defaultInstance.lean:20:20: error: failed to synthesize instance
  Foo Bool (?m x)
defaultInstance.lean:20:20: error: don't know how to synthesize implicit argument
  @f … ?m ?m …
context:
x : Bool
⊢ Type
defaultInstance.lean:20:17: error: failed to infer definition type
defaultInstance.lean:22:35: error: failed to create type class instance for 
  Foo Bool (?m x)
defaultInstance.lean:22:35: error: don't know how to synthesize implicit argument
  @f … ?m ?m …
context:
x : Bool
⊢ Type
defaultInstance.lean:22:32: error: failed to infer definition type