Could you please explain how you have tested this?

Yeah, it would be nice if you could come up with an example, show the bad error message on current master and then the improved one after your change. Thanks!

Oops sorry I was in a hurry yesterday. For example when I have code like this:

main :: forall a. Int
main = 1

It will compile with warning:

in module Main
Type variable e is ambiguous, since it is unused in the polymorphic type which introduces it.
in type declaration for main

After this PR, the warning becomes:

in module Main
at src/Main.purs line 3, column 1 - line 3, column 22
Type variable e is ambiguous, since it is unused in the polymorphic type which introduces it.
in type declaration for main

It's especially useful for IDE integration.