[nightly] Syntactically invalid declaration files emitted under extremely specific conditions

[MichaelMitchell-at]

🔎 Search Terms

declaration syntax invalid comment docstring

🕗 Version & Regression Information

• Bisected this to 2b4e7e3 cc @dragomirtitian @weswigham

⏯ Playground Link

https://www.typescriptlang.org/dev/bug-workbench/?target=7&composite=true&ts=5.6.0-dev.20240608#code/PTAEAEGMHsFsAdoGcCWAXApgLlGgTgK4YBQIES+KkaO+RpY4AJhpADYCGeHaK0AdrUIky4JAAtoAdwCisdA3KTZ8tJiYAxFG2ygOAOib60SYovAAzbRn4dYugyeIoE0PGlABvUACNoADwwmACEA0ABfUAs8OFAAIn1gP0CQgLiAbjMMf0R3UBh+ClAAfVAAXl8AoND-TMUAQQ8UfhZ-UAAOAGZSACoe4lAe0CkeAZ7gM1ErHVt7HGTqgMNjU2zcjxZ2Lgx8gSKF1P8cF3WACgSktIBKfRqAHk8B0GeD49d3c8TkuJv7-gJYD4MHgAHyZcJg55Q6Ew2HQsgAPQAKuIUEhQGiMS1sh1OljcKj0dMSOZibNdMllk41m4NqxOHgdlskOj7kiQV4ns8AG4cNhEHBIzLQgoUQjUNynXn83RIq7g4g0vKbBk7CwEfjUPj8Sr+O7sqV8gWgOU4Nlg4hAA

💻 Code

// @composite: true
// @strict: true
// @declaration: true
// @showEmit
// @showEmittedFile: a.d.ts

// @filename: a.ts
import { boxedBox } from "./boxedBox";

export const _ = boxedBox;

// At index 83
/**
 * wat
 */

// @filename: boxedBox.d.ts
export declare const boxedBox: import("./box").Box<{
    boxed: import("./box").Box<number>;
}>;                        // ^This is index 83 in this file

// @filename: box.d.ts
export declare class Box<T> {
    value: T;
    constructor(value: T);
}
export declare function box<T>(value: T): Box<T>;

🙁 Actual behavior

a.d.ts is emitted as

export declare const _: import("./box").Box<{
    boxed: import("./box").Box /**
     * wat
     */<number>;
}>;
/**
 * wat
 */

🙂 Expected behavior

a.d.ts is emitted as

export declare const _: import("./box").Box<{
    boxed: import("./box").Box<number>;
}>;
/**
 * wat
 */

Additional information about the issue

Given the extremely specific conditions under which this bug arises, somehow we hit the lottery by encountering it. Basically what I observed is that there is a point where this code is reached:

TypeScript/src/compiler/emitter.ts

Lines 5715 to 5728 in d8086f1

	function emitCommentsAfterNode(node: Node, savedContainerPos: number, savedContainerEnd: number, savedDeclarationListContainerEnd: number) {
	const emitFlags = getEmitFlags(node);
	const commentRange = getCommentRange(node);
	// Emit trailing comments
	if (emitFlags & EmitFlags.NoNestedComments) {
	commentsDisabled = false;
	}
	emitTrailingCommentsOfNode(node, emitFlags, commentRange.pos, commentRange.end, savedContainerPos, savedContainerEnd, savedDeclarationListContainerEnd);
	const typeNode = getTypeNode(node);
	if (typeNode) {
	emitTrailingCommentsOfNode(node, emitFlags, typeNode.pos, typeNode.end, savedContainerPos, savedContainerEnd, savedDeclarationListContainerEnd);
	}
	}

in the following state:

• node refers to Box, the one that's part of import("./box").Box that comes just before the syntax error.
• getSourceFileOfNode(node) evaluates to the b.d.ts which is different from currentSourceFile, which is a.ts
• We use node.pos and node.end to figure out which comment range to get the comment from. But these indexes are relative to b.d.ts, not a.ts! So we attempt to look for a comment in a.ts. Most of the time no comment would be found at these positions in a.ts, but by sheer luck, there happens to be a comment in a.ts starting right at node.end!

Imagine our surprise when my teammate's compilation started failing after touching a completely different part of a different file. They were just unlucky enough to have shifted a token into the exact right spot to surface the bug.

[fatcerberus]

This sounds similar to #58698 which was supposed to have been fixed by #58706