Type merging for imported types only works if the type is imported from "node_modules/@types/" dir

Consider the following example.

In typings_module:
vars.d.ts:

interface UserProperties {
}

declare global {
    const variable: UserProperties;
}

index.d.ts:

export as namespace ModuleNamespace;
export * from "./vars"

Now, we want to specify properties for the UserProperties interface.
a_project_file.d.ts:

declare module "typings_module" {
	export interface UserProperties {
		test: number;
	}
}

export {}

The bug is the type merging works only if the "typings_module" files are inside "node_modules/@types/" and does not work if the same files are referenced from another location via the "typeRoots" TSC option in "tsconfig.json".

TypeScript Version: 3.9.6

Search Terms: "type merging", "declaration files"

It's hard for us to guess about file layouts from prose; can you make a sample repo or zip file?

The layout is simple: a typings module with two files (vars.d.ts and index.d.ts, see their content above). The thing is if I symlink a dir with those files in node_modules/@types, type merging works, but when the typings are plugged in via the "typeRoots" project options, the merging breaks. Except for the merging, the typings work fine in both configurations.

It's hard for us to guess about file layouts from prose; can you make a sample repo or zip file?

Here is the real example where I discovered the problem: https://github.com/DefinitelyTyped/DefinitelyTyped/pull/46236/files#diff-411587a1c5c471aa01bb2b97932f74f1R62

I'll try to create an MWE.

typescript-issue-39691-mwe.zip

Here is the MWE, please. Check out the client/index.ts file. To make it work, I do ln -s ../../../types/typings-module in the client/node_modules/@types/ dir.

The issue here is that when the typings-module is included through custom type root the user source file extending interface (extension.d.ts in the example) cannot resolve "typings-module" from that source file, so it does not merge those two things together.
The reason being module resolution does not take into account the custom type roots only node_modules/@types which is what results in this. That issue is tracked by #37958 and is breaking change and i have been investigating this but it cannot be taken for 4.2.
Keeping this alive but marking for 4.3

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Type merging for imported types only works if the type is imported from "node_modules/@types/" dir #39691

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Type merging for imported types only works if the type is imported from "node_modules/@types/" dir #39691

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RyanCavanaugh commented on Jul 21, 2020

ezsh commented on Jul 21, 2020

RyanCavanaugh commented on Jul 21, 2020

ezsh commented on Jul 21, 2020

ezsh commented on Jul 21, 2020

sheetalkamat commented on Jan 8, 2021

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