Ensure ErrorOr cannot implicitly invoke explicit ctors of the underlying type.

An unpleasant surprise while migrating unique_ptrs (see changes in
lib/Object): ErrorOr<int*> was implicitly convertible to
ErrorOr<std::unique_ptr<int>>.

Keep the explicit conversions otherwise it's a pain to convert
ErrorOr<int*> to ErrorOr<std::unique_ptr<int>>.

I'm not sure if there should be more SFINAE on those explicit ctors (I
could check if !is_convertible && is_constructible, but since the ctor
has to be called explicitly I don't think there's any need to disable
them when !is_constructible - they'll just fail anyway. It's the
converting ctors that can create interesting ambiguities without proper
SFINAE). I had to SFINAE the explicit ones because otherwise they'd be
ambiguous with the implicit ones in an explicit context, so far as I
could tell.

The converting assignment operators seemed unnecessary (and similarly
buggy/dangerous) - just rely on the converting ctors to convert to the
right type for assignment instead.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217048 91177308-0d34-0410-b5e6-96231b3b80d8

unittests/Support/ErrorOrTest.cpp

@@ -60,5 +60,36 @@ TEST(ErrorOr, Covariant) {
 
   ErrorOr<std::unique_ptr<B> > b1(ErrorOr<std::unique_ptr<D> >(nullptr));
   b1 = ErrorOr<std::unique_ptr<D> >(nullptr);
+
+  ErrorOr<std::unique_ptr<int>> b2(ErrorOr<int *>(nullptr));
+  ErrorOr<int *> b3(nullptr);
+  ErrorOr<std::unique_ptr<int>> b4(b3);
 }
+
+// ErrorOr<int*> x(nullptr);
+// ErrorOr<std::unique_ptr<int>> y = x; // invalid conversion
+static_assert(
+    !std::is_convertible<const ErrorOr<int *> &,
+                         ErrorOr<std::unique_ptr<int>>>::value,
+    "do not invoke explicit ctors in implicit conversion from lvalue");
+
+// ErrorOr<std::unique_ptr<int>> y = ErrorOr<int*>(nullptr); // invalid
+//                                                           // conversion
+static_assert(
+    !std::is_convertible<ErrorOr<int *> &&,
+                         ErrorOr<std::unique_ptr<int>>>::value,
+    "do not invoke explicit ctors in implicit conversion from rvalue");
+
+// ErrorOr<int*> x(nullptr);
+// ErrorOr<std::unique_ptr<int>> y;
+// y = x; // invalid conversion
+static_assert(!std::is_assignable<ErrorOr<std::unique_ptr<int>>,
+                                  const ErrorOr<int *> &>::value,
+              "do not invoke explicit ctors in assignment");
+
+// ErrorOr<std::unique_ptr<int>> x;
+// x = ErrorOr<int*>(nullptr); // invalid conversion
+static_assert(!std::is_assignable<ErrorOr<std::unique_ptr<int>>,
+                                  ErrorOr<int *> &&>::value,
+              "do not invoke explicit ctors in assignment");
 } // end anon namespace