Improve type discrimination algorithm

[ahejlsberg]

Implements what I suggest here.

We might consider back-porting this to the old code base.

Fixes #1020.

[Copilot]

Pull Request Overview

This PR enhances the type discrimination algorithm in the checker, adds new tests for missing discriminants, and updates baselines for error outputs.

• Added missingDiscriminants.ts tests to cover cases where discriminants are partially missing.
• Updated reference baselines under submodule/compiler and compiler to reflect the revised discrimination behavior.
• Modified relater.go to change how TernaryMaybe is applied within the discrimination loop.

Reviewed Changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
testdata/tests/cases/compiler/missingDiscriminants.ts	New test cases for missing discriminants behavior
testdata/baselines/reference/submodule/compiler/discriminateWithMissingProperty.errors.txt.diff	Cleared outdated submodule error baseline
testdata/baselines/reference/submodule/compiler/discriminateWithMissingProperty.errors.txt	Removed old submodule error baseline content
testdata/baselines/reference/compiler/missingDiscriminants.types	New type-check baseline for missing discriminants tests
testdata/baselines/reference/compiler/missingDiscriminants.symbols	New symbol baseline for missing discriminants tests
testdata/baselines/reference/compiler/missingDiscriminants.errors.txt	New error baseline for missing discriminants tests
internal/checker/relater.go	Adjusted discrimination logic to separate property-check and match

Comments suppressed due to low confidence (1)

internal/checker/relater.go:1228

• The inserted closing brace here appears to end the for loop prematurely, changing the intended nesting and control flow. Remove or reposition it so that the loop and subsequent logic maintain their original scopes.

}

[jakebailey]

Ported it over in microsoft/TypeScript#61828 and it fails the self test. I wonder why this doesn't fail in CI here.

[jakebailey]

Ah, because the failure isn't in src/compiler, which is what we test here given we don't have build mode.

[Eigilak]

The man, the myth, the legend 🫡