c++: Add TARGET_EXPR comments

Discussing the 98469 patch and class prvalues with Jakub led me to
double-check our handling of TARGET_EXPR in constexpr.c, and add a note
about why we don't strip them in parameter initialization.  And another to
clarify that we're handling an INIT_EXPR in a place we do strip them.

gcc/cp/ChangeLog:

	* constexpr.c (cxx_bind_parameters_in_call): Add comment.
	(cxx_eval_store_expression): Add comment.

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 0c12f60..4a5e638 100644 (file)
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -1591,6 +1591,9 @@ cxx_bind_parameters_in_call (const constexpr_ctx *ctx, tree t,
       if (TREE_ADDRESSABLE (type))
        /* Undo convert_for_arg_passing work here.  */
        x = convert_from_reference (x);
+      /* Normally we would strip a TARGET_EXPR in an initialization context
+        such as this, but here we do the elision differently: we keep the
+        TARGET_EXPR, and use its CONSTRUCTOR as the value of the parm.  */
       arg = cxx_eval_constant_expression (ctx, x, /*lval=*/false,
                                          non_constant_p, overflow_p);
       /* Don't VERIFY_CONSTANT here.  */
@@ -5388,6 +5391,9 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
 
   if (!preeval)
     {
+      /* We're handling an INIT_EXPR of class type, so the value of the
+        initializer can depend on the object it's initializing.  */
+
       /* Create a new CONSTRUCTOR in case evaluation of the initializer
         wants to modify it.  */
       if (*valp == NULL_TREE)