c++: bad 'this' conversion for nullary memfn [PR106760]

Here we notice the 'this' conversion for the call f<void>() is bad, so
we correctly defer deduction for the template candidate, but we end up
never adding it to 'bad_cands' since missing_conversion_p for it returns
false (its only argument is 'this' which has already been determined to
be bad).  This is not a huge deal, but it causes us to longer accept the
call with -fpermissive in release builds, and a tree check ICE in checking
builds.

So if we have a non-strictly viable template candidate that has not been
instantiated, then we need to add it to 'bad_cands' even if no argument
conversion is missing.

	PR c++/106760

gcc/cp/ChangeLog:

	* call.cc (add_candidates): Relax test for adding a candidate
	to 'bad_cands' to also accept an uninstantiated template candidate
	that has no missing conversions.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/conv3.C: New test.

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

1 files changed, 13 insertions, 0 deletions

diff --git a/gcc/testsuite/g++.dg/ext/conv3.C b/gcc/testsuite/g++.dg/ext/conv3.C
new file mode 100644
index 0000000..7324d56
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/conv3.C
@@ -0,0 +1,13 @@
+// PR c++/106760
+// { dg-additional-options "-fpermissive" }
+
+struct S {
+  template<class> int f();
+  template<class> int g(...);
+};
+
+int main() {
+  const S s;
+  s.f<void>(); // { dg-warning "discards qualifiers" }
+  s.g<void>(); // { dg-warning "discards qualifiers" }
+}