_p
-		      (atype, TREE_TYPE (init)));
 	  eltinit = cp_build_array_ref (input_location, init, idx, complain);
 	  if (!lvalue_p (init))
 	    eltinit = move (eltinit);
-	  eltinit = force_rvalue (eltinit, complain);
+	  eltinit = (perform_implicit_conversion_flags
+		     (elttype, eltinit, complain,
+		      LOOKUP_IMPLICIT|LOOKUP_NO_NARROWING));
 	  eltinit = cxx_eval_constant_expression (&new_ctx, eltinit, lval,
 						  non_constant_p, overflow_p);
 	}