Prevent ill-formed instantiation of __invoke_of<...> during the evaluation of a bind expression. Fixes PR22003.

The SFINAE on the function __mu(Fn, Args...) that evaluates nested bind
expressions always tries to deduce the return type for Fn(Args...) even when Fn
is not a nested bind expression. This can cause hard compile errors when the
instantation of Fn(Args...) is ill-formed. This patch prevents the instantation
of __invoke_of<Fn, Args...> unless Fn is actually a bind expression.

Bug reportand patch from Michel Morin.

http://llvm.org/bugs/show_bug.cgi?id=22003


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@224753 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/utilities/function.objects/bind/func.bind/func.bind.bind/invoke_function_object.pass.cpp

@@ -0,0 +1,49 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// <functional>
+
+// template<CopyConstructible Fn, CopyConstructible... Types>
+//   unspecified bind(Fn, Types...);
+// template<Returnable R, CopyConstructible Fn, CopyConstructible... Types>
+//   unspecified bind(Fn, Types...);
+
+// http://llvm.org/bugs/show_bug.cgi?id=22003
+
+#include <functional>
+
+struct DummyUnaryFunction
+{
+    template <typename S>
+    int operator()(S const & s) const { return 0; }
+};
+
+struct BadUnaryFunction
+{
+    template <typename S>
+    constexpr int operator()(S const & s) const
+    {
+        // Trigger a compile error if this function is instantiated.
+        // The constexpr is needed so that it is instantiated while checking
+        // __invoke_of<BadUnaryFunction &, ...>.
+        static_assert(!std::is_same<S, S>::value, "Shit");
+        return 0;
+    }
+};
+
+int main(int argc, char* argv[])
+{
+    // Check that BadUnaryFunction::operator()(S const &) is not
+    // instantiated when checking if BadUnaryFunction is a nested bind
+    // expression during b(0). See PR22003.
+    auto b = std::bind(DummyUnaryFunction(), BadUnaryFunction());
+    b(0);
+    auto b2 = std::bind<long>(DummyUnaryFunction(), BadUnaryFunction());
+    b2(0);
+}