D1182: Implement improved error messages for ambiguous type variables (#10733)

[Jan Stolarek]

> In general we shouldn't commit anything that breaks validate, because
> this causes problems for other developers.  The right thing to do would
> be to mark it expect_broken before committing.
Sorry for that. I was actually thinking about marking the test as expect_broken, but then the 
problem would be completely hidden> I wanted to discuss a possible solution with Simon and Edward 
first but it looks like Thomas already found a workaround.

Jan