Assingment type inference is broken with map, forEach

[pakettiale]

🔎 Search Terms

"union narrowing", "map", "forEach"

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about type inference and narrowing

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=3.3.3#code/DYUwLgBAhgXBB2BXAtgIxAJwgHwY4wEAvHgQNwBQFUlFA2gIwA0EATCwMwC6AdAGYB7DAFEoAYwAWACnjEAfBADeFCKujEIDSgF8AlFRpVBWKWIHwAzpAAeEAXwiMm7brqUq1UDVoraDlIA

💻 Code

let a: number | null = null;

a;

[1, 2, 3].forEach(n => {
    a = 1;
})

a;

for (const x of [1,2,3]) {
    a = 1;
}

a;

🙁 Actual behavior

Variable a on line 9 is inferred as null even though it is assigned a number value in forEach

🙂 Expected behavior

Expect it to be inferred as number | null or preferably number.

On line 15 it is demonstrated that after for (const x of xs) { a = 1; } the type of a is inferred as number | null. I would expect map and forEach to follow the same logic.

Additional information about the issue

This can be worked around by defining a on line 1 like this

const a = null as null | number;

Now a is inferred as number | null in both cases

[pakettiale]

This is duplicate of #9998