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Untagged variants: no warning for patterns shadowed by a literal case, and a spurious non-exhaustive warning for bool payloads

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Assessment

Difficulty
5/5
Estimated time
Over a week
Newbie friendliness
35/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
ocaml
Domain
compilers

Research direction

Start by reading parmatch.ml, which the issue identifies as the site of the exhaustiveness check, and find the adjacent unused-case check for the warning proposal. Review the related issues #8631, #8632, and #8777 for runtime value comparisons and overlap cases. Done means dead untagged payload patterns are warned about and the bool example is exhaustive without an unreachable failure.

Written by the indexing model from the issue text.

Description

Summary

An untagged variant may have a literal case whose runtime value a payload case can also hold (#8632, #8777):

@unboxed type t = | @as("a") A | S(string)

Matching tests declared literals first, so the pattern S("a") can never match. The value "a" always goes to A, or to a wildcard if there is no A arm. The type checker treats S("a") and A as different, which causes two problems.

1. No warning for patterns that can never match

Checked on master. None of these produce a warning, and each S(...) arm marked dead is dead in the generated JS:

let f1 = x => switch x { | A => 1 | S("a") => 2 | S(_) => 3 }  // S("a") dead
let f2 = x => switch x { | S("a") => 2 | A => 1 | S(_) => 3 }  // dead even as first arm
let f3 = x => switch x { | S("a") => 2 | _ => 3 }              // compiles to `return 3`
let f4 = x => switch x { | S("a" | "b") => 2 | _ => 3 }        // the "a" alternative is dead

The same applies when:

  • the literal comes from the constructor name (Primary | Color(string): Color("Primary") is dead);
  • an int literal overlaps a float payload (@as(1) One | F(float): F(1.0) is dead);
  • a nested untagged constant has the same value (@as("x") X | W(inner) where inner has @as("x") P: W(P) is dead).

In a refutable binding such as let l = (S("a") as v) => v, the pattern never matches at all, so every call with S("a") throws Match_failure.

Proposal: warn on a constructor pattern S(p) of an untagged variant when every value p can match is the runtime value of a literal case in the same variant. Use warning 11 (unused case) for a whole arm and warning 12 (redundant sub-pattern) for or-pattern alternatives. Suggested message: "this pattern can never match: "a" is the value of constructor A".

Notes:

  • The check is local to each pattern and doesn't depend on arm order, so it can run beside check_unused without touching the exhaustiveness algorithm.
  • Values must be compared the way they are at runtime, as in #8631: 1 and 1.0 are equal, bigint spellings differ (1_0n = 10n), ints use their 32-bit value, and a constructor without @as is represented by its own name.
  • Guards (S(s) if s == "a") are out of scope.

2. Spurious non-exhaustive warning for bool payloads

With #8777 a bool payload can partly overlap a literal:

@unboxed type b = | @as(true) Yes | Ans(bool)
let g = x => switch x { | Yes => 1 | Ans(false) => 2 }

This match covers every value, yet the compiler reports warning 8 with the counterexample Ans(true) and emits an unreachable Match_failure throw. The only way to silence it is to add the dead arm Ans(true) => ..., which point 1 would then flag. The two parts therefore need to land together, or the bool case ends up with no warning-free way to write the match.

Proposal: during the exhaustiveness check only, treat a literal-case pattern as also covering the payload values equal to it, e.g. check Yes as Yes | Ans(true). This is harder than point 1 because it changes parmatch.ml. For string and number payloads it rarely matters, since their domain is infinite, but for bool it does.

Related: #8632, #8777, #8631, rescript-lang/rescript-lang.org#1337

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