String union types in props inside generic types

TypeScript Version: 3.9.2 & 4.0.0-beta

Code

type A = {
    foo: number
}

type B<T extends string> = {
    [key in T]?: number
}

type C<T extends string> = A & B<T>

const f = <T extends string>(prop: T, value: number): C<T> => {
    const c: C<T> = {
        foo: 6
    }

    c[prop] = value

    return c
}

type Props = 'bar' | 'baz'

const o = f<Props>('bar', 6)

Expected behavior:
It should work :)

Actual behavior:
line 16: Type 'number' is not assignable to type 'C[T]'.

Playground Link: click

首先，使用列出的 TypeScript 版本运行提供的 Playground 重现，并确认第 16 行的诊断信息。跟踪编译器中的泛型索引赋值，然后添加覆盖测试，使该示例能够在没有所报告错误的情况下编译；issue 中未指定源文件或测试。