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Unwrap NoInfer types when narrowing #58292

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merged 8 commits into from
May 31, 2024

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Andarist
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fixes #58266

@typescript-bot typescript-bot added the For Uncommitted Bug PR for untriaged, rejected, closed or missing bug label Apr 23, 2024
Comment on lines 27684 to 27685
declaredType = isNoInferType(declaredType) ? (declaredType as SubstitutionType).baseType : declaredType;
initialType = isNoInferType(initialType) ? (initialType as SubstitutionType).baseType : initialType;
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My first instinct was that I should re-wrap the result of narrowing with NoInfer but quoting the original NoInfer PR:

Other than blocking inference, NoInfer markers have no effect on T. Indeed, T and NoInfer are considered identical types in all other contexts.

So I don't see a reason why that would be necessary. I also tested something like this (TS playground) to see if this would be needed and I don't think it is:

declare function inner<T>(arg: T): T;
declare function outer<T>(a: T, cb: (arg: NoInfer<T>) => void): void;

outer({ foo: "bar" }, (arg) => {
  arg
  // ^? (parameter) arg: NoInfer<{ foo: string; }>
  const result = inner(arg); // inferred despite `arg` being a `NoInfer` type!
  //    ^? const result: NoInfer<{ foo: string; }>
});

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I agree it's fine to erase NoInfer in variable, parameter, and property references, but I think it would be simpler and more consistent to do it first thing in the getNarrowableTypeForReference function. I think that's all you'd need.

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moved it to getNarrowableTypeForReference, works like a charm - thanks!

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See my comment above.

@typescript-bot typescript-bot added For Backlog Bug PRs that fix a backlog bug and removed For Uncommitted Bug PR for untriaged, rejected, closed or missing bug labels May 1, 2024
@Andarist Andarist requested a review from ahejlsberg May 1, 2024 21:37
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@sandersn @RyanCavanaugh could this one land before the upcoming RC?

@sandersn sandersn merged commit f5b2d9b into microsoft:main May 31, 2024
28 checks passed
skeate added a commit to skeate/TypeScript that referenced this pull request Jun 1, 2024
* upstream/main: (37 commits)
  Added NoTruncation flag to completions (microsoft#58719)
  Clone node to remove location even when it has been modified if needed (microsoft#58706)
  Properly account for `this` argument in intersection apparent type caching (microsoft#58677)
  Fix: Include Values of Script Extensions for Unicode Property Value Expressions in Regular Expressions (microsoft#58615)
  In `reScanSlashToken` use `charCodeChecked` not `codePointChecked` (microsoft#58727)
  Shorten error spans for errors reported on constructor declarations (microsoft#58061)
  Mark file as skips typechecking if it contains ts-nocheck (microsoft#58593)
  Fixed an issue with broken `await using` declarations in `for of` loops (microsoft#56466)
  Do not expand type references in keyof and index access (microsoft#58715)
  Improve the performance of isolatedDeclarations quickfix  (microsoft#58722)
  Unwrap `NoInfer` types when narrowing (microsoft#58292)
  Recover from type reuse errors by falling back to inferred type printing (microsoft#58720)
  Fixing self import (microsoft#58718)
  Enable JS emit for noCheck and noCheck for transpileModule (microsoft#58364)
  Revert PR 55371 (microsoft#58702)
  Update dependencies (microsoft#58639)
  Fix baselines after PR 58621 (microsoft#58705)
  Do not infer `yield*` type from contextual `TReturn` (microsoft#58621)
  `await using` normative changes (microsoft#58624)
  Handling statements from a known source file (microsoft#58679)
  ...
@Andarist Andarist deleted the fix/noinfer-flow-type branch September 19, 2024 16:02
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NoInfer isn't erased and breaks type narrowing
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