Stop warning 20 on calls of open JS object methods by cknitt · Pull Request #8921 · rescript-lang/rescript

cknitt · 2026-10-19T19:49:36Z

Calling a method of an open JS object type, e.g. cd(e)["getData"]("text/plain") with cd: string => {..}, reported warning 20 "this argument will not be used by the function". The method's type is a fresh type variable, which the warning reads as "the function never returns", but the JS method does receive the argument. The warning is now skipped when the applied expression is an object property read (Texp_object_get); other fresh-variable cases such as failwith("x")(1) still warn.

Fixes #8159.

New super_errors fixture unused_argument_object_method.res covers both the object method call (no warning) and the failwith case (warning kept).

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https://claude.ai/code/session_01Kr4cjoUi2f5VpMvE6wT9bo

@get

Calling a method read from an open JS object type, such as `obj["getData"]("text/plain")` with `obj: {..}`, no longer reports warning 20 "this argument will not be used by the function". The method's type is a fresh type variable, which the warning took as a sign that the function never returns, but the call passes its arguments to the JS method. Calls whose function is a fresh type variable for other reasons, such as `failwith("x")(1)`, still warn. Example (rescript): @get external clipboardData: string => {..} = "clipboardData" let text = clipboardData("event")["getData"]("text/plain") Diagnostic (text): Before: Warning number 20 on "text/plain": this argument will not be used by the function. After: no warning. Area: typechecker tests Output: unchanged Formatting: unchanged Formats: none PPX: unchanged Editor: unchanged Tests: +tests/build_tests/super_errors/fixtures/unused_argument_object_method.res +tests/build_tests/super_errors/expected/unused_argument_object_method.res.expected Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01Kr4cjoUi2f5VpMvE6wT9bo Signed-off-by: Christoph Knittel <ck@cca.io>

codecov · 2026-10-19T19:56:34Z

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.14%. Comparing base (58da277) to head (67cea1e).
⚠️ Report is 10 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8921      +/-   ##
==========================================
+ Coverage   81.03%   81.14%   +0.10%     
==========================================
  Files         455      456       +1     
  Lines       63640    63758     +118     
==========================================
+ Hits        51573    51735     +162     
+ Misses      12067    12023      -44

Files with missing lines	Coverage Δ
compiler/ml/typecore.ml	`86.81% <100.00%> (+0.22%)`	⬆️

... and 34 files with indirect coverage changes

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pkg-pr-new · 2026-10-19T20:01:47Z

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commit: 67cea1e

github-actions · 2026-10-19T20:03:47Z

Developer playground preview: https://rescript-lang.github.io/rescript/dev-playground/?version=pr-8921

cknitt · 2026-10-20T06:05:42Z

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chatgpt-codex-connector · 2026-10-20T06:05:58Z

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📝 Code Review	✅ Completed 2026-10-20T13:53:17.099987Z	`67cea1e`	Manual request

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@get

Calling an object field still reports warning 20 "this argument will not be used by the function" unless the read added the field to an open object row. The exemption for open JS object methods covered every object field read, so calls of fields that cannot return, such as `failwith("boom")["m"](x)` or `{"m": failwith("boom")}["m"](x)`, lost the warning. The read now records the fresh variable it creates for a field added to an open row, and the application skips the warning only when its callee is that variable. Example (rescript): @get external clipboardData: string => {..} = "clipboardData" let text = clipboardData("event")["getData"]("text/plain") let fromFailwith = () => failwith("boom")["m"](Console.log("b")) Diagnostic (text): Before: no warning on either call. After: warning 20 on Console.log("b") only, as on master. Review: Why a global slot rather than a typed-tree flag? The callee's read is the last read typed before its application, and the slot is set on every read and compared by physical equality with the callee's type, so it only matches the callee's own read. A typed-tree field would change Texp_object_get and the .cmt format for a warning. Area: typechecker tests Output: unchanged Formatting: unchanged Formats: none PPX: unchanged Editor: unchanged Tests: ~tests/build_tests/super_errors/fixtures/unused_argument_object_method.res ~tests/build_tests/super_errors/expected/unused_argument_object_method.res.expected Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01Kr4cjoUi2f5VpMvE6wT9bo Signed-off-by: Christoph Knittel <ck@cca.io>

Arguments that read object fields, from closed or open objects, in first or last position or through a nested method call, keep the call of an open JS object method free of warning 20. The exemption is decided before any argument is typed, so their reads cannot replace the callee's. Example (rescript): let closedFirst = clipboardData("e")["m"](closed["x"], "a") let nested = clipboardData("e")["m"](clipboardData("f")["n"]("y")) Diagnostic (text): no warning, before and after this commit. Area: tests Output: unchanged Tests: ~tests/build_tests/super_errors/fixtures/unused_argument_object_method.res ~tests/build_tests/super_errors/expected/unused_argument_object_method.res.expected Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01Kr4cjoUi2f5VpMvE6wT9bo Signed-off-by: Christoph Knittel <ck@cca.io>

cknitt · 2026-10-20T13:33:29Z

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The fixture for warning 20 on object methods writes its never-returning expressions as `throw(Failure(...))` instead of the deprecated `failwith`, so its expected output holds only warning 20 once `failwith` reports warning 3. It also covers an open object that is kept in a binding and whose method is read before it is called: that call reports no warning 20 either. Example (rescript): let retained = clipboardData("e") let first = retained["m"] let second = retained["m"]("used") Diagnostic (text): no warning on `"used"`. Review: Does the retained case need a record of every open-row field variable? No: a binding keeps the object type at the binding's level, below the level of the later call, so the call's fresh-variable check does not apply and no warning is reported. Area: tests Output: unchanged Tests: ~tests/build_tests/super_errors/fixtures/unused_argument_object_method.res ~tests/build_tests/super_errors/expected/unused_argument_object_method.res.expected Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01Kr4cjoUi2f5VpMvE6wT9bo Signed-off-by: Christoph Knittel <ck@cca.io>

github-actions · 2026-10-20T13:50:31Z

Semantic diff: https://cdn.rescript-lang.org/semantic-diff/pr-8921/index.html?v=67cea1ed8

cknitt · 2026-10-20T13:50:36Z

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chatgpt-codex-connector · 2026-10-20T13:53:15Z

Codex Review: Didn't find any major issues. Hooray!

Reviewed commit: 67cea1ed83

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Stop warning 20 on calls of open JS object methods - #8921

Stop warning 20 on calls of open JS object methods#8921
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