Spreading constrained generic parameters allows for invalid/unsafe return types

[WadePeterson]

🔎 Search Terms

spread generic
spread generic unsafe
spread constrained generic invalid

🕗 Version & Regression Information

This is the behavior in every version I tried, and I reviewed the FAQ for entries about "generics" and "spread"

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/JYOwLgpgTgZghgYwgAgILIN4Chm+XALmQGcwpQBzHPAIyJAFcBbG6LAXyyxgZATGAB7EMkikAjAAoAJnDCE0ASiLpseZFAhgGUEWvV4AdMdnyANNQPI6yAOQxgANwi3kAejfIEgqJv4AbAE9kaCgfYiIAFUCABxRbUnIQCldgYmQQQTB8YmJgChA4Gn8UMEFRWPjGFmhbQ0tkTk5uXn4hETEwACYAHkiQgA9IEGl01AA+GTkFSOU0TAbNbV0Fq1xjQ1M4CzXrInsnF3dPaUEIdMzs0J8zawZs4gALQQZ-aUMNLR0QdNBHOH8wGkyEENAAVhB+A0mlggA

💻 Code

interface A {
    a: string
    b: number
}

function test1(data: A): A {
    return {
        ...data,
        b: 'five' // correctly errors: Type 'string' is not assignable to type 'number'.
    }
}

function test2<T extends A>(data: T): A {
    return {
        ...data,
        b: 'five' // does not error, but should. returns invalid object
    }
}

🙁 Actual behavior

test2 function compiles without error, but when called at runtime, will always return an invalid object, with property b of type string instead of number

🙂 Expected behavior

test2 function has a compiler error preventing spreading of the b property with an invalid value.

Additional information about the issue

I found a similar issue, #60623

However, this issue has a more complex case, with an example function returning a generic. In my example, test2 declares a static return type, and unambiguously returns an invalid value that does not satisfy that type.

[jcalz]

This is still #10727; TS doesn't have a native spread type operator. The closest thing it has is intersection, which is simple and often good enough... except when you overwrite properties with incompatible types.