C++ PATCH for c++/91923 - failure-to-SFINAE with class type NTTP in C++17

[Marek Polacek]

We reject this well-formed test because the following error was triggering
even in a SFINAE context, while it should not:
https://en.cppreference.com/w/cpp/language/sfinae says that
"attempting to give an invalid type to a non-type template parameter" is
a SFINAE error.

I wonder if the tf_error check should be used more in that function.  But
this is enough for this particular testcase.

Bootstrapped/regtested on x86_64-linux, ok for trunk?

2019-09-27  Marek Polacek  <pola...@redhat.com>

        PR c++/91923 - failure-to-SFINAE with class type NTTP in C++17.
        * pt.c (invalid_nontype_parm_type_p): Only emit errors when
        tf_error.

        * g++.dg/cpp0x/nontype5.C: New test.

diff --git gcc/cp/pt.c gcc/cp/pt.c
index e5d64989b32..e71403460b0 100644
--- gcc/cp/pt.c
+++ gcc/cp/pt.c
@@ -25235,8 +25235,9 @@ invalid_nontype_parm_type_p (tree type, tsubst_flags_t 
complain)
     {
       if (cxx_dialect < cxx2a)
        {
-         error ("non-type template parameters of class type only available "
-                "with %<-std=c++2a%> or %<-std=gnu++2a%>");
+         if (complain & tf_error)
+           error ("non-type template parameters of class type only available "
+                  "with %<-std=c++2a%> or %<-std=gnu++2a%>");
          return true;
        }
       if (dependent_type_p (type))
diff --git gcc/testsuite/g++.dg/cpp0x/nontype5.C 
gcc/testsuite/g++.dg/cpp0x/nontype5.C
new file mode 100644
index 00000000000..c31134581aa
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp0x/nontype5.C
@@ -0,0 +1,17 @@
+// PR c++/91923 - failure-to-SFINAE with class type NTTP in C++17.
+// { dg-do compile { target c++11 } }
+
+template<typename T>
+constexpr bool is_integral_(...) {
+    return false;
+}
+template<typename T, T = 1>
+constexpr bool is_integral_(long) {
+    return true;
+}
+
+static_assert(is_integral_<int>(42), "");
+static_assert(!is_integral_<void>(42), "");
+
+struct S {};
+static_assert(!is_integral_<S>(42), "");