[libcxx] [test] Don't ask whether Incomplete& can be assigned to.

[STL_MSFT]

Summary

[libcxx] [test] Don't ask whether Incomplete& can be assigned to.

This is the subject of an active NB comment. Regardless of what the Working
Paper currently says, asking this question is morally wrong, because the
answer can change when the type is completed. C1XX now detects such
precondition violations and complains about them; perhaps Clang should too.

[EricWF]

LGTM.

Although is_assignable can often get accidentally instantiated with an incomplete type by way of std::tuple<Incomplete&> or similar, which is the rational for this test case.

test/std/utilities/meta/meta.unary/meta.unary.prop/is_assignable.pass.cpp

 #endif
     test_is_not_assignable<A, B> ();
     test_is_not_assignable<void, const void> ();
     test_is_not_assignable<const void, const void> ();
     test_is_not_assignable<int(), int> ();
 
 //  pointer to incomplete template type
   test_is_assignable<X<D>*&, X<D>*> ();
-    test_is_not_assignable<Incomplete&, Incomplete const&>();
 }