[isolatedDeclarations] function expressions as an argument to a function call that is exported are reported as needing an annotation

[bradzacher]

🔎 Search Terms

"isolatedDeclarations", "function return type", "exported variable"

🕗 Version & Regression Information

• This changed between versions 5.6.3 and 5.7.3

⏯ Playground Link

https://www.typescriptlang.org/play/?isolatedDeclarations=true#code/JYWwDg9gTgLgBAKjgQwM5wEoFNkGN4BmUEIcA5FDvmQNwBQokscA3ivsBAHZwC+cREuRAQARgA9aDcNHhsxqLFABuSvgOKkyIiQFpKeGFLpZxTeLm6p4AYRKQuWLvAC8cBUtVQAFN4CUcC4AfKx0cHCUMACuUDwAPEFxAPRB9Lx+9CZmsnCWXNZwMAAWwFwA5oHsMJxc3gRRXBzccP6h4ZExPHYyjs5pGXRAA

💻 Code

import * as React from 'react';
import { action } from 'mobx';
import { observer } from 'mobx-react';

export const Component = observer(() => {
  return <></>;
});

export const thing = action(function () {
  return Component;
});

🙁 Actual behavior

import * as React from 'react';
import { action } from 'mobx';
import { observer } from 'mobx-react';

export const Component = observer(() => {
             ~~~~~~~~~ Variable must have an explicit type annotation with --isolatedDeclarations.
                                  ~~~~~ Function must have an explicit return type annotation with --isolatedDeclarations.
  return <></>;
});

export const thing = action(function () {
             ~~~~~ Variable must have an explicit type annotation with --isolatedDeclarations.
                            ~~~~~~~~ Function must have an explicit return type annotation with --isolatedDeclarations.
  return Component;
});

🙂 Expected behavior

import * as React from 'react';
import { action } from 'mobx';
import { observer } from 'mobx-react';

export const Component = observer(() => {
             ~~~~~~~~~ Variable must have an explicit type annotation with --isolatedDeclarations.
  return <></>;
});

export const thing = action(function () {
             ~~~~~ Variable must have an explicit type annotation with --isolatedDeclarations.
  return Component;
});

Additional information about the issue

The function expressions are not directly exported so they should not be reported on at all.
This is causing ts-fix to insert many unnecessary annotations when codemodding the codebase at Canva because there's no way for it to tell that annotating the variable will silence the error on the function -- so it adds both annotations.

[KisaragiEffective]

Here's self-contained example based on yours:

export declare function observer<T>(component: T): T;
export declare function action<T>(fn: T): T;

// ok
() => 1;
// also ok
observer(() => 1);

// error on both variable and function
export const named = observer(() => 1);
// also error on both variable and function
export const thing = action(() => named);
// error on variable, but not function
export const thing2 = action((): typeof named => named);

maybe changed in #59282 , according history from https://github.com/microsoft/TypeScript/blame/2778e84ed88894156e2c5a17d74a25c73cc96ffe/tests/baselines/reference/isolatedDeclarationErrors.errors.txt#L3