Debug failure at resolveExternalModule after allowArbitraryExtensions

[jakebailey]

As of 89e928e (#51435), the perf runner is Debug asserting: https://typescript.visualstudio.com/TypeScript/_build/results?buildId=142178&view=logs&j=fd490c07-0b22-5182-fac9-6d67fe1e939b

>> /home/setup/vsts-agent/_work/7/s/built/local/tsc.js:109755
>>       throw e;
>>       ^
>> 
>> Error: Debug Failure.
>>     at resolveExternalModule (/home/setup/vsts-agent/_work/7/s/built/local/tsc.js:44740:35)
>>     at resolveExternalModuleNameWorker (/home/setup/vsts-agent/_work/7/s/built/local/tsc.js:44702:61)
>>     at resolveExternalModuleName (/home/setup/vsts-agent/_work/7/s/built/local/tsc.js:44699:12)
>>     at checkImportDeclaration (/home/setup/vsts-agent/_work/7/s/built/local/tsc.js:76954:35)
>>     at checkSourceElementWorker (/home/setup/vsts-agent/_work/7/s/built/local/tsc.js:77425:16)
>>     at checkSourceElement (/home/setup/vsts-agent/_work/7/s/built/local/tsc.js:77244:7)
>>     at forEach (/home/setup/vsts-agent/_work/7/s/built/local/tsc.js:37:22)
>>     at checkSourceFileWorker (/home/setup/vsts-agent/_work/7/s/built/local/tsc.js:77608:7)
>>     at checkSourceFile (/home/setup/vsts-agent/_work/7/s/built/local/tsc.js:77575:5)
>>     at checkSourceFileWithEagerDiagnostics (/home/setup/vsts-agent/_work/7/s/built/local/tsc.js:77669:5

This seems to be carried through to PRs too (like https://typescript.visualstudio.com/TypeScript/_build/results?buildId=142192&view=logs&j=fd490c07-0b22-5182-fac9-6d67fe1e939b&t=fb1961db-d514-5215-fe81-a42cdc309f86 😢)

cc @weswigham

[jakebailey]

This checkDefined is what's failing:

else if (resolvedModule.resolvedUsingTsExtension && !shouldAllowImportingTsExtension(compilerOptions, currentSourceFile.fileName)) {
    const tsExtension = Debug.checkDefined(tryExtractTSExtension(moduleReference));
    error(errorNode, Diagnostics.An_import_path_can_only_end_with_a_0_extension_when_allowImportingTsExtensions_is_enabled, tsExtension);
}

[jakebailey]

And moduleReference is '@angular/core', yet it was resolved using a TS extension?

[jakebailey]

Minimal repro:

// @filename: tsconfig.json
{
    "compilerOptions": {
        "paths": {
            "@angular/*": ["./@angular/*"]
        }
    }
}

// @filename: @angular/core/package.json
{
    "name": "@angular/core",
    "typings": "index.d.ts"
}

// @filename: @angular/core/index.ts
export {};

// @filename: @angular/core/testing/test.ts
import "@angular/core";

[jakebailey]

The general pattern seems to be:

• loadNodeModuleFromDirectory finds index.d.ts, then calls loadNodeModuleFromDirectoryWorker with tests/cases/@angular/core/index.d.ts
• nodeLoadModuleByRelativeName is then called on tests/cases/@angular/core/index.d.ts, which then eventually calls loadModuleFromFileNoImplicitExtensions, which used to return undefined, but now returns a result that says "I found a match and it was resolved using a ts extension"
• Things fall apart because a non-file import was resolved using a ts extension.