Unexpected "used before its declaration" error when implementing a exported type of a merged namespace

Bug Report

🔎 Search Terms

used before its declaration vscode

🕗 Version & Regression Information

• tested in 4.7.3, 5.0.4 and nightly

⏯ Playground Link

Playground link with relevant code

💻 Code

namespace Foo {
    export interface Bar { }
}

class X implements Foo.Bar {

}

const Foo = 1;

🙁 Actual behavior

In VSCode it reports:

namespace Foo
Block-scoped variable 'Foo' used before its declaration.ts(2448)
Untitled-1(9, 7): 'Foo' is declared here.
Variable 'Foo' is used before being assigned.ts(2454)

You may not see the error immediately if you paste the code into a new file. It will appear when you start typing something.

Note that the error cannot be reproduced in the playground.

🙂 Expected behavior

No error.

Start with the provided TypeScript example and Playground link, then compare the editor diagnostic with the Playground behavior for the merged namespace and exported type. Trace the type-checking path that reports the "used before its declaration" and "used before being assigned" errors, and add a regression test showing that this code produces no error.