Strange merging behavior of module augmentation and pattern ambient module

[ajafff]

TypeScript Version: 3.4.1

Search Terms:

Code

// @filename: types.ts
declare module '*.foo' {
    let foo: string;
}

// @filename: test.ts
import {foo as a1, bar as a2} from 'a.foo';
import {foo as b1, bar as b2} from 'b.foo'; // expected error: 'b.foo' has no exported member 'bar'

declare module 'a.foo' {
    let bar: number;
}

tsconfig.json is the default config from tsc --init

Expected behavior:

I want to augment a.foo with an exported member bar. Importing foo and bar from module a.foo succeeds while bar is not present in module b.foo

Actual behavior:

The module augmentation a.foo merges with the pattern ambient module *.foo. Every module matching the pattern now has an exported member bar.

To limit the effect to only a.foo I need to convert the module augmentation to an ambient module and redeclare all members of the pattern ambient module because there's merging of ambient modules and pattern ambient modules.

Note: This is not an actual issue in one of my projects, I just happened to notice this surprising behavior while playing around with module resolution.