Unclear error message when pattern matching on ambiguous notation
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bug
error message
P-medium
- Dominant language
- Lean
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Description
notation a " :: " b => Fin.add a b
#check [42] matches _ :: _
/-
bug.lean:2:20: error: invalid pattern, constructor or constant marked with '[match_pattern]' expected
-/
FWIW, this used to work in Lean 3:
notation (name := fincons) a `::` b := fin.add a b
#check show nat, from match [42] with | x::xs := 1 | _ := 0 end
(This came up in mathlib with the Sym.cons' function.)
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First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
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- Open a pull request that references the issue number.
Research direction
Start by reproducing the example in bug.lean and compare it with the included Lean 3 notation example. Trace how the notation a :: b => Fin.add a b is handled during pattern matching, then verify that the ambiguous-notation case produces a clearer diagnostic or the intended matching behavior.
Written by the indexing model from the issue text.
Assessment
- Domain
- compilers
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Active
- Clarity
- Mostly clear
- Newbie friendliness
- 45/100