Assertion failure in FilePath.lexicallyNormalized()

[jglogan]

% swift --version
Apple Swift version 6.2 (swift-6.2-RELEASE)
Target: arm64-apple-macosx26.0
Build config: +assertions

Using swift-system 1.6.3, attached file contains minimal reproducer.

Test code:

import SystemPackage
import Testing

@Test func pathNormalize() throws {
    let path = FilePath("dir/../../../etc/pwned")
    #expect(path.lexicallyNormalized() == "../../etc/pwned")
}

% swift test
[1/1] Planning build
Building for debugging...
[1/1] Write swift-version--2028C2505FF3CBA9.txt
Build complete! (0.13s)
Test Suite 'All tests' started at 2026-01-11 13:48:30.341.
Test Suite 'All tests' passed at 2026-01-11 13:48:30.342.
	 Executed 0 tests, with 0 failures (0 unexpected) in 0.000 (0.001) seconds
􀟈  Test run started.
􀄵  Testing Library Version: 6.2 (3fdabe5392108d8)
􀄵  Target Platform: arm64-apple-macosx
􀟈  Test pathNormalize() started.
SystemPackage/FilePathParsing.swift:231: Assertion failed
error: Exited with unexpected signal code 5

20260111-lexicalnormalize-bug.tgz

[jglogan]

If I get time I'll root cause this, I did drop some debug prints into _normalizeSpecialDirectories() and see this:

◇ Test pathNormalize() started.
DEBUG: start normalize special
DEBUG: readIdx=0 writeIdx=0 compEnd=3 nextStart=4
DEBUG: readIdx=4 writeIdx=4 compEnd=6 nextStart=7
DEBUG: drop parent
DEBUG: readIdx=7 writeIdx=0 compEnd=9 nextStart=10
DEBUG: swap readIdx=7 writeIdx=0
DEBUG: swap readIdx=8 writeIdx=1
DEBUG: swap readIdx=9 writeIdx=2
DEBUG: readIdx=10 writeIdx=3 compEnd=12 nextStart=13
DEBUG: skip drop parent, prior not parent
SystemPackage/FilePathParsing.swift:239: Assertion failed