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Constraint check compares a type parameter's TypeVar, not its instantiation: nested generic calls warn on code tsc accepts

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
48/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Active
Tech stack
rust, typescript
Domain
backend, compilers

Research direction

Start by tracing generic call argument type checking in the Rust compiler, using the reported TypeVar("F") versus instantiated argument type as the clue. Compare the nested generic call with the direct inQueryConnection(calculate) case, then run or add a regression case based on the reproducer. Done means the nested call compiles without the constraint warning while preserving correct behavior.

Written by the indexing model from the issue text.

Description

perry compile warns on code tsc --strict accepts, whenever the argument to a generic function is itself the result of a generic call. The compiler appears to check the uninstantiated type variable against the parameter's constraint instead of the type it was instantiated to, and no type variable can satisfy a function-shape constraint.

Benign: it is a warning, codegen continues, and the binary runs correctly. This is a diagnostic-quality bug, not a blocker — but it fires on ordinary decorator-style composition, so it is noisy on real code.

Reproducer

interface Rel { n: number }

function inQueryConnection<F extends (this: Rel, ...args: never[]) => Promise<any>>(fn: F): F {
  return function (this: Rel, ...args: unknown[]) {
    return fn.apply(this, args as never[]);
  } as unknown as F;
}

function withDeferred<F extends (this: Rel, ...args: never[]) => unknown>(fn: F): F {
  return function (this: Rel, ...args: never[]) {
    return fn.apply(this, args);
  } as unknown as F;
}

async function calculate(this: Rel, op?: string): Promise<number> {
  return op ? 1 : this.n;
}

export const Calculations = {
  calculate: inQueryConnection(withDeferred(calculate)),
};

console.log(`[repro] ${await Calculations.calculate.call({ n: 7 })}`);

Observed

Warning: Constraint violation in function 'inQueryConnection': TypeMismatch {
  type_param: "F",
  expected: Function(FunctionType { params: [("this", Named("Rel"), false),
            ("_rest", Array(Never), false)], return_type: Promise(Any),
            is_async: false, is_generator: false }),
  actual: TypeVar("F") }

The binary then compiles, links and prints [repro] 7, which is correct.

Expected

No warning. withDeferred(calculate) has type typeof calculate, which satisfies inQueryConnection's constraint. tsc --ignoreConfig --strict --target es2022 --module esnext --moduleResolution bundler --noEmit exits 0 on the same file (TypeScript 7.1.0-dev.20260920.1).

What narrows it

  • It is not a bound mismatch. Widening the outer bound from => Promise<any> to => unknown, so both bounds are identical, still warns — the message only changes to return_type: Unknown. So Perry compares against the correct bound and still rejects TypeVar("F").
  • actual: TypeVar("F") is the tell: the argument is reported as the type variable, not as the type it was instantiated to.
  • Calling inQueryConnection(calculate) directly — a non-generic argument — does not warn. The nesting is required.

Environment

  • Perry main @ 20fde48d, built from source, Linux x86_64, LLVM 22.1.8, rustc 1.101.0-nightly
  • Also reproduces on npm @perryts/perry 0.5.1520 and on from-source 0.5.1662

Found while compiling a TypeScript web framework ahead-of-time; the original site is a query-method wrapper composed exactly this way.

Dominant language
Rust
Stars
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Forks
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Avg merge
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Merged PRs (30d)
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Getting set up

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

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