Assertion signature on contextual 'this'-parameter doesn't narrow and has no error

TypeScript Version: 4.2.0-dev.20210101

Search Terms: asserts this

Expected behavior:

Consistent behavior. Either

• this.assert(v) narrows v
• this.assert has an error about missing type annotation, no narrowing, this.returnNever() should not mark the following code as unreachable

Actual behavior:

No narrowing from asserts, no error on call expression, never-returning function has control flow effect.

Code

declare function makeContextualThisParameter<T>(cb: (this: T) => any): void;

makeContextualThisParameter<{assert(v: any): asserts v}>(function() {
    const v = Boolean();
    this.assert(v); // expected error here
    v; // or narrowing `v: true` here
});

makeContextualThisParameter<{returnNever(): never}>(function() {
    this.returnNever();
    debugger; // this is marked as unreachable
});

{
  "compilerOptions": {
    "noImplicitAny": true,
    "strictNullChecks": true,
    "strictFunctionTypes": true,
    "strictPropertyInitialization": true,
    "strictBindCallApply": true,
    "noImplicitThis": true,
    "noImplicitReturns": true,
    "alwaysStrict": true,
    "esModuleInterop": true,
    "declaration": true,
    "experimentalDecorators": true,
    "emitDecoratorMetadata": true,
    "moduleResolution": 2,
    "target": "ES2017",
    "jsx": "React",
    "module": "ESNext"
  }
}

Playground Link: Provided

Reproduce the contextual this examples in the issue with TypeScript 4.2.0-dev.20210101 and the listed compiler options, using the provided Playground link as the starting point. Trace how assertion signatures and never-returning calls affect narrowing and reachability, then add regression coverage showing the expected diagnostic or narrowing behavior.