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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 (11 comments) (1 reaction) (0 assignees)TypeScript (48,455 stars) (6,726 forks)batch import
BugDomain: check: Control FlowHelp Wanted

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

Tech stack
typescript
Domain
developer experience
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
half day
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 basicsstrictNullCheckscontrol flow analysis
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
25
Research direction
Investigate how the non null assertion (!) on property access is handled in control flow narrowing. Compare the narrowing behavior when using the non null assertion versus a simple truthy check (&&). The fix likely involves the checker's handling of NonNullExpression in narrowing of property types. Examine files such as checker.ts for the type narrowing logic, specifically how property access after a non null assertion is treated.