Port PR 62656: Fix TS2783 false positive for union types in object spread expressions

[Copilot]

This PR ports microsoft/TypeScript#62656 to fix a false positive TS2783 error when spreading union types in object literals.

Problem

TypeScript incorrectly reported error TS2783 ("'x' is specified more than once, so this usage will be overwritten") when spreading union types in object literals, even when the property wasn't guaranteed to exist in all constituents of the union.

For example, this code incorrectly produced an error:

type Thing = {
    id: string;
    label: string;
};

function find(id: string): undefined | Thing {
    return things.find(thing => thing.id === id);
}

fun({
    id: 'foo',
    ...find('foo') ?? { label: 'Foo' },  // ❌ False positive: 'id' is specified more than once
});

The error claimed id would "always" be overwritten, but this was incorrect. The spread expression evaluates to Thing | { label: string }, and the fallback object { label: 'Foo' } doesn't have an id property at all, so id is only overwritten when find() returns a Thing.

Root Cause

The checkSpreadPropOverrides function only checked if a property had SymbolFlags.Optional before reporting the error. It didn't account for properties that exist in some constituents of a union type but not others. These properties are marked with CheckFlags.Partial (specifically ReadPartial or WritePartial), indicating they're only present in a subset of the union's constituents.

Solution

Added an additional check for CheckFlags.Partial in checkSpreadPropOverrides. Properties marked as partial in union types should not trigger the "always overwrites" error since they don't exist in all code paths.

// Only report error if property is both non-optional AND not partial
if right.Flags&ast.SymbolFlagsOptional == 0 && right.CheckFlags&ast.CheckFlagsPartial == 0 {
    // Report error...
}

Testing

• All existing tests pass with no baseline changes
• Manually verified correct behavior for:
  • Union types where property exists in only some constituents (no error - fixed)
  • Union types where property exists in all constituents (error - preserved)
  • Optional properties in unions (no error - preserved)
  • Properties with union types like x: number | undefined (error - preserved)

The fix preserves all existing correct behavior while eliminating the false positive for partial properties in union types.

Fixes microsoft/TypeScript#61223

Original prompt

Port PR 62656

Note

Custom agent used: Strada to Corsa Port Expert
A Go and TypeScript expert who can easily figure out how to port PRs from one language to another

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