auto x = "foo"w ~ 'x'; auto y = "foo"d ~ 'x'; auto z = 'x' ~ "foo"w; auto w = 'x' ~ "foo"d; All four give an error about not being able to concat a w/dchar[] with an int (int?!). Pretty amazing.
*** Issue 3126 has been marked as a duplicate of this issue. ***
Closing as WONTFIX as this has been fixed in D2.