parametric arguments that are not used in fields are forgotten before they're needed

[dank-openai]

Describe the bug

pub struct SomeType<FIRST: u32, SECOND: u32> { data: uN[FIRST] }

pub fn dostuff<FIRST: u32, SECOND: u32, COMPUTED: u32 = {SECOND}>
    (a: SomeType<FIRST, SECOND>) -> SomeType<FIRST, SECOND> {
    a
}

#[test]
fn test_dostuff() {
    let a = SomeType<u32:4, u32:4> { data: u4:0 };
    let result = dostuff(a);
    assert_eq(result.data, u4:0);
}

error:

0113: pub fn dostuff<FIRST: u32, SECOND: u32, COMPUTED: u32 = {SECOND}>
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^----^ TypeInferenceError: Parametric expression `SECOND` referred to `SECOND` which is not present in the parametric environment;

To Reproduce
Copy above and build.

Expected behavior
I expect this to compile without trouble, and that COMPUTED is a parametric argument equal to SECOND (which I would then use later in the parametric section or body of the function).

Additional context
There appears to be too-early parametric type erasure. Any parametric types not used to define a field of a struct are erased before function parametric type inference occurs.

This problem appears to be caused by the fact that SECOND is not used to define a field in struct SomeType. This can be demonstrated two ways:

1. Change: COMPUTED: u32 = {FIRST}. This compiles (because FIRST is used by data). Now change data: uN[SECOND] and it fails to compile (because FIRST is no longer used by data.)
2. (don't do # 1 before doing:) Add another field to the struct, and define the field using SECOND, e.g. like unused: uN[SECOND]. Resolve the compile errors that result from this new field. Now it compiles.

[richmckeever]

TI2 will fix this. The function parametrics and struct parametrics will be orthogonal in this example, other than the value being propagated to the struct. Parametric struct support in TI2 is not checked in yet, but it may not be worth fixing this in TI1 unless you need it now.

[dank-openai]

Parametric struct support in TI2 is not checked in yet, but it may not be worth fixing this in TI1 unless you need it now.

I figured out a workaround that I think will be sufficient. Thanks.

When might TI2 be released?