6810 is fine like that

the "Constraint check failure for test.T" is missing, but that's a separate problem. I think it gets eaten by the BetterErrors transformation.

tests/misc/projects/Issue6810/compile-fail.hxml.stderr

@@ -1,4 +1,8 @@
-Fail.hx:8: characters 3-7 : Constraint check failure for test.T
-Fail.hx:8: characters 3-7 : FakeVoid should be haxe.NotVoid
-Fail.hx:7: characters 3-7 : Constraint check failure for test.T
-Fail.hx:7: characters 3-7 : Void should be haxe.NotVoid
+Fail.hx:7: characters 8-12 : error: Void should be haxe.NotVoid
+Fail.hx:7: characters 8-12 :  have: (...) -> Void
+Fail.hx:7: characters 8-12 :  want: (...) -> haxe.NotVoid
+Fail.hx:7: characters 8-12 : For function argument 'f'
+Fail.hx:8: characters 8-16 : error: FakeVoid should be haxe.NotVoid
+Fail.hx:8: characters 8-16 :  have: (...) -> FakeVoid
+Fail.hx:8: characters 8-16 :  want: (...) -> haxe.NotVoid
+Fail.hx:8: characters 8-16 : For function argument 'f'