Bug Report

I have a ready, very easy fix for this bug, so if you indeed recognize this as a bug, please assign it to me, and I'll submit a PR.

🔎 Search Terms

checkModuleDeclaration node type never
checker.ts
isExternalModuleAugmentation
Property 'parent' does not exist on type 'never' (ts2339)

🕗 Version & Regression Information

This is a logical bug in checker.ts that should arise in pretty much any TypeScript version that supports type guards/predicaments. However, since isAmbientModule()'s result is assigned to a variable, and control flow of aliased conditional expressions is introduced only in v4.4.0, this issue will only appear in v4.4.0+ (or any other version with pull request #44730)

Judging by related commits that I've found by using git blame -L37720,37722 src/compiler/checker.ts and
git log -L37720,37722:src/compiler/checker.ts, the bug was introduced somewhere around v2.8-rc:

• 4a963a2 – Dec 22, 2015, seems to be v1.7.5 or v1.8.0-beta – Added the isExternalModuleAugmentation() check (which will become redundant in the next commit)
• 5e593ac – Mar 6, 2018, seems to be v2.8-rc – Added a type predicate to isExternalModuleAugmentation(), which created a redundancy with parent isAmbientModule check.

⏯ Playground Link

Close recreation of the issue

💻 Code

checker.ts : line 37664

function checkModuleDeclaration(node: ModuleDeclaration) {
    // ...

    // Control flow analysis of aliased conditional expressions 
    // (e.g. type guard result saved in a variable) are introduced
    // only in v4.4.0 https://github.com/microsoft/TypeScript/pull/44730
    const isAmbientExternalModule = isAmbientModule(node);

   // ...

    if (isAmbientExternalModule) {
        // Before v4.4.0, node here is of type 'TNode'
        // After v4.4.0, node here is of type 'AmbientModuleDeclaration'
        if (isExternalModuleAugmentation(node)) {
            // isExternalModuleAugmentation() contains a redundant isAmbientModule() check
        }
        else if (isGlobalSourceFile(node.parent)) {
        //                          ^^^^^^^^^^^^
        // Here, tsc v4.5.0-dev.20210824 (and all others) give an error:
        // Property 'parent' does not exist on type 'never'.ts(2339)
        // Because of the redundant check in if (isExternalModuleAugmentation(node)) ...,
        // control flows deduces node's type to be 'never'.

        // ...
        } else {
            // ...
        }
    }
    // ...
}

🙁 Actual behavior

Covered in the code examples/playground.

🙂 Expected behavior

Covered in the code examples/playground.