Why doesn't this typecheck?

Foo.idr

module Foo

%default total

interface C a where
  T' : Type
T : (a : Type) -> C a => Type
T a = T' {a}

interface (C a, Show (T a)) => D a where
  p : (x : T a) -> (show x = show x)
{-
Type checking ./Foo.idr
Foo.idr:10:11-34:
   |
10 | interface (C a, Show (T a)) => D a where
   |           ~~~~~~~~~~~~~~~~~~~~~~~~
When checking type of Foo.p:
When checking argument x to type constructor =:
        Can't find implementation for Show (T a)
-}