Microsoft/TypeScript

Noncrashing property access through non-null assertion operator should narrow that property to NonNullable

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#15,655 opened on May 8, 2017

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BugDomain: check: Control FlowHelp Wanted

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Description

TypeScript Version: 2.3 (Playground)

Code

Using strictNullChecks.

interface Foo {
    optional?: number;
}

interface Bar {
    foo?: Foo;
}

function test(bar: Bar) {
    if (bar.foo!.optional) {
        let num: number = bar.foo.optional;
    }

    if (bar.foo && bar.foo.optional) {
        let num: number = bar.foo.optional;
    }
}

Expected behavior: No errors or warnings.

Actual behavior:

  • num in first if block is number | undefined
  • bar.foo in first if block can be undefined

skarmavbild 2017-05-08 kl 12 58 06

Contributor guide

Research direction
Investigate how the TypeScript compiler handles type narrowing for property access via non null assertion. Look at the control flow graph and type inference for optional properties.
Tech stack
typescript
Domain
compilers
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-2 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
TypeScript type systemcontrol flow analysis
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
30