c++: lambda and the current instantiation [PR82980]

c++: lambda and the current instantiation [PR82980]

When a captured variable is type-dependent, we've expressed the type of the
capture field and proxy with a decltype variant.  But if the type is "the
current instantiation", we need to be able to see that so that we can do
lookup inside it just like we could with the captured variable itself.

I also tried looking through lambda capture in
cp_parser_postfix_dot_deref_expression, but this way seems cleaner.  I plan
to treat more types as deducible in stage 1.

I considered also using this in do_auto_deduction, but think that would be
wrong: [temp.dep.expr] says an id-expression is type-dependent if it is
"associated by name lookup with a variable declared with a type that
contains a placeholder type where the initializer is type-dependent".  That
doesn't clearly exclude deducing a dependent type from the initializer, but
it seems like a barrier, and other implementations agree.

	PR c++/82980

gcc/cp/ChangeLog:

	* lambda.cc (type_deducible_expression_p): New.
	(lambda_capture_field_type): Check it.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/lambda/lambda-current-inst1.C: New test.