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Keep both checks in x !== true && x !== false - #9113
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Why:
The JS simplifier rewrote `x !== true && x !== false` to `false` (and
`x === true || x === false` to `true`) as if `x` were a boolean. In an
untagged variant `x` can be any payload, so matches with both boolean
literal cases, or a `bool` payload beside one, lost their other checks
(#9098). The boolean-only rule is removed; the general constant rules
below it still fold every case that holds for any `x`.
JS modified (untagged match with both boolean literals):
Example:
@unboxed type t = D(Date.t) | @as(true) Lt | @as(false) Lf | R(RegExp.t)
let g = (v: t) => switch v { | D(_) => 2 | _ => 4 }
let h = (v: t) => switch v { | Lt | Lf => 1 | _ => 3 }
Before (js):
function g(v) {
return 4;
}
function h(v) {
return 1;
}
After (js):
function g(v) {
if (v === true || v === false || !(v instanceof Date)) {
return 4;
} else {
return 2;
}
}
function h(v) {
if (v === true || v === false) {
return 1;
} else {
return 3;
}
}
JS modified (bool payload beside a boolean literal):
Example:
@unboxed type t = Bo(bool) | S(string) | @as(true) Lt
let f = (v: t) => switch v { | Bo(false) => 2 | _ => 6 }
Before (js):
if (v === true) {
After (js):
if (v !== false) {
Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01Kbf3gGkPma9LAZimuQjnrP
Signed-off-by: Christoph Knittel <[email protected]>
Changelog🐛 Bug fix
Semantic Diff1 commit · Full page Keep both boolean checks in x !== true && x !== falsef61e918 · cknitt + Claude · Oct 28, 2026, 15:27 UTC
JS modified (untagged match with both boolean literals) @unboxed type t = D(Date.t) | @as(true) Lt | @as(false) Lf | R(RegExp.t)
let g = (v: t) => switch v { | D(_) => 2 | _ => 4 }
let h = (v: t) => switch v { | Lt | Lf => 1 | _ => 3 }
function g(v) {
- return 4;
+ if (v === true || v === false || !(v instanceof Date)) {
+ return 4;
+ } else {
+ return 2;
+ }
}
function h(v) {
- return 1;
+ if (v === true || v === false) {
+ return 1;
+ } else {
+ return 3;
+ }
}JS modified (bool payload beside a boolean literal) @unboxed type t = Bo(bool) | S(string) | @as(true) Lt
let f = (v: t) => switch v { | Bo(false) => 2 | _ => 6 }
-if (v === true) {
+if (v !== false) { |
Codecov Report✅ All modified and coverable lines are covered by tests. Additional details and impacted files@@ Coverage Diff @@
## master #9113 +/- ##
==========================================
+ Coverage 81.46% 81.47% +0.01%
==========================================
Files 460 460
Lines 64675 64668 -7
==========================================
+ Hits 52689 52691 +2
+ Misses 11986 11977 -9
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cristianoc
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Looks good. Removing the boolean-only rule preserves the non-boolean payload checks, and the regression tests cover both examples from #9098 plus the combined literal arm. CI is green. The committed JS tests also pass in a minimal runtime harness; I did not rebuild the compiler locally.
Untagged variants with boolean literal cases could be miscompiled: a match with both
@as(true)and@as(false)cases that doesn't name them dropped its other checks (switch v { | D(_) => 2 | _ => 4 }compiled toreturn 4), and aboolpayload beside one boolean literal checked the wrong thing (Bo(false)matchedS("hello")).The cause is a rule in
Js_exp_make.simplify_and_that foldedx === b1 && x === b2andx !== b1 && x !== b2on boolean constants as ifxwere a boolean, sox !== true && x !== falsebecamefalse(and, through negation insimplify_or_,x === true || x === falsebecametrue). In an untagged variantxcan be any payload. The rule is removed. The general constant rules that follow it already fold the combinations that hold for everyx(equal constants,x === a && x === bwith different constants, mixed===/!==), so the only change in output is that the unsound case is kept.The new test
tests/tests/src/untagged_bool_literals_test.resruns both programs from the issue and a match withLt | Lfin one case, on every constructor; all three tests fail with the previous compiler. No other generated test output changes.Fixes #9098
🤖 Generated with Claude Code
https://claude.ai/code/session_01Kbf3gGkPma9LAZimuQjnrP