Longest match logic means more colGt required

[digama0]

macro (priority := high) "let " x:ident : tactic => `(let' $x := ?_)

example : True := by
  let x
  exact Nat
  
-- expected ':=' or '|'

This is a regression caused by #1658, even though it was intended to improve the situation for exactly this example. (Perhaps the mathlib let/have should be defined in a test case...) It works if you put (let x) in parens but as written it gets parsed as let x exact Nat where exact and Nat are identifiers, followed by a parse error because the := is missing. As you can see, priority := high makes no difference for the parse.