C++ PATCH for c++/71117 (wrong overload resolution with generic lambda)

[Jason Merrill]

My fix for add_template_conv_candidate, implementing my suggestion for
core 2189, broke calls to a function object that inherits from a
generic lambda.  The committee hasn't had a chance to consider this
issue yet, but for now let's disable the template conversion candidate
when there's a viable operator() candidate.

Tested x86_64-pc-linux-gnu, applying to trunk.

commit b56248900900e3a9f85670d3cf0bf4acdfae34f4
Author: Jason Merrill <ja...@redhat.com>
Date:   Thu Jul 14 16:32:27 2016 -0400

        PR c++/71117 - core 2189 and generic lambda
    
        * call.c (add_template_conv_candidate): Disable if there are
        viable candidates.

diff --git a/gcc/cp/call.c b/gcc/cp/call.c
index a3c5008..889852f 100644
--- a/gcc/cp/call.c
+++ b/gcc/cp/call.c
@@ -3204,6 +3204,12 @@ add_template_conv_candidate (struct z_candidate 
**candidates, tree tmpl,
                             tree return_type, tree access_path,
                             tree conversion_path, tsubst_flags_t complain)
 {
+  /* Making this work broke PR 71117, so until the committee resolves core
+     issue 2189, let's disable this candidate if there are any viable call
+     operators.  */
+  if (any_strictly_viable (*candidates))
+    return NULL;
+
   return
     add_template_candidate_real (candidates, tmpl, NULL_TREE, NULL_TREE,
                                 NULL_TREE, arglist, return_type, access_path,
diff --git a/gcc/testsuite/g++.dg/cpp0x/conv-tmpl1.C 
b/gcc/testsuite/g++.dg/cpp0x/conv-tmpl1.C
index 7f866da..eb40dd6 100644
--- a/gcc/testsuite/g++.dg/cpp0x/conv-tmpl1.C
+++ b/gcc/testsuite/g++.dg/cpp0x/conv-tmpl1.C
@@ -1,3 +1,4 @@
+// Test for Core 2189.
 // { dg-do compile { target c++11 } }
 
 template <class T>
diff --git a/gcc/testsuite/g++.dg/cpp1y/lambda-generic-conv2.C 
b/gcc/testsuite/g++.dg/cpp1y/lambda-generic-conv2.C
new file mode 100644
index 0000000..5528455
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/lambda-generic-conv2.C
@@ -0,0 +1,26 @@
+// PR c++/71117
+// { dg-do compile { target c++14 } }
+
+template <class T> T&& declval() noexcept;
+template <class, class>
+constexpr bool is_same = false;
+template <class T>
+constexpr bool is_same<T, T> = true;
+
+template <class F>
+struct indirected : F {
+    indirected(F f) : F(f) {}
+    template <class I>
+    auto operator()(I i) -> decltype(declval<F&>()(*i)) {
+        return static_cast<F&>(*this)(*i);
+    }
+};
+
+int main() {
+    auto f = [](auto rng) {
+        static_assert(is_same<decltype(rng), int>, "");
+        return 42;
+    };
+    indirected<decltype(f)> i(f);
+    static_assert(is_same<decltype(i(declval<int*>())), int>, "");
+}