Port #62275: Discard types that reduce to never before discriminating by discriminable items

[Copilot]

Ports microsoft/TypeScript#62275.

Intersection types that reduce to never (e.g., conflicting properties like { enableB: true } & { enableB: false }) were participating in discriminant-based narrowing, preventing proper contextual typing and causing parameters to fall back to any.

• Filter out never-reducing types alongside primitives in discriminateTypeByDiscriminableItems:

// Before
if t.flags&TypeFlagsPrimitive == 0 {

// After
if t.flags&TypeFlagsPrimitive == 0 && c.getReducedType(t).flags&TypeFlagsNever == 0 {

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.

[Copilot]

Pull request overview

Ports TypeScript PR microsoft/TypeScript#62275 to improve contextual typing and discriminant-based narrowing by ensuring union constituents that reduce to never don’t participate in discriminateTypeByDiscriminableItems, preventing erroneous narrowing that could degrade contextual typing to any.

Changes:

• Exclude union constituents whose reduced type is never when initializing the candidate set in discriminateTypeByDiscriminableItems.
• Update reference baselines to reflect correct contextual typing (notably, setItem’s parameter no longer falls back to any) and removal of the prior TS7006 error in the referenced test.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
internal/checker/relater.go	Filters out constituents that reduce to never before discriminant-based narrowing.
testdata/baselines/reference/submodule/compiler/contextuallyTypedByDiscriminableUnion2.types.diff	Updates diff baseline to reflect reduced mismatch after the fix.
testdata/baselines/reference/submodule/compiler/contextuallyTypedByDiscriminableUnion2.types	Updates reference .types output showing improved contextual typing (no any fallback).
testdata/baselines/reference/submodule/compiler/contextuallyTypedByDiscriminableUnion2.errors.txt.diff	Removes prior error diff content associated with TS7006.
testdata/baselines/reference/submodule/compiler/contextuallyTypedByDiscriminableUnion2.errors.txt	Removes the previously expected TS7006 error output.