Declaration emit writes = Infinity initializers for overflowing number literals (invalid .d.ts, TS1254) #64760

🔎 Search Terms

declaration emit Infinity, d.ts Infinity, TS1254 ambient const initializer, 1e400 declaration, number literal overflow declaration

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about declaration emit, Infinity and numeric literal types (none apply).
• Tried: 6.0.3 (Playground) and main (7.1.0-dev, b5c34ba). Both emit the invalid = Infinity initializers; 6.0.3 also emits declare const _default: Infinity; for the default export.

⏯ Playground Link

https://www.typescriptlang.org/play/?declaration=true#src=export%20const%20a%20%3D%201e400%3B%0Aexport%20const%20b%20%3D%20-1e400%3B%0Aexport%20class%20C%20%7B%0A%20%20%20%20static%20readonly%20e%20%3D%201e400%3B%0A%7D%0Aexport%20default%201.7976931348623159e308%3B%0A

💻 Code

export const a = 1e400;
export const b = -1e400;
export class C {
    static readonly e = 1e400;
}
export default 1.7976931348623159e308;

🙁 Actual behavior

With --declaration, the emitted .d.ts is:

export declare const a = Infinity;
export declare const b = -Infinity;
export declare class C {
    static readonly e = Infinity;
}
declare const _default = Infinity;
export default _default;

Checking that file reports TS1254 on every initializer: "A 'const' initializer in an ambient context must be a string or numeric literal or literal enum reference." Infinity is an identifier, not a literal.

🙂 Expected behavior

The emitted .d.ts type-checks, for example:

export declare const a: number;
export declare const b: number;
export declare class C {
    static readonly e: number;
}
declare const _default: number;
export default _default;

Additional information about the issue

Related: #42905 (Infinity in type positions, fixed in 7.1), #63831 (-1e500 as const emitted as -Infinity) and #56272 (whether overflowing literals should have an Infinity literal type at all). This issue is narrower: whatever the type is, the emitted .d.ts should be valid.

I have a fix with a compiler test ready: https://github.com/sidcraftscode/TypeScript/tree/fix/dts-non-finite-literals. I'm opening a PR that references this issue.