Swift extractor internal error: assertion 'found != SwiftMangler::preloadedExtensionOrFilePrivateValueIndexes.end()' failed (declaration not found within parent)

[inequitas]

Description

CodeQL's Swift analysis fails with an internal error during GitHub code scanning (default setup) on a public Swift/SwiftPM macOS project:

CodeQL encountered an unexpected internal error with the following message:
Assertion failed: found != SwiftMangler::preloadedExtensionOrFilePrivateValueIndexes.end(). declaration not found within parent
Some or all of the Swift analysis may have failed.

Environment

• GitHub code scanning default setup, language: swift
• GitHub-hosted macOS runner
• Occurred ~2026-07-20, reproducible across scans

Reproduction

Public repo that triggers it: https://github.com/inequitas/performancemonitor — a Swift Package Manager macOS menu-bar app (SwiftUI/AppKit), Swift 5.10 tools, no third-party dependencies. The codebase uses fileprivate/private declarations and extensions extensively, which — given the assertion name (preloadedExtensionOrFilePrivateValueIndexes) — may be related.

Impact

Swift analysis fails/is incomplete, so code scanning produces no reliable Swift results; we've had to disable CodeQL for the repo.

Happy to help narrow down a minimal repro if useful.

[inequitas]

Some root-cause investigation from the reporting side, in case it's useful.

The assertion is in swift/extractor/mangler/SwiftMangler.cpp (getExtensionOrFilePrivateValueIndex, ~L145-146). indexExtensionsAndFilePrivateValues walks a parent's members to give each extension and each fileprivate ValueDecl a stable index; the assertion fires when a decl that qualifies as fileprivate/extension isn't found again while re-enumerating its parent's members.

The fileprivate-value branch is recent — it looks like it was added in d09e2f66c ("Assign indexes to fileprivate ValueDecls"), whereas the extension-only path had a similar fix earlier in 06d0d4841. The new fileprivate branch seems to hit the same class of problem for decls whose parent's getAllMembers() / top-level enumeration doesn't return the exact Decl* — likely candidates being accessors of computed properties (not in getAllMembers) and top-level private globals surfaced as PatternBindingDecl rather than VarDecl.

Candidate minimal repro — not locally verified (I don't have the CodeQL CLI set up to confirm, so please treat this as a hypothesis rather than a confirmed reduction):

private func f() {}
private let x = 0
private extension Int { var y: Int? { nil } }

Happy to help verify or narrow this down further if it's useful.