autoBoundErrorMsg.lean:1:34-1:39: error: don't know how to synthesize implicit argument
  @Eq ?m a b
context:
α : Sort ?u
a b : ?m
h : ∀ {a b : α}, a = b
⊢ Sort ?u
when the resulting type of a declaration is explicitly provided, all holes (e.g., `_`) in the header are resolved before the declaration body is processed