generic function parameter infers boolean as true

<!-- Please try to reproduce the issue with `typescript@next`. It may have already been fixed. --> **TypeScript Version:** 3.3.0-dev.201xxxxx <!-- Search terms you tried before logging this (so others can find this issue more easily) --> **Search Terms:** true boolean generic function extends default **Code** ```ts const fn = <T extends boolean>(flag: T | false = false) => flag fn({} as boolean) // The only workaround? fn<boolean>({} as boolean) ``` **Expected behavior:** Expected `fn<boolean>(flag?: boolean)` to be inferred **Actual behavior:** Instead got `fn<true>(flag?: boolean)` as the inferred type. The issue stems from `T | false`, which is required for `= false` to work. **Playground Link:** [Click here](http://www.typescriptlang.org/play/#src=const%20fn%20%3D%20%3CT%20extends%20boolean%3E(flag%3A%20T%20%7C%20false%20%3D%20false)%20%3D%3E%20flag%0D%0Afn(%7B%7D%20as%20boolean)%20%2F%2F%20call%20inferred%20as%20fn%3Ctrue%3E()%20when%20it%20should%20be%20fn%3Cboolean%3E()%0D%0A) **Related Issues:** Possibly https://github.com/Microsoft/TypeScript/issues/28154