No Int defaulting in closure capture

[jfecher]

The code

n = 0
f () =
    g x = print x
    g n

fails with:

b.an:4:5        error: 15 matching impls found for Print a
    g n

/.../ante/stdlib/prelude.an:356:1        note: Candidate 1
impl Print u8  with printne x = print_unsigned (cast x)

/.../ante/stdlib/prelude.an:357:1        note: Candidate 2
impl Print u16 with printne x = print_unsigned (cast x)

/.../ante/stdlib/prelude.an:358:1        note: Candidate 3 (12 more hidden)
impl Print u32 with printne x = print_unsigned (cast x)

The Print a constraint arising from n: a given Int a to be solved at the scope of main - after the Print a constraint is tried so there is no defaulting to i32 yet. There are a few options here:

• Default to i32 earlier (when?)
• Delay the solving of Print a, this is the most attractive / seemingly correct option. Looking at the desugared form of the program without closures it is clear the closure environment can be generalized within the lambda:

n = 0
closure = n, fn env -> 
    g x = print x
    g env

env, f = closure
f env

• Some midground, ad-hoc solution. E.g. When an error involves a defaulting type variable, keep some metadata to default it then and there to see if the error goes away. Any later uses of the variable would appear as i32.
• Prompt the user for 'type annotations needed by this point' or similar. This could also be confusing, given f is in the process of typechecking we would likely issue this error for f, but it is n that needs a concrete type.