[C++] [PR84231] overload on cond_expr in template

[C++] [PR84231] overload on cond_expr in template

A non-type-dependent COND_EXPR within a template is reconstructed with
the original operands, after one with non-dependent proxies is built to
determine its result type.  This is problematic because the operands of
a COND_EXPR determined to be an rvalue may have been converted to denote
their rvalue nature.  The reconstructed one, however, won't have such
conversions, so lvalue_kind may not recognize it as an rvalue, which may
lead to e.g. incorrect overload resolution decisions.

If we mistake such a COND_EXPR for an lvalue, overload resolution might
regard a conversion sequence that binds it to a non-const reference as
viable, and then select that over one that binds it to a const
reference.  Only after template substitution would we rebuild the
COND_EXPR, realize it is an rvalue, and conclude the reference binding
is ill-formed, but at that point we'd have long discarded any alternate
candidates we could have used.

This patch modifies the logic that determines whether a
(non-type-dependent) COND_EXPR in a template is an lvalue, to rely on
its type, more specifically, on the presence of a REFERENCE_TYPE
wrapper.  In order to avoid a type bootstrapping problem, the
REFERENCE_TYPE that wraps the type of some such COND_EXPRs is
introduced earlier, so that we don't have to test for lvalueness of
the expression using the very code that we wish to change.

for  gcc/cp/ChangeLog

	PR c++/84231
	* tree.c (lvalue_kind): Use presence/absence of REFERENCE_TYPE
	only while processing template decls.
	* typeck.c (build_x_conditional_expr): Move wrapping of
	reference type around type...
	* call.c (build_conditional_expr_1): ... here.  Rename
	is_lvalue to is_glvalue.
	* parser.c (cp_parser_fold_expression): Catch REFERENCE_REF_P
	INDIRECT_REF of COND_EXPR too.

for  gcc/testsuite/ChangeLog

	PR c++/84231
	* g++.dg/pr84231.C: New.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@258271 138bc75d-0d04-0410-961f-82ee72b054a4