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noImplicitReturns does not take account of a never return #18841

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noImplicitReturns does not take account of a never return#18841
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Description

@daveholladay
Dave Holladay (daveholladay)
opened on Sep 29, 2017

TypeScript Version: 2.5.2

Code

function foo(): never {
    throw new Error()
}

function bar(): number {
    if (Math.random() > 0.5)
        return 1;

    foo();
    // compiler should realise we can't get here.
}

Expected behavior:
Compiles without warnings when compiled with --noImplicitReturns

Actual behavior:
error TS7030: Not all code paths return a value.

Activity

  1. Daniel Rosenwasser (DanielRosenwasser)
    added
    In DiscussionNot yet reached consensus
    SuggestionAn idea for TypeScript
    on Oct 1, 2017

  2. DanielRosenwasser commented on Oct 1, 2017

    @DanielRosenwasser
    Daniel Rosenwasser (DanielRosenwasser)
    on Oct 1, 2017
    Member

    If we did this, every single expression statement with a function call would need to tie into the control flow graph which would likely be prohibitively expensive, but we should discuss it anyway.

  3. weswigham commented on Oct 1, 2017

    @weswigham
    Wesley Wigham (weswigham)
    on Oct 1, 2017
    Member

    Ron Buckton (@rbuckton) started looking into expanding flow control analysis to allow it to operate for throw expressions in #18798, so we're already looking into it.

  4. rbuckton commented on Oct 2, 2017

    @rbuckton
    Ron Buckton (rbuckton)
    on Oct 2, 2017
    Contributor

    Brian Terlson (@bterlson) mentioned a similar issue, wanting to be able to use process.exit(), but I don't think never is the correct type for this case.

  5. mhegazy commented on Oct 4, 2017

    @mhegazy
    Mohamed Hegazy (mhegazy)
    on Oct 4, 2017
    Contributor

    This is a duplicate of #10470. The main issue really here is how these two features are implemented. implicit return checks happen earlier in the binder. where as control flow analysis happens later on when we are checking. merging the two is not a trivial task.

  6. Mohamed Hegazy (mhegazy)
    added
    DuplicateAn existing issue was already created
    and removed
    In DiscussionNot yet reached consensus
    SuggestionAn idea for TypeScript
    on Oct 4, 2017

  7. mhegazy commented on Oct 19, 2017

    @mhegazy
    Mohamed Hegazy (mhegazy)
    on Oct 19, 2017
    Contributor

    Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.

  8. Mohamed Hegazy (mhegazy)
    closed this as completedon Oct 19, 2017

  9. ethanresnick commented on Feb 12, 2018

    @ethanresnick
    Ethan Resnick (ethanresnick)
    on Feb 12, 2018
    Contributor

    Is there a remaining open issue to track this? If not, should there be? I realize it's hard to implement, but maybe it's worth keeping open as a request? (#10470 has also been closed.)

  10. RyanCavanaugh commented on Feb 12, 2018

    @RyanCavanaugh
    Ryan Cavanaugh (RyanCavanaugh)
    on Feb 12, 2018
    Member

    Ethan Resnick (@ethanresnick) We'd prefer that open issues be actionable rather than serve as an infinite-scroll TODO list. Issues don't need to be open for us to be able to see activity on them

  11. microsoft
    locked and limited conversation to collaborators on Jul 3, 2018
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