ThisType would disturb Generics' inferring type when assign an expression to a type variable

**TypeScript Version:** 3.9.2, and it does exist in 'Nightly' version, too. **Search Terms:** ThisType, Generics, Inferring problem **Code** ```ts type customFunc = (...args: any) => any declare function SimpleVue< D, C extends { [K in string]: customFunc }, M = { [K in string]: customFunc }, CC = { [K in keyof C]: ReturnType<C[K]> }, ReturnedD = D extends customFunc ? ReturnType<D> : never, >(option: { data: D, methods: M, computed: C } & ThisType<M & ReturnedD & CC>): ReturnedD const instance = SimpleVue({ data() { return { firstname: 'Type', lastname: 'Challenges', amount: 10, } }, computed: { fullname() { return this.firstname + ' ' + this.lastname } }, methods: { hi() { alert(this.fullname.toLowerCase()) } } }); ``` **Expected behavior:** We expect `ReturnedD = D extends customFunc ? ReturnType<D> : never` infers `ReturnedD` to `ReturnType<D>` at runtime since `D` fits the shape of `customFunc`: ``` () => { firstname: string; lastname: string; amount: number; } ``` And the code should show no error in TypeScript environment. **Actual behavior:** The actual behavior is that `ReturnedD` is inferred to `never` type since `D extends customFunc` here is false at runtime. You can also check it with the returned type of `instance` as well. **Playground Link:** Check out the link [here](https://www.staging-typescript.org/play?ts=3.9.2#code/C4TwDgpgBAxgrgZ2AewLYDE4DsZQLxQAUAdKQIYBOA5ggFxRlYgCU+AfA0wFBcAmEMADaVoAM2wxgAS2RYoAZSmowgiADU4EADxcoUACIAaXVADCUCAA9gELLwRQA3lADaAaShS5SCl6oBdengkNEwcKABfYz0AWXwnVw8vKB8-QNhEFAwJSOizcwJnd085AGsIEGRRM3SAJQhgOAosABVwbVN3fw4ok3rG5ohefXiRqxs7B2CssNwAfih+ptb2rX0OeiwIADcICmM2QmQwaVl6RxNeMmAyeiMTVAaAC2R7ehi8mDQwOBteelMXAiUAAZFAWk8pAg2pAtHEwUtBsNQfk2Mx6IitsMeF8sEgSkhGDBoARFMpVBoIIQLnorjdCKwaXooBQGssnCZmVBRFIKEgsGRHvQAOQwiDCvJc4T8wUQEWmJ5kQSqLBUCAICWc5mC5DYYD0ACMAAZJZETL09F9lL8hucteJlQLHgyOVy9KyBnJgJCEMQeXzgE7oABqKDCsNQUPeqHEaWB2VaiLmvKPb2vOiu5mQ…