Test matrix: ERESOLVE on (eslint 7, parser 6) cells under strict peer-deps

[captaindonald]

Three cells in the node-minors and eslint-8- matrices flake at the
after_install step (npm install --no-save eslint@7 @typescript-eslint/parser@6 babel-eslint@…)
under NPM_CONFIG_LEGACY_PEER_DEPS=false:

• node-minors: (16, 7, 10) — node 16, eslint 7, babel-eslint 10
• eslint-8-: (19, 7, 8, 6) — node 19, eslint 7, babel-eslint 8, typescript-eslint 6
• eslint-8-: (19, 7, 9, 6) — node 19, eslint 7, babel-eslint 9, typescript-eslint 6

Failure

npm warn peer eslint@"^7.32.0 || ^8.2.0" from eslint-config-airbnb-base@15.0.0
npm warn peer eslint@"^7.0.0 || ^8.0.0" from @typescript-eslint/parser@6.21.0
npm ERR! code ERESOLVE
npm ERR! Cannot destructure property 'name' of 'node' as it is undefined.

(The Cannot destructure … line is npm's own crash while it tries to print
the ERESOLVE diagnostic.)

Root cause

Both workflows set NPM_CONFIG_LEGACY_PEER_DEPS=false for the affected cells:

• node-minors: when node-version >= 16 && eslint >= 7
• eslint-8-: when typescript-eslint >= 6

Under strict peer-deps, the bare npm install resolves with the latest
eslint matching devDependencies.eslint (currently ^9.7), then the
after_install step tries to swap it down to eslint@7. The combination
of eslint-config-airbnb-base@15.0.0 (peers eslint@^7.32 || ^8.2),
@typescript-eslint/parser@6 (peers eslint@^7 || ^8), and other transitive
peer constraints leaves npm an unsatisfiable graph.

npm's tree resolver is nondeterministic on unsatisfiable peer graphs;
depending on cache state and resolution order, the install sometimes lands
with peer warnings and sometimes throws ERESOLVE. These cells aren't
deterministically broken — they lose the coin flip often enough to keep
showing up red.

Cross-check against #3979 (Feb 2026) and #4006

All three are intermittent. They've been seen both green and red across
the two PRs:

• (16, 7, 10): green on Fix ESLint v10 RuleContext API removal (follow-up to #3972) #3979, red on early [Tests] drop ESLint-10-incompatible 'type:' property from RuleTester error assertions #4006 runs, green again on the latest [Tests] drop ESLint-10-incompatible 'type:' property from RuleTester error assertions #4006 tip
• (19, 7, 9, 6): green on Fix ESLint v10 RuleContext API removal (follow-up to #3972) #3979, red on early [Tests] drop ESLint-10-incompatible 'type:' property from RuleTester error assertions #4006 runs, green again on the latest [Tests] drop ESLint-10-incompatible 'type:' property from RuleTester error assertions #4006 tip
• (19, 7, 8, 6): red on Fix ESLint v10 RuleContext API removal (follow-up to #3972) #3979 and red on early [Tests] drop ESLint-10-incompatible 'type:' property from RuleTester error assertions #4006 runs, green on the latest [Tests] drop ESLint-10-incompatible 'type:' property from RuleTester error assertions #4006 tip — longest historical red streak of the three, but still not deterministic

Resolution paths

1. Drop strict peer-deps for these cells. Change the LEGACY_PEER_DEPS
   guard in both workflows so that eslint <= 8 paths always use
   LEGACY_PEER_DEPS=true. Surgical, no source changes, accepts that npm
   will warn rather than crash.
2. Pin eslint-config-airbnb-base. It's the tightest peer constraint
   in the conflict (^7.32 || ^8.2). Pinning to a hypothetical newer
   version with broader peers, or replacing it with
   eslint-config-airbnb-base-typescript, would loosen the graph.
3. Trim the matrix. Eslint 7 + parser 6 is the only combo failing;
   removing those cells (or only the ones still red after option 1) is
   the smallest change.

Discovered while triaging CI on #4006.

[captaindonald]

Commit e47c827 reverted with 079dadc.

Issue remains unresolved.

[captaindonald]

Tried option 1 (LEGACY_PEER_DEPS=true everywhere on these workflows) in #4006: turned 60+ previously-passing cells red. Reverted. The strict-peer setting was apparently load-bearing for the dep tree on a lot more cells than just the three flaky ones — flipping it changed which transitive versions resolved, and tests started failing on otherwise-fine cells.

Option 1 is out. Options 2 and 3 still open.

[ljharb]

Option 2 is out too - and the airbnb config is blocked on eslint-plugin-import, which means so is this plugin. Separately, option 3 would never be an option.