Imported symbol as a type is not merged with a local variable

[Conaclos]

🔎 Search Terms

Declaration merging, isolatedModule, type import conflict

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about declaration merging

⏯ Playground Link

https://www.typescriptlang.org/play?isolatedModules=true&ts=5.4.0-beta#code/JYWwDg9gTgLgBDAnmApnA3nAwhcEB2K+8AvnAGZS5wDkUKAhgMYw0DccA9J3ACoDKcFFCpQAznAIIAFsAkB3WQBs0weHLihIsFABM4DCQwTIUAKCYEx8HHkLEAXNlyR78AD5x8AVyVK4ALxevkpsZmZa0PBIqBhwAGJYcGSU1HSMLOxcPAIGfhDyEjCyRhJqNEYmqBZW8IlOiXCePn6BwX5hQA

💻 Code

import type { Component } from 'react'; // TS errors on this while it is imported as a type
const Component: Component | null = null;

import type { FC } from 'react'; // TS allows this as it's a type
const FC: FC | null = null;

🙁 Actual behavior

TypeScript reports the following error:

Import declaration conflicts with local declaration of Component.

🙂 Expected behavior

Because Component is imported as a type, it should be merged with the variable Component without emitting an error.

Additional information about the issue

A related issue was recently fixed and discussed in a design meeting.

Fixing the current issue could make TypeScript behavior more consistent.

[whzx5byb]

This is intended, since import type {} will also create type information which can be used in typeof if possible. You can write:

// @filename: a.ts
export const Foo = 'Foo';

// @filename: b.ts
import type { Foo } from './a';
type Bar = typeof Foo
  // ^ type Bar = "Foo"

You are not allowed to define a variable Foo here, because it will change the result of typeof Foo.

Workaround: use explicit type definition with import()

// @filename: b.ts
type Foo = typeof import('./a').Foo;
const Foo = 1;
// merged as expected!

[Conaclos]

Thanks! That's so ugly ;(

[fatcerberus]

import type {} will also create type information which can be used in typeof if possible.

More specifically, it imports all meanings of the imported symbol, even value meanings, it just doesn't allow using them in emitting positions.