Unused impl generics only fail when the impl is used

[michaeljklein]

Aim

Attempted to compile the following program, where T and let N: u32 are unused in the impl for Foo:

trait Foo {
    fn foo(self) -> Self;
}

impl<T, let N: u32> Foo for () {
    fn foo(self) -> Self {
        ()
    }
}

fn main() {
    // only errors with this line
    println(().foo());
}

Expected Behavior

Expected the program to fail when the impl that cannot be instantiated was defined (i.e. without println(().foo());)

Bug

The program only fails when the impl is used:

error: Type annotation needed
    ┌─ src/main.nr:273:13
    │
273 │     println(().foo());
    │             ------ Could not determine type of generic argument
    │
    = Call stack:
      1. src/main.nr:273:13

To Reproduce

Workaround

None

Workaround Description

No response

Additional Context

No response

Project Impact

None

Blocker Context

No response

Nargo Version

nargo version = 0.39.0 noirc version = 0.39.0+68c32b4ffd9b069fe4b119327dbf4018c17ab9d4 (git version hash: 68c32b4, is dirty: false)

NoirJS Version

No response

Proving Backend Tooling & Version

No response

Would you like to submit a PR for this Issue?

None

Support Needs

No response