5.6 regression: Inference with mixed in type intersection no longer compiles

[trevorade]

🔎 Search Terms

Inference

🕗 Version & Regression Information

• This changed between versions 5.5.4 and 5.6.0-beta (still happening in 5.6.1-rc)

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.6.1-rc#code/C4TwDgpgBAYgrgOwMbAJYHsEGF0FsyYQLAA8AClALxQCGCIAfFVABRgBO6YAzgFxRkAlFSYBRADYRcRYFAA+UBHHHiA3ACh1qYhHYAzGkmgSpMgILBg7VACM4wCNygBvdVCioAJngDSEEAD8-NxW2gDm8orK4pFKuDa6Gu5IABao4p7sREFQiADWCOgA7ghJUNx5qGA5NujoknQaAL6anhBI4jRZUHqIKBgIUBCS0sTkDCxuUMA0YfzwyGiYOPiEYxQAZFAmo8AWoXYO3AwANFM0ltaHjvyb2yPml7b2jmeC-DsyGuptHV3QvUWA3uADkaNI2JwePxnAhwRAciFrAgwk13vdTMRvsNMcAWBIwdITi40RoAPRkqAAEXQjkU6FkSDwYHS0G0UAArAA6ABsXKgAGUUuhlJ4oEz8KyueogA

💻 Code

type FunctionComponent<P = any> = (props: P) => Element | null;

interface ElementAttributes {
 idomKey?: string | null | number;
 children?: unknown;
 skip?: boolean;
}

declare function element<P>(
 tag: FunctionComponent<P & ElementAttributes>,
 attributes: P & ElementAttributes,
): Element;

declare function ElName(props: {name?: string}): Element;

element(ElName, {});
// Does not compile in 5.6. Should compile.

🙁 Actual behavior

Last line errors with:

Argument of type '(props: { name?: string | undefined; }) => Element' is not assignable to parameter of type 'FunctionComponent<ElementAttributes>'.
 Types of parameters 'props' and 'props' are incompatible.
 Type 'ElementAttributes' has no properties in common with type '{ name?: string | undefined; }'.(2345)

🙂 Expected behavior

Compiles as it did in previous versions

Additional information about the issue

Confirmed that #59709 does not resolve this issue.

(Google note: relevant source location http://shortn/_Ce1E5yV592)

[RyanCavanaugh]

#57909 strikes once more cc Mateusz Burzyński (@Andarist)

[Andarist]

This one is not beyond repair 😉 #59772

[RyanCavanaugh]

There are definitely going to be multiple correct possible inferences when all sites for a type parameter are the nested in the same intersection, and a "smaller" inference for P seems justifiable from the principle of being most likely to be sound in practice.

It seems like the "right" fix here is to change element to use a constraint instead:

declare function element<P extends ElementAttributes>(
 tag: FunctionComponent<P>,
 attributes: P,
): Element;

which works as expected.

[typescript-bot]

This issue has been marked as "Not a Defect" and has seen no recent activity. It has been automatically closed for house-keeping purposes.