This is the same issue as PR86429, just in potential_constant_expression_1
rather than cxx_eval_constant_expression.  As in that case, when we're
trying to evaluate a constant expression within a lambda, we don't have a
constant closure object to refer to, but we can try to refer directly to the
captured variable.

gcc/cp/ChangeLog
2020-05-05  Jason Merrill  <jason@redhat.com>

	PR c++/90212
	* constexpr.c (potential_constant_expression_1): In a lambda
	function, consider a captured variable directly.