Suggest AllowAmbiguousTypes

The error message I get with GHC HEAD on the example program in #8390 (closed):

tf.hs:38:10:
    Could not deduce (Fun g a b0)
      arising from the ambiguity check for an instance declaration
    from the context (Fun f b c, Fun g a b)
      bound by an instance declaration:
                 (Fun f b c, Fun g a b) => Fun (Compose f g) a c
      at tf.hs:38:10-56
    The type variable ‛b0’ is ambiguous
    In the ambiguity check for:
      forall f g a c b. (Fun f b c, Fun g a b) => Fun (Compose f g) a c
    In the instance declaration for ‛Fun (Compose f g) a c’

could perhaps benefit from an appended line:

    (To defer this ambiguity check to use sites, enable AllowAmbiguousTypes)

Trac metadata

Trac field	Value
Version	7.7
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture