c++: Fix operator() lookup in lambdas [PR95451]

c++: Fix operator() lookup in lambdas [PR95451]

During name lookup, name-lookup.c uses:
            if (!(!iter->type && HIDDEN_TYPE_BINDING_P (iter))
                && (bool (want & LOOK_want::HIDDEN_LAMBDA)
                    || !is_lambda_ignored_entity (iter->value))
                && qualify_lookup (iter->value, want))
              binding = iter->value;
Unfortunately as the following testcase shows, this doesn't work in
generic lambdas, where we on the auto b = ... lambda ICE and on the
auto d = lambda reject it even when it should be valid.  The problem
is that the binding doesn't have a FUNCTION_DECL with
LAMBDA_FUNCTION_P for the operator(), but an OVERLOAD with
TEMPLATE_DECL for such FUNCTION_DECL.

The following patch fixes that in is_lambda_ignored_entity, other
possibility would be to do that before calling is_lambda_ignored_entity
in name-lookup.c.

2021-02-26  Jakub Jelinek  <jakub@redhat.com>

	PR c++/95451
	* lambda.c (is_lambda_ignored_entity): Before checking for
	LAMBDA_FUNCTION_P, use OVL_FIRST.  Drop FUNCTION_DECL check.

	* g++.dg/cpp1y/lambda-generic-95451.C: New test.