tcfail131.hs:7:11: error:
• Couldn't match expected type ‘Integer’ with actual type ‘b’
‘b’ is a rigid type variable bound by
the type signature for:
g :: forall b. Num b => b -> b
at tcfail131.hs:6:3-22
• In the first argument of ‘f’, namely ‘x’
In the expression: f x x
In an equation for ‘g’: g x = f x x
• Relevant bindings include
x :: b (bound at tcfail131.hs:7:5)
g :: b -> b (bound at tcfail131.hs:7:3)