Compiler bug: Using an undefined Int variable with Inspect.toStr

[dxrcy]

foo : Int I32

main =
    _ = Inspect.toStr foo
    Task.ok {}

An internal compiler expectation was broken.
[...]
ambient functions don't unify
Location: crates/compiler/unify/src/unify.rs:202:18

foo : Num *

main =
    _ = Inspect.toStr foo
    Task.ok {}

thread 'main' panicked at crates/compiler/mono/src/borrow.rs:396:34:
internal error: entered unreachable code:
        No borrow signature for LambdaName { name: `15.IdentId(5)`, niche: Niche(Captures([])) } layout.

        Tip 1: This can happen when you call a function with less arguments than it expects.
        Like `Arg.list!` instead of `Arg.list! {}`.
        Tip 2: `roc check yourfile.roc` can sometimes give you a helpful error.

roc check only warns about the _ = ... part.

Qualifying number kind X as Num X does not affect the result, and neither does converting X to List X.

Changing foo to a Str, Bool, Frac *, Dec * or List Str prevents the compiler bug, and it throws a runtime error NoImplementationNamed (which appears to be intended behaviour).

Similarly, using Num.toStr or another function also throws NoImplementationNamed.

This makes me think that it is specifically an issue with Num and Inspect.toStr....