Return type of a function gets incorrectly inferred when nesting a case in an if

[archaeron]

Return type of a function gets incorrectly inferred when nesting a case in an if.

I have reproduced this with typescript@next (3.5.0-dev.20190529)

Search Terms:
switch + return type
switch + noImplicitReturns

Code

with the compiler option noImplicitReturns:

export type Q = { type: "A" } | { type: "B" };

export function f1(q: Q | undefined): string {
  if (q) {
    switch (q.type) {
      case "A":
        return "A";
      case "B":
        return "B";
    }
  } else {
    return "nothing";
  }
}

export function f2(q: Q | undefined): string {
  if (q) {
    return f2_help(q);
  } else {
    return "nothing";
  }
}

function f2_help(q: Q): string {
  switch (q.type) {
    case "A":
      return "A";
    case "B":
      return "B";
  }
}

Expected behavior:

I'd expect both f1 and f2 to compile, since I can get f1 by inlining f2_help into f2.

Actual behavior:

But f1 gives me an error:

Error:

testcase.ts:3:39 - error TS7030: Not all code paths return a value.

3 export function f1(q: Q | undefined): string {
                                        ~~~~~~

Found 1 error.

Playground Link:

NOTE: this only fails with noImplicitReturns turned on in the option (the option does not get saved in the url)

Playground

Repository reproducing the issue

https://github.com/archaeron/typescript-testcase

Related Issues:

Might be related: #31495

[RyanCavanaugh]

Duplicate #11572

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.