fix: "Types in a binary operation should match, but found T and T"

[jfecher]

Description

Problem*

Resolves #4635
Resolves #4502

Summary*

This was more difficult to fix than it originally seemed. The main issue was between interactions with unbound type variables, type aliases, type rules for operators, and operator traits.

Removing the "infer unbound type variables to be numeric" rule on operators causes a lot of stdlib code to break where it'd be unreasonable to have type annotations. This caused unbound type variables to be bound to the first object type whose impl it was checked against when calling verify trait impl.

I eventually settled on just delaying the verify trait impl check for operators until the end of a function when more types are known.

Additional Context

Documentation*

Check one:

• No documentation needed.
• Documentation included in this PR.
• [For Experimental Features] Documentation to be submitted in a separate PR.

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[nventuro]

I just found a simpler scenario that also triggers this bug, in case you want to test against it or add it as a regression:

struct Wrapper {
    inner: Field,
}

trait FromField {
    fn from_field(value: Field) -> Self;
}

trait ToField {
    fn to_field(self) -> Field;
}

impl ToField for Wrapper {
    fn to_field(self) -> Field {
        self.inner
    }
}

impl FromField for Wrapper {
    fn from_field(value: Field) -> Wrapper {
        Wrapper { inner: value }
    }
}

#[test]
fn test() {
    let wapper = Wrapper { inner: 37 };
    assert_eq(FromField::from_field(wapper.to_field()), wapper);
}

This errors out with Types in a binary operation should match, but found Wrapper and Wrapper

[jfecher]

@nventuro I think the original example is sufficient. I'll mention though that the new error message with this PR for your snippet is:

error: No matching impl found for `Wrapper: Eq`
   ┌─ /.../short/src/main.nr:28:5
   │
28 │     assert_eq(FromField::from_field(wapper.to_field()), wapper);
   │     ----------------------------------------------------------- No impl for `Wrapper: Eq`
   │

[nventuro]

Ah yes, I didn't bother adding that impl for the minimum reproducible example 😅