tc168.hs:17:1: error:
• Could not deduce (C a1 (a, b0))
from the context: C a1 (a, b)
bound by the inferred type for ‘g’:
C a1 (a, b) => a1 -> a
at tc168.hs:17:1-16
The type variable ‘b0’ is ambiguous
• In the ambiguity check for the inferred type for ‘g’
To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
When checking the inferred type
g :: forall a b a1. C a1 (a, b) => a1 -> a