Use -Dnsel_P2505R=0 to disable monadic operations (#61, addendum to #…

…60, thanks @szaszm)

and conditionally compile `invoke` tests, only test if `invoke` itself
is compiled

README.md

@@ -133,7 +133,7 @@ Define this macro to 1 to experience the by-design compile-time errors of the li
 
 By default, *expected lite* provides monadic operations as described in [P2505R5](http://wg21.link/p2505r5). You can disable these operations by defining the following macro.
 
--D<b>nsel\_P2505R</b>=
+-D<b>nsel\_P2505R</b>=0
 
 You can use the R3 revision of P2505, which lacks `error_or`, and uses `remove_cvref` for transforms, by defining the following macro.
 

test/expected.t.cpp

@@ -2171,6 +2171,7 @@ CASE( "issue-58" )
     EXPECT( !unexpected.has_value() );
 }
 
+#if nsel_P2505R >= 3
 CASE( "invoke" )
 {
     struct A {
@@ -2197,6 +2198,7 @@ CASE( "invoke" )
     EXPECT( nonstd::expected_lite::detail::invoke(&A::get2, ref, 'a') == 12 );
     EXPECT( nonstd::expected_lite::detail::invoke(&A::get2, cref, 'a') == 7 );
 }
+#endif // nsel_P2505R >= 3
 
 // -----------------------------------------------------------------------
 //  using as optional