value_or() requires that U is convertible to T

Due to Vladimir Batov.

test/Jamfile.v2

@@ -41,5 +41,6 @@ import testing ;
     [ compile-fail optional_test_ref_fail_init_from_Urefref.cpp ]
     [ compile-fail optional_test_ref_fail_assign_from_Trefref.cpp ]
     [ compile-fail optional_test_ref_fail_assign_from_Urefref.cpp ]
+    [ compile-fail optional_test_fail_explicit_convert_in_value_or.cpp ]
   ;
 }