Inconsistent Function Subtyping: `(input?: string) => void` Incorrectly Assignable to `() => void`

[sklationd]

🔎 Search Terms

"optional function inconsistency", "runtime error with function assign", "bivariant", "strictFunctionTypes'

🕗 Version & Regression Information

This is the behavior in every version I tried, and I reviewed the FAQ for entries about "Why are function parameters bivariant?", "Why are functions with fewer parameters assignable to functions that take more parameters?"

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/C4TwDgpgBA8gCgFQJIwHIEEAyB9AYgVVQGEoBeKACgEMAnAcwH4AuKAZ2BoEsA7OgSjIA+KADcA9pwAmAbgBQAGwjAoYsME5juVebgCu3AMYt4yNFjyES5CjzC7gzNhx78hsqB6gHNrMYoB08mJ0Ntx2Dv6s8pwGEBQADAA0UACMfHxysqCQUACiALKIAJoWxGSUAqTC4lJyisoQALZqIHqGLAXFpVYVQlAA3gC+mdnQqDAI2CYoGDgEZda29iwARmJ+EFTcldUSMgpKUNxiwDBqGlo6+kZQ45PTZnOW5aHhq+uKWztePhuBwa97BlZLImi02gZyqp1JptBDpFAAPSItgACzEunkkiOJygK2gYCorFYEEksmOp3OsKuhnKYNA8PJJzOMMuEIoHF0EAySJRNH06ka0AgNBoYhoKgMBl0NFJINkQA

💻 Code

let a = (input?: string) => input?.slice(0, 1);
let b = () => {};
let c = (input: number) => console.log(input);

b = a; // should not be okay
c = b; // okay
c(3);  // runtime error!
// c = a <- type error

🙁 Actual behavior

Assigning a function of type (input?: string) => void to a function of type () => void passes the type checker but causes a runtime error.

🙂 Expected behavior

Assigning (input?: string) => void to () => {} should trigger a type error because (input?: string) => void is not a subtype of () => {}; rather, the reverse is true. As a result, this should prevent a runtime error.

Additional information about the issue

Could this be a known issue, or is it an intentional design choice? If intentional, what is the rationale behind it?

[MartinJohns]

Duplicate of #13043.