FlexibleInstances / UndecidableInstances confusion

Consider the following code:

class Foo a

bar :: Foo a => Int
bar = 0

Compiling this code with no extensions correctly reports an error that bar is inherently ambiguous.

However, if I enable -XUndecidableInstances, the ambiguity error disappears, though I still cannot declare instance Foo a. On the other hand, if I enable -XFlexibleInstances, the ambiguity error remains, even though I can now declare instance Foo a.

I believe that the problem is due to confusion in the note [The ambiguity check for type signatures] in typecheck/TcMType.lhs. That note refers to UndecidableInstances where I believe it means FlexibleInstances. The code immediately below (line 1378, in function checkAmbiguity) also uses UndecidableInstances where it would seem to want FlexibleInstances.

I would fix this myself, but I don't feel confident enough in this wing of GHC to know this is the right change!

Trac metadata

Trac field	Value
Version	7.5
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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