Detect self-referencing mapped type constraints

[Naji Ullah (Naji-Ullah)]

With this PR we report a circularity error when the type parameter of a mapped type is referenced in its own constraint. Previously such circularities were detected only when base constraint resolution happened to revisit the type parameter (as in { [P in P]: X }), but not when the reference occurred in a position whose base constraint doesn't depend on it, such as keyof P. In the example

type Test = keyof {
    [P in keyof P]: unknown
};

declare const test: Test;

export function getTest() {
    return test;
}

no error was reported, and P escaped from the mapped type into the generated declaration file:

export declare function getTest(): keyof P;

We now report

Type parameter 'P' has a circular constraint.

on keyof P, the same error we already report for { [P in P]: X }, and we use the error type as the constraint so P no longer escapes (the declaration above becomes getTest(): any). The check happens where the declared constraint of a mapped type parameter is resolved, so it also covers instantiations of generic mapped types. It applies only to mapped types, so ordinary type parameters such as T extends keyof T are unaffected. Any reference to the type parameter within its own constraint is considered circular, including one that doesn't affect the resulting keys, such as { [P in keyof { a: P }]: X }.

The new test circularMappedTypeConstraint2.ts covers the example above, variants such as keyof P | "x", Extract<keyof P, string>, keyof Cond<P>, and a generic mapped type and its instantiation, as well as cases that must remain error-free (T extends keyof T, a shadowed inner P, and an as clause referencing P). No existing baselines are affected.

AI disclosure: I used an AI coding assistant (Claude Code) while investigating and writing this fix. I've reviewed the change and the tests, I understand them, and I'll be handling review feedback myself.

Fixes #62176.

[Copilot]

Copilot review overview

🟢 Approval recommended

The implementation is narrowly scoped and its diagnostic, recovery, instantiation, shadowing, and declaration-emit behavior are covered by regression baselines.

Review effort: Balanced
Findings: None

What changed in this PR

Adds circular-constraint detection for self-referencing mapped type parameters and prevents them from leaking into declaration output.

Changes:

• Detect mapped type parameters referenced within their own constraints.
• Substitute the error type after reporting circularity.
• Add regression tests and compiler baselines for errors, types, symbols, and declaration emit.

File	Description
tsc/​internal/​checker/​checker.go	Implements self-reference detection and error recovery.
tsc/​testdata/​tests/​cases/​compiler/​circularMappedTypeConstraint2.ts	Adds positive and negative regression cases.
tsc/​testdata/​baselines/​reference/​compiler/​circularMappedTypeConstraint2.errors.txt	Records expected diagnostics.
tsc/​testdata/​baselines/​reference/​compiler/​circularMappedTypeConstraint2.js	Verifies JavaScript and declaration output.
tsc/​testdata/​baselines/​reference/​compiler/​circularMappedTypeConstraint2.types	Records inferred types.
tsc/​testdata/​baselines/​reference/​compiler/​circularMappedTypeConstraint2.symbols	Records symbol resolution.

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[Naji Ullah (Naji-Ullah)]

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