[bug:7.5.3] Mapper class explicite field regression

🔎 Search Terms

Mapper class explicite inference for primitive

🕗 Version & Regression Information

This changed between versions 5.7.3 and 5.6.3

⏯ Playground Link

if need, here is a very simple and epurate playground without all the tests.
SMALL_PLAYGROUND

💻 Code

I have added some additional tests in order to gather more information and understand what is happening.
Bugs and fixes are the two adresse to verify.

declare function rollable( n:number ):number

export abstract class Abstract {
	[key: string]: number
	#Abstract!: never;
}

class Test {
	bug = new class {
		property1 = 1;
		property2 = rollable( 2 );
	}();

	fix = new class {
		property1:number = 1;
		property2 = rollable( 2 );
	}();

	moreBug1 = new class {
		property1 = 1 as const;
		property2 = rollable( 2 );
	}();

	moreBug2 = new class extends Abstract {
		property1 = 1 as const;
		property2 = rollable( 2 );
	}();
}
type Mapper<T> ={
	[k in keyof T]: [k, T[k]];
}
type Mapper2<T> ={
	[k in keyof T as string extends k ? never : k]: [k, T[k]];
}
type mapper_bug = Mapper<Test['bug']>; // work in version: 5.6.3
type mapper_fix = Mapper<Test['fix']>; // expected : property1: ["property1", number];
//       ^?
type mapper_moreTest1 = Mapper<Test['moreBug1']>;
type mapper_moreTest2 = Mapper<Test['moreBug2']>;
type mapper2_bug = Mapper2<Test['bug']>;
type mapper2_fix = Mapper2<Test['fix']>;

🙁 Actual behavior

the mapper ignore the field declaration !!

🙂 Expected behavior

the mapper should consider the field declaration !

Additional information about the issue

No, but I just spent a whole day trying to understand before realizing that it's a TypeScript issue! I am exhausted.

please test the play ground in both version : 5.6.3 and 5.7.3

Bisects to #59282

class C {
	annotated: number = 1;
	unannotated = 2;
}
type Mapper<T> = {
	[k in keyof T]: k
}
type mapper_bug = Mapper<C>; // work in version: 5.6.3
//    ^?
//   Actual
//   { annotated: "annotated"; unannotated: number }
//   Expected
//   { annotated: "annotated"; unannotated: "unannotated" }

This is fixed in the current native TypeScript build (7.1.0-dev.20260918.1).

In TypeScript 5.7.3, Quick Info for Mapper<C> displays unannotated: number. The current build displays unannotated: ["unannotated", number], matching the annotated field's mapped tuple shape.

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[bug:7.5.3] Mapper class explicite field regression #61540

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🕗 Version & Regression Information

⏯ Playground Link

💻 Code

🙁 Actual behavior

🙂 Expected behavior

Additional information about the issue

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[bug:7.5.3] Mapper class explicite field regression #61540

Description

🔎 Search Terms

🕗 Version & Regression Information

⏯ Playground Link

💻 Code

🙁 Actual behavior

🙂 Expected behavior

Additional information about the issue

Activity

Andarist commented on Apr 7, 2025

RyanCavanaugh commented on Apr 8, 2025

RyanCavanaugh commented on Sep 19, 2026

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