c++: shortcut bad reference binding [PR94894]

In case of l/rvalue or cv-qual mismatch during reference binding, we
try to give more helpful diagnostics by computing a bad conversion that
allows the mismatch.  But in doing so, we may end up considering and
instantiating a conversion function that could induce a hard error and
in turn cause us to reject otherwise valid code.  We could just give up
on producing a better diagnostic here, but ideally we'd preserve the
better diagnostics for invalid code while avoiding unnecessary template
instantiations for valid code.

To that end, this patch adapts the bad conversion shortcutting mechanism
from r12-3346-g47543e5f9d1fc5 to additionally handle this situation.
The main observation from there is that during overload resolution, if we
know we have a strictly viable candidate then we don't need to distinguish
between an unviable and non-strictly viable candidate.  Thus we don't
need to distinguish between an invalid and bad conversion either, which
is what this patch exploits.  Of course, we don't know whether we have a
strictly viable candidate until after the fact, so we still need to
remember when we deferred distinguishing between an invalid and bad
conversion.  This patch adds a special conversion kind ck_deferred_bad
for this purpose.

	PR c++/94894
	PR c++/105766
	PR c++/106201

gcc/cp/ChangeLog:

	* call.cc (enum conversion_kind): Add ck_deferred_bad enumerator.
	(has_next): Return false for it.
	(reference_binding): Return a ck_deferred_bad conversion instead
	of an actual bad conversion when LOOKUP_SHORTCUT_BAD_CONVS is set.
	Remove now obsolete early exit for the incomplete TO case.
	(implicit_conversion_1): Don't mask out LOOKUP_SHORTCUT_BAD_CONVS.
	(add_function_candidate): Set LOOKUP_SHORTCUT_BAD_CONVS iff
	shortcut_bad_convs.
	(missing_conversion_p): Also return true for a ck_deferred_bad
	conversion.
	* cp-tree.h (LOOKUP_SHORTCUT_BAD_CONVS): Define.

gcc/testsuite/ChangeLog:

	* g++.dg/conversion/ref8.C: New test.
	* g++.dg/conversion/ref9.C: New test.

1 files changed, 5 insertions, 0 deletions

diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index cf51c39..3278b41 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -5877,6 +5877,11 @@ enum overload_flags { NO_SPECIAL = 0, DTOR_FLAG, TYPENAME_FLAG };
 #define LOOKUP_REVERSED (LOOKUP_REWRITTEN << 1)
 /* We're initializing an aggregate from a parenthesized list of values.  */
 #define LOOKUP_AGGREGATE_PAREN_INIT (LOOKUP_REVERSED << 1)
+/* We're computing conversions as part of a first pass of overload resolution
+   wherein we don't try to distinguish an unviable candidate from a
+   non-strictly viable candidate and thus can avoid computing unnecessary
+   bad conversions.  */
+#define LOOKUP_SHORTCUT_BAD_CONVS (LOOKUP_AGGREGATE_PAREN_INIT << 1)
 
 /* These flags are used by the conversion code.
    CONV_IMPLICIT   :  Perform implicit conversions (standard and user-defined).