c++: decltype of (non-captured variable) [PR83167]

c++: decltype of (non-captured variable) [PR83167]

For decltype((x)) within a lambda where x is not captured, we dubiously
require that the lambda has a capture default, unlike for decltype(x).
But according to [expr.prim.id.unqual]/3 we should just ignore the lambda
in this case.  This patch narrowly fixes this issue by disabling the
capture_decltype handling and falling back to the ordinary handling when
the innermost lambda has no capture-default.  In fact, we can restrict
the special handling to only by-copy lambdas since that's what
[expr.prim.id.unqual]/3 is concerned with; for by-ref implicit captures
both code paths should give the same result anyway.

During review some other issues were discovered which are documented in
a new FIXME.

	PR c++/83167

gcc/cp/ChangeLog:

	* semantics.cc (capture_decltype): Inline into its only caller ...
	(finish_decltype_type): ... here.  Update nearby comment to refer
	to recent standard.  Add FIXME.  Restrict uncaptured variable type
	transformation to happen only for lambdas with a by-copy
	capture-default.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/lambda/lambda-decltype4.C: New test.