c++: decltype of (by-value captured reference) [PR79620]

c++: decltype of (by-value captured reference) [PR79620]

The capture_decltype handling in finish_decltype_type wasn't looking
through implicit INDIRECT_REF (added by convert_from_reference), which
caused us to incorrectly resolve decltype((r)) to float& below.  This
patch fixes this, and adds an assert to outer_automatic_var_p to help
prevent against such bugs.

We still don't fully accept the example ultimately because for the
decltype inside the lambda's trailing return type, at that point we're
in lambda type scope but not yet in lambda function scope that the
capture_decltype handling looks for (which is an orthogonal bug).

	PR c++/79620

gcc/cp/ChangeLog:

	* cp-tree.h (STRIP_REFERENCE_REF): Define.
	* semantics.cc (outer_var_p): Assert REFERENCE_REF_P is false.
	(finish_decltype_type): Look through implicit INDIRECT_REF when
	deciding whether to call capture_decltype.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/lambda/lambda-decltype3.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>