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.

4 files changed, 24 insertions, 8 deletions

diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index 354773f..68cf878 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -8541,7 +8541,16 @@ convert_like_internal (conversion *convs, tree expr, tree fn, int argnum,
 	unsigned len = CONSTRUCTOR_NELTS (expr);
 	tree array;
 
-	if (len)
+	if (tree init = maybe_init_list_as_array (elttype, expr))
+	  {
+	    elttype = cp_build_qualified_type
+	      (elttype, cp_type_quals (elttype) | TYPE_QUAL_CONST);
+	    array = build_array_of_n_type (elttype, len);
+	    array = build_vec_init_expr (array, init, complain);
+	    array = get_target_expr (array);
+	    array = cp_build_addr_expr (array, complain);
+	  }
+	else if (len)
 	  {
 	    tree val; unsigned ix;
 
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);
 	}
diff --git a/gcc/cp/init.cc b/gcc/cp/init.cc
index 6ccda365..af6e30f 100644
--- a/gcc/cp/init.cc
+++ b/gcc/cp/init.cc
@@ -4475,7 +4475,9 @@ build_vec_init (tree base, tree maxindex, tree init,
   /* Look through the TARGET_EXPR around a compound literal.  */
   if (init && TREE_CODE (init) == TARGET_EXPR
       && TREE_CODE (TARGET_EXPR_INITIAL (init)) == CONSTRUCTOR
-      && from_array != 2)
+      && from_array != 2
+      && (same_type_ignoring_top_level_qualifiers_p
+	  (TREE_TYPE (init), atype)))
     init = TARGET_EXPR_INITIAL (init);
 
   if (tree vi = get_vec_init_expr (init))
@@ -4601,7 +4603,14 @@ build_vec_init (tree base, tree maxindex, tree init,
     {
       if (lvalue_kind (init) & clk_rvalueref)
 	xvalue = true;
-      base2 = decay_conversion (init, complain);
+      if (TREE_CODE (init) == TARGET_EXPR)
+	{
+	  /* Avoid error in decay_conversion.  */
+	  base2 = decay_conversion (TARGET_EXPR_SLOT (init), complain);
+	  base2 = cp_build_compound_expr (init, base2, tf_none);
+	}
+      else
+	base2 = decay_conversion (init, complain);
       if (base2 == error_mark_node)
 	return error_mark_node;
       itype = TREE_TYPE (base2);
diff --git a/gcc/cp/tree.cc b/gcc/cp/tree.cc
index 4d5e3f5..751c9ad 100644
--- a/gcc/cp/tree.cc
+++ b/gcc/cp/tree.cc
@@ -785,8 +785,6 @@ build_vec_init_elt (tree type, tree init, tsubst_flags_t complain)
   releasing_vec argvec;
   if (init && !BRACE_ENCLOSED_INITIALIZER_P (init))
     {
-      gcc_assert (same_type_ignoring_top_level_qualifiers_p
-		  (type, TREE_TYPE (init)));
       tree init_type = strip_array_types (TREE_TYPE (init));
       tree dummy = build_dummy_object (init_type);
       if (!lvalue_p (init))