[pushed] c++: Fix constrained conversion op.

[Jason Merrill]

We don't want to promote a conversion from viable == 0 to viable == -1.
Found in ranges-v3.

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

gcc/cp/ChangeLog
2020-02-28  Jason Merrill  <ja...@redhat.com>

        * call.c (build_user_type_conversion_1): Don't look at the second
        conversion of a non-viable candidate.
---
 gcc/cp/call.c                               |  4 ++++
 gcc/testsuite/g++.dg/cpp2a/concepts-conv1.C | 15 +++++++++++++++
 2 files changed, 19 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-conv1.C

diff --git a/gcc/cp/call.c b/gcc/cp/call.c
index e07ee85c06e..85bbd043a1d 100644
--- a/gcc/cp/call.c
+++ b/gcc/cp/call.c
@@ -4083,6 +4083,10 @@ build_user_type_conversion_1 (tree totype, tree expr, 
int flags,
 
       for (cand = candidates; cand != old_candidates; cand = cand->next)
        {
+         if (cand->viable == 0)
+           /* Already rejected, don't change to -1.  */
+           continue;
+
          tree rettype = TREE_TYPE (TREE_TYPE (cand->fn));
          conversion *ics
            = implicit_conversion (totype,
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-conv1.C 
b/gcc/testsuite/g++.dg/cpp2a/concepts-conv1.C
new file mode 100644
index 00000000000..aa29acbcb13
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-conv1.C
@@ -0,0 +1,15 @@
+// { dg-do compile { target concepts } }
+
+template <class T> concept False = false;
+
+template <class T>
+struct A
+{
+  explicit operator bool ();
+  explicit operator bool () requires False<T>;
+};
+
+int main()
+{
+  int i { A<int>() };          // { dg-error "" }
+}

base-commit: 77e596cf3c1b7bc11ba946394ed8d62a49157b49
-- 
2.18.1