No error accessing non-existent method on string with strictNullChecks on

[steadicat]

TypeScript Version: 3.0.1

Search Terms: strictNullChecks, inferred any, no error, parameter, argument, implicit any, property does not exist

Code

function getString(required: {}): string {
  return '';
}

function functionThatReturnsAString() {
  const optional: {} | null = null;
  return getString(optional);
}

const s = functionThatReturnsAString(); // s is correctly inferred as 'string'
s.foo(); // no error?

Expected behavior:

Expected to get error Property 'foo' does not exist on type 'string' on the last line.

Actual behavior:

No error on the last line.

With all the strict settings on, there's a different error on the argument to the getString call, but that should not affect the return type of functionThatReturnsAString. The function could be in a completely separate part of the codebase. Inspecting that should not be required when trying to debug the s.foo() call.

Playground Link: https://www.typescriptlang.org/play/#src=function%20getString(required%3A%20%7B%7D)%20%7B%0D%0A%20%20return%20''%3B%0D%0A%7D%0D%0A%0D%0Afunction%20functionThatReturnsAString()%20%7B%0D%0A%20%20const%20optional%3A%20%7B%7D%20%7C%20null%20%3D%20null%3B%0D%0A%20%20return%20getString(optional)%3B%0D%0A%7D%0D%0A%0D%0Aconst%20s%20%3D%20functionThatReturnsAString()%3B%0D%0As.foo()%3B%0D%0A

Tested with all strict options on. (Note that Playground has been finnicky today.)

Related Issues:

[ghost]

I believe this is because of this line in resolveCall in checker.ts:
return produceDiagnostics || !args ? resolveErrorCall(node) : getCandidateForOverloadFailure(node, candidates, args, !!candidatesOutArray);

If produceDiagnostics is true, we just return anySignature (from resolveErrorCall) and don't get downstream errors. But if not, we get the best candidate we can (which happens to be the only one). Maybe we could just always getCandiateForOverloadFailure?

[sheetalkamat]

Fixed by @Andy-MS in #26646

[ghost]

That PR didn't fix this, the line I pointed out still exists.

I would leave this issue open since there's still the possibility for confusing differences between services and the compiler.

[sheetalkamat]

But I see error reported by running tsc as well as from language service so its fixed?

[ghost]

We get s: string, but there's no compile error when calling a method that doesn't exist on string -- because the diagnostics-producing TypeChecker has different behavior than the one used for services.

[sheetalkamat]

[ghost]

@sheetalkamat You probably don't have strictNullChecks enabled since getString(optional) should be an error.

[ghost]

Duplicate of #19686 .