Port #61683: Use getNonMissingTypeOfSymbol in compareProperties

[Copilot]

• Change getTypeOfSymbol to getNonMissingTypeOfSymbol in compareProperties function (checker.go:27072)
• Build and verify the change compiles
• Run full test suite and accept all baseline changes
• exactOptionalPropertyTypesIdentical baseline accepted
• multipleBaseInterfaesWithIncompatibleProperties2 (exactoptionalpropertytypes=true) baseline accepted — diff file deleted (output now matches TS reference)

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[Copilot]

Pull request overview

This PR ports microsoft/TypeScript#61683 to fix a bug in property comparison under exactOptionalPropertyTypes mode. The issue caused optional properties with and without explicit | undefined to be incorrectly treated as identical in conditional type resolution.

Changes:

• Updated compareProperties to use getNonMissingTypeOfSymbol instead of getTypeOfSymbol to properly strip the internal "missing" type marker before comparison
• Added test baseline showing the now-correctly-detected type error for the scenario described in the issue

Reviewed changes

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

File	Description
internal/checker/checker.go	Changed property type comparison to use getNonMissingTypeOfSymbol for both source and target properties
testdata/baselines/reference/submodule/compiler/exactOptionalPropertyTypesIdentical.errors.txt	Added expected error output showing the fix correctly detects type incompatibility
testdata/baselines/reference/submodule/compiler/exactOptionalPropertyTypesIdentical.errors.txt.diff	Removed the diff file indicating no previous errors were expected

[Jake Bailey (jakebailey)]

Copilot You still need to run the tests and accept all baseline changes

[Copilot]

Copilot You still need to run the tests and accept all baseline changes

Ran the full test suite and accepted all baseline changes in 0683d859. There was one additional test affected: multipleBaseInterfaesWithIncompatibleProperties2(exactoptionalpropertytypes=true) — its .diff file was deleted because our output now correctly matches the TypeScript reference baseline. All tests pass.