Fix for "Outlier case in Excess Property detection in Union (a case that is -more- strict than it should be)" #44856

[craigphicks]

Changes to be committed:
modified: src/compiler/checker.ts
new file: tests/baselines/reference/_44856.js
new file: tests/baselines/reference/_44856.symbols
new file: tests/baselines/reference/_44856.types
new file: tests/cases/compiler/_44856.ts

• fix for issue Outlier case in Excess Property detection in Union (a case that is -more- strict than it should be) #44856

• minor modifications to checker.ts

  • Preexisting algorithm:

    • In the case of checking for excess properties being assigned to a union, the logic forks:
      • If there is an index in the union whose type qualifies it as discriminant index, then don't allow excess properties (roughly)
      • otherwise, allow excess properites which satisfy some member of the union

  • Change:

    • Don't consider an index with a boolean type to qualify for being a discriminant index.

• Before:

  • In src/compiler/checker.ts, in the function createUnionOrIntersectionProperty the preexisting code is

if ((isLiteralType(type) || isPatternLiteralType(type) || type === uniqueLiteralType) {
        checkFlags |= CheckFlags.HasLiteralType;
    }

• isLiteralType(type) returns true for a boolean type, and setting checkFlags |= CheckFlags.HasLiteralType results in a higher up decision to qualify the corresponding index as a discriminated index.

• After:

if ((isLiteralType(type) &&
        (!fromIsDiscriminantProperty || !(type.flags & TypeFlags.Boolean && type.flags & TypeFlags.Union)))
    || isPatternLiteralType(type) || type === uniqueLiteralType) {
    checkFlags |= CheckFlags.HasLiteralType;
}

• the fromIsDiscriminantProperty is to provide the context in which createUnionOrIntersectionProperty.
• That is necessary because createUnionOrIntersectionProperty is called in other contexts that require the existing behavior.
  • Here is one such scenario from objectSpread.ts

function f<T, U>(t: T, u: U) {
    return { ...t, ...u, id: 'id' }
}
let overwriteId: { id: string, a: number, c: number, d: string } =
    f({ a: 1, id: true }, { c: 1, d: 'no' })

• Without introducing the condition fromIsDiscriminantProperty, overwriteId would have type never.

Fixes #44856

[typescript-bot]

This PR doesn't have any linked issues. Please open an issue that references this PR. From there we can discuss and prioritise.

[sandersn]

@typescript-bot run dt
@typescript-bot user test this inline

[typescript-bot]

Heya @sandersn, I've started to run the diff-based community code test suite on this PR at 78b9245. You can monitor the build here.

Update: The results are in!

[typescript-bot]

Heya @sandersn, I've started to run the parallelized Definitely Typed test suite on this PR at 78b9245. You can monitor the build here.

[typescript-bot]

@sandersn
Great news! no new errors were found between main..refs/pull/49004/merge