[DSLX] IR conversion fails when calling static impl method on parametric struct inside a proc

[mikex-oss]

Describe the bug
In DSLX, calling a static method defined inside an impl block of a parametric struct from inside a parametric proc causes IR conversion (ir_converter_main / GetConversionRecords) to fail with:

Error: INTERNAL: IrConversionError: ... Failed to convert IR because type was missing for AST node: bits[N]

To Reproduce

struct Foo<N: u32> {
    val: bits[N],
}

impl Foo<N> {
    fn new(x: bits[N]) -> Self {
        Foo<N> { val: x }
    }
}

proc Bar<N: u32> {
    init {
        Foo<N>::new(bits[N]:0)
    }
    config() {
        ()
    }
    next(state: Foo<N>) {
        state
    }
}

proc main {
    init { () }
    config() {
        spawn Bar<u32:32>();
    }
    next(state: ()) {
        ()
    }
}

Command:
bazel run //xls/dslx/ir_convert:ir_converter_main -- /path/to/repro.x

Output:
Error: INTERNAL: IrConversionError: /path/to/repro.x:6:15-6:22 Failed to convert IR because type was missing for AST node: bits[N] (kind: type annotation)
=== Source Location Trace: ===
xls/dslx/ir_convert/function_converter.cc:113
xls/dslx/ir_convert/function_converter.cc:5482
xls/dslx/ir_convert/function_converter.cc:868
xls/dslx/ir_convert/function_converter.cc:3529
xls/dslx/ir_convert/ir_converter.cc:365
xls/dslx/ir_convert/ir_converter.cc:413
xls/dslx/ir_convert/ir_converter.cc:663
xls/dslx/ir_convert/ir_converter.cc:734
xls/dslx/ir_convert/ir_converter_main.cc:159

Expected behavior
ir_converter_main recognizes static functions in impl Foo<N> as scoped to a parametric struct, skips un-instantiated root conversion, and monomorphizes Foo<N>::new with the parametric environment from Bar<u32:32> (N=32).

Additional context
Possibly related: #2082 and #2043

[JayPatel095]

Still reproduces on current main (434714e). Looking into it.

[richmckeever]

Note that, while you should indeed be able to refer to N, it's not really idiomatic anymore to say Foo<N>::new(bits[N]:0) as opposed to just Foo<N>::new(0), which does work, i.e. the type is inferred from the formal param.

[JayPatel095]

IsMethodOnParametricStruct returns true if self exists, so static impl functions on parametric structs were treated as fully concrete despite having an empty ParametricEnv. Fix in four locations in #4831 by replacing with Function::IsFunctionOnParametricStruct.