A tuple containing a spread not at the trailing edge has its minimum length calculated incorrectly

Bug Report

🔎 Search Terms

• tuple
• spread

🕗 Version & Regression Information

It happens in v4.9.5, no idea about other versions.

⏯ Playground Link

https://www.typescriptlang.org/play?jsx=0#code/C4TwDgpgBAYgcgRigHgCoD4oF4oAoB0hAhgE4DmAzgFxQXAkCWAdmQNoC6AlNpqgNwAoAaEiw4AJhQZseKIXylKNVgGMA9kwoBXALYQAJjTqMWAGjmEyJNVrDVa9Zm3bnmYLcCOOzUZvogAHl4mZOxQ3Fi8ggLqmsBQAGZMCDTwCMhMugBGECSYOLhQROZZ4TxQAAzRsXSJTOKpEhnZufmyxVClEZhVQA

💻 Code

type FN1 <T> = (...args: string[]) => T;

type FN2 <T> = ( ...args: [consumed: string, ...groups: string[], input: string, index: string] ) => T;

const fn1: FN1<number> = ( a, b ) => 0;

const fn2: FN2<number> = ( a, b ) => 0;

🙁 Actual behavior

TS complains about my fn2, with the following error:

Type '(a: string, b: string) => number' is not assignable to type 'FN2<number>'.
  Types of parameters 'b' and 'groups' are incompatible.
    Type '[...groups: string[], input: string, index: string]' is not assignable to type '[b: string]'.
      Source has 2 element(s) but target allows only 1.(2322)

If I'm interpreting it correctly it is saying like "hey you defined two arguments, but maybe this tuple only has one item in it.

🙂 Expected behavior

If I'm interpreting the error correctly then the error is wrong because there's no scenario where that tuple has just one item in it, minimum it will always have 3.

在链接的 TypeScript Playground 中，使用 issue 中的 tuple 类型和 function 类型重现该报告。跟踪 compiler 对位于非末尾位置的 spread 进行 tuple 最小长度计算的过程。完成的标准是：示例不再报告源 tuple 可能只有一个元素，同时对于确实过短的 tuple，现有的诊断行为仍然正确。