Broken inference for generic classes passed to a generic function taking an optional class

[LukeAbby]

🔎 Search Terms

class inference, optional class, broken inference

🕗 Version & Regression Information

This changed in PR #31116

However, this was the first time that there was even a distinction between a generic and non-generic class during inference to my knowledge. This likely means this has existed ever since the ability to infer a generic class has existed.

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/C4TwDgpgBAggdiAwgezgZ2AJwK4GNjKZQC8UcEA7lABQB09AhpgOZoBcUDCA2gLoCUJAHxRkAIwBWEfAG4AUHIBm2OPgCWqKACUIcACYRMAMQA2DVgFk1ADzVwAPACEGaCIjNo0UCNeC69XvBIqBg4+IRQAD5QKgaKdhB6JDH6EPHkekLUclC5UM6uAPwcBW4eaAA0coIA3gC+8nK45VAocMAMCZj2ACoi9Qo6qcZmljZ21G0dXfxyAPRzeUvLeQB6sCzYALa6wKKKUKCQUADkRxDIB1Od5JgnUGpecMh7LmhqzHAMYibQBFBgJgMHZ+IiXQ7gaAnWJpBJ6E60IA

💻 Code

type AnyConstructor = new (...args: any[]) => object;

function RenderFlagsMixin<BaseClass extends AnyConstructor | undefined = undefined>(
    Base?: BaseClass,
) {};

class Container<T> {}
//             ^ Only happens when the class is generic.

RenderFlagsMixin(Container)
//               ^ Argument of type 'typeof Container' is not assignable to parameter of type 'undefined'.

🙁 Actual behavior

BaseClass is inferred as undefined despite the call being the value Container.

🙂 Expected behavior

BaseClass should be inferred as typeof Container.

Additional information about the issue

No response

[jcalz]

Oof, in TS5.5+, the following code crashes the compiler:

function RenderFlagsMixin<T extends new (...args: any[]) => object>(Base?: T) { };
class Container<T> { t?: T }
RenderFlagsMixin(Container)

Maybe this isn't 100% related to this bug report but just noting that I ran into this when trying to investigate.

[LukeAbby]

More information on that crash:

• It appears to require a minimum of "strictNullChecks": true in the tsconfig.
• tsc when run alone will crash but tsc index.ts will not.
• The commit with the regression is e1874f3.
• It doesn't appear the t?: T is strictly necessary.
• The traceback on 5.8.3 is:

TypeError: Cannot read properties of undefined (reading '0')
    at getObjectTypeInstantiation (.data/TypeScript/lib/_tsc.js:62220:175)
    at instantiateTypeWorker (.data/TypeScript/lib/_tsc.js:62479:16)
    at instantiateTypeWithAlias (.data/TypeScript/lib/_tsc.js:62459:20)
    at instantiateType (.data/TypeScript/lib/_tsc.js:62437:29)
    at instantiateList (.data/TypeScript/lib/_tsc.js:62061:24)
    at instantiateTypes (.data/TypeScript/lib/_tsc.js:62075:12)
    at chooseOverload (.data/TypeScript/lib/_tsc.js:75485:39)
    at resolveCall (.data/TypeScript/lib/_tsc.js:75253:16)
    at resolveCallExpression (.data/TypeScript/lib/_tsc.js:75690:12)
    at resolveSignature (.data/TypeScript/lib/_tsc.js:76118:16)

[MartinJohns]

• tsc when run alone will crash but tsc index.ts will not.

When running tsc index.ts it won't use your local tsconfig, so strictNullChecks has the default value (false). Without this flag the type undefined doesn't exist anymore.

[Andarist]

I put up a PR o fix this here: #61635
when it comes to the crash, that is already addressed by already open #59972