We represent 'this' in a default member initializer with a PLACEHOLDER_EXPR.
Normally in constexpr evaluation when we encounter one it refers to
ctx->ctor, but when we're creating a temporary of class type, that replaces
ctx->ctor, so a PLACEHOLDER_EXPR that refers to the type of the member being
initialized needs to be replaced before that happens.

gcc/cp/ChangeLog
2020-03-31  Jason Merrill  <jason@redhat.com>

	PR c++/94205
	* constexpr.c (cxx_eval_constant_expression) [TARGET_EXPR]: Call
	replace_placeholders.
	* typeck2.c (store_init_value): Fix arguments to
	fold_non_dependent_expr.