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The following code fails to compile:

{-# LANGUAGE DataKinds, TypeOperators, TypeFamilies,
             KindSignatures, ConstraintKinds #-}

import GHC.TypeLits

type a / b = FDiv a b
type a ** b = FMul a b

type family FDiv a b where
  FDiv 11648 128 = 91

type family FMul a b where
  FMul 64 91 = 5824

type family FGCD a b where
  FGCD 128 448 = 64
  FGCD 128 5824 = 64

type family FLCM a b where
  FLCM 128 5824 = 11648

data CT (m :: Nat) (m' :: Nat)
type H0 = 128
type H1 = 448
type H0' = 11648
type H1' = 5824

main' = 
  let x = undefined :: CT H0 H0'
  in foo x :: CT H1 H1'

foo x = bug x

type Ctx2 e r s e' r' =
  (e ~ FGCD r e', r' ~ FLCM r e', e ~ FGCD r s)

type Ctx1 e r s e' r' =
  (Ctx2 e r s e' r', e' ~ (e ** (r' / r)))

bug :: (Ctx1 e r s e' r')
  => CT r r' -> CT s s'
bug = undefined

with the error

    Could not deduce ((~) Nat (FGCD r e'0) (FGCD r s))
    ...
    The type variable ‘e'0’ is ambiguous
    When checking that ‘foo’ has the inferred type
      foo :: forall (r :: Nat) (s :: Nat) (s' :: Nat) (e' :: Nat).
             ((~) Nat (FGCD r s) (FGCD r e'),
              (~) Nat (FMul (FGCD r s) (FDiv (FLCM r e') r)) e') =>
             CT r (FLCM r e') -> CT s s'
    Probable cause: the inferred type is ambiguous

However, if I move the e' ~ ... constraint from Ctx1 to Ctx2 or to the context of bug, it compiles as expected. Somehow, GHC misses the constraint when it is in Ctx1.

I don't think this is #10338, but someone who knows more about the innards of GHC can verify.