Mapped types not instantiated when defined in JSDoc

[ilogico]

TypeScript Version: 3.9.2, 3.8.3, next

Search Terms: mapped types jsdoc

Code

/**
 * @template T
 * @param {T} a
 * @param {{[K in keyof T]: (value: T[K]) => void }} b
 */
function f(a, b) {
}

f({ x: 42 }, { x(param) { } });

Expected behavior:
I expected param to be of type number, like it is when the function is defined in TS.
Actual behavior:
param is of type { x: number }[K], even though K has no meaning/can't be bound in this context.

Playground Link: playground

Related Issues:

This occurs when defining the function in JS through JSDoc, even when used in TS.
When defined in TS, it works in JS as well.

[sandersn]

Very likely some part of instantiation checks for a node that has a JSDoc equivalent, but doesn't use the getEffective* family of functions.

[sandersn]

Note that a separate @typedef also gives param: number, so defining the mapped type in jsdoc works sometimes. It may have something to do with @template in the same jsdoc as @param:

/**
 * @template T
 * @typedef {{[K in keyof T]: (value: T[K]) => void }} KT
 */
/**
 * @template T
 * @param {T} a
 * @param {KT<T>} b
 */
function f(a, b) {
}

f({ x: 42 }, { x(param) { } });

[sandersn]

The special-case code from https://github.com/microsoft/TypeScript/pull/24204/files isn't correct; it always jumps to parameter declaration when searching for type parameters in a @param's type, even when this skips type parameters like K.

The fix is to getOuterTypeParameters work correctly with @param, I think, instead of pretending that they are really at the location of the parameter. Failing that, getOuterTypeParameter at least needs code to jump from @param to the parameter in the same way that the existing code does over-eagerly.