TS 7.0.2 (tsgo): augmentation of a type-only re-exported interface resolves order-dependently — same file set passes via explicit include list, fails via directory glob

[DoodleBears]

Versions

• Fails: typescript@7.0.2 (tsgo)
• Works: typescript@6.0.3 on the identical program and dependency tree
• Module resolution: bundler; strict, verbatimModuleSyntax on

Context

Real-world case from a TanStack Start 1.168 app:

• @tanstack/router-core declares the interface:

  // @tanstack/router-core/dist/esm/router.d.ts
  export interface Register { /* ... */ }

• @tanstack/react-router only re-exports it:

  // @tanstack/react-router/dist/esm/index.d.ts
  export type { Register /* ... */ } from '@tanstack/router-core';

• App code augments the re-exporting module name:

  // src/server.ts
  declare module "@tanstack/react-router" {
    interface Register {
      server: { requestContext: { env: Env; ctx: Ctx } }
    }
  }

• A third package's declaration imports the re-export and feeds it through a conditional type:

  // @tanstack/react-start .../server-entry.d.ts
  import { Register } from '@tanstack/react-router';
  export type ServerEntry = { fetch: RequestHandler<Register> };
  // start-server-core request-handler.d.ts
  export type RequestOptions<TRegister> = EarlyHintsOptions & InlineCssOptions &
    (TRegister extends { server: { requestContext: infer TRequestContext } }
      ? { context: TRequestContext & BaseContext }   // augmented branch
      : { context?: BaseContext });                  // unaugmented fallback

Actual behavior (7.0.2)

Whether the conditional picks the augmented or unaugmented branch depends on how the same root files enter the program:

Program configuration	Result under 7.0.2	Under 6.0.3
"include": ["src"] (+ exclude patterns), ~26 .ts/.tsx roots	❌ unaugmented branch — excess-property error at the handler call site	✅ augmented
byte-identical root set enumerated as an explicit include array of file paths	✅ augmented	✅ augmented
single-file programs / small synthetic programs using the same packages	✅ augmented	✅ augmented

Each row is deterministic across repeated runs. Probing RequestOptions<Register> with a discriminating assignment confirms TS 6 reduces it to { context: TRequestContext & BaseContext } while TS 7 reduces it to { context?: BaseContext } in the failing configuration.

Notably, a second augmentation of the declaring module (declare module "@tanstack/router-core") does land somewhere — conflicting duplicate declarations across files produce TS2717 "Subsequent property declarations must have the same type", naming the first declaration's shape — yet the conditional still resolves to the unaugmented branch, suggesting two distinct Register symbols end up existing per program state.

Minimization attempts

A synthetic two-package repro (local @fake/core declaring the interface + @fake/reexport doing export type { Register }, one augmenting file + one consumer probing the conditional) compiles fine under 7.0.2 even with 150 additional modules mixed into the glob. The flip appears to require the scale/structure of the real program (route-tree codegen that also augments Register.router, dozens of cross-importing route modules, etc.).

Workaround we shipped

Typing the single affected call site explicitly against our own shape and keeping the augmentation for documentation value:

const fetchHandler = handler.fetch as unknown as (
  request: Request,
  opts: { context: QuesonaWebRequestContext }
) => Promise<Response>

Happy to provide the full failing program snapshot or run diagnostics (--explainFiles output comparing both configurations shows an identical resolved file set for the router packages) if useful.