Port TS#62656: Fix TS2783 false positive for union types in object spread

[Copilot]

Ports microsoft/TypeScript#62656.

checkSpreadPropOverrides reported TS2783 ("specified more than once, so this usage will be overwritten") for properties that only exist in some constituents of a spread union type. These partial properties are marked with CheckFlags.Partial, but the function only checked SymbolFlags.Optional.

// Before: only skipped optional properties
if right.Flags&ast.SymbolFlagsOptional == 0 {

// After: also skip partial properties (present in some but not all union constituents)
if right.Flags&ast.SymbolFlagsOptional == 0 && right.CheckFlags&ast.CheckFlagsPartial == 0 {

Example that previously false-positived:

type Thing = { id: string; label: string };
fun({
    id: 'foo',  // ❌ TS2783: 'id' is specified more than once
    ...find('foo') ?? { label: 'Foo' },  // spread is Thing | { label: string }
});

The { label: 'Foo' } fallback has no id, so the overwrite is not guaranteed — error was incorrect.

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[Copilot]

Pull request overview

Ports the TypeScript fix for TS2783 false positives when spreading union types where a property exists in only some union constituents (“partial” properties).

Changes:

• Update checkSpreadPropOverrides to also skip CheckFlags.Partial properties when checking for guaranteed overwrites.
• Update baseline error output to remove the incorrect TS2783/TS2785 report for the union-spread repro.
• Remove the corresponding baseline .diff content now that the expected output is aligned.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
testdata/baselines/reference/submodule/compiler/spreadUnionPropOverride.errors.txt.diff	Removes the no-longer-applicable baseline diff for the previously incorrect diagnostic.
testdata/baselines/reference/submodule/compiler/spreadUnionPropOverride.errors.txt	Updates expected errors to no longer include the TS2783 false positive for id.
internal/checker/checker.go	Fixes the overwrite check by excluding properties marked CheckFlags.Partial (not guaranteed in all union constituents).