[gcc r15-1717] c++: bad 'this' conversion for nullary memfn [PR106760]

[Patrick Palka via Gcc-cvs]

https://gcc.gnu.org/g:50073ffae0a9b8feb9b36fdafdebd9885f6d7dc8

commit r15-1717-g50073ffae0a9b8feb9b36fdafdebd9885f6d7dc8
Author: Patrick Palka <ppa...@redhat.com>
Date:   Fri Jun 28 19:45:21 2024 -0400

    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 <ja...@redhat.com>

Diff:
---
 gcc/cp/call.cc                   |  3 ++-
 gcc/testsuite/g++.dg/ext/conv3.C | 13 +++++++++++++
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index 7bbc1fb0c78..83070b2f633 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -6742,7 +6742,8 @@ add_candidates (tree fns, tree first_arg, const vec<tree, 
va_gc> *args,
 
       if (cand->viable == -1
          && shortcut_bad_convs
-         && missing_conversion_p (cand))
+         && (missing_conversion_p (cand)
+             || TREE_CODE (cand->fn) == TEMPLATE_DECL))
        {
          /* This candidate has been tentatively marked non-strictly viable,
             and we didn't compute all argument conversions for it (having
diff --git a/gcc/testsuite/g++.dg/ext/conv3.C b/gcc/testsuite/g++.dg/ext/conv3.C
new file mode 100644
index 00000000000..7324d560056
--- /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" }
+}