[BUG] Cannot materialize a capturing closure

[ehein6]

Bug description

I get errors when trying to use Optional to wrap a capturing closure in an argument or a parameter. See example code below.

• Optional capturing closure as parameter leads to compiler crash
• OptionalReg capturing closure leads to error in pass manager
• Optional capturing closure as argument leads to compile-time error message

The error is marked "TODO" but I didn't see an issue filed to fix this anywhere.

Blocking KERN-542. Tracing.mojo is using a workaround to avoid accepting a capturing closure as a parameter.

Steps to reproduce

Test program:

# Change `OptionalReg` to `Optional` for a compiler crash
struct StructWithFuncParam[func: OptionalReg[fn () capturing -> String] = None]:
    var s: String
 
    fn __init__(inout self):
        self.s = func.value()()
 
    fn print_me(inout self):
        print(self.s)
 
 
struct StructWithFuncArg:
    var s: String
 
    fn __init__(
        inout self, func: OptionalReg[fn () capturing -> String] = None
    ):
        self.s = func.value()()
 
    fn print_me(inout self):
        print(self.s)
 
 
fn main():
    var f = "foo"
 
    @parameter
    fn foo() -> String:
        return f
 
    # error: 'kgen.param.constant' op cannot be used to materialize capturing closures; use `kgen.create_closure` instead
    var x = StructWithFuncParam[foo]()
    x.print_me()
    # CHECK: "foo"
 
    # error: TODO: capturing closures cannot be materialized as runtime values
    # var z = StructWithFuncArg(foo)
    # z.print_me()

System information

mojo 0.0.0 (4bb704c4)
modular 0.0.0 (4bb704c4)
macOS Sonoma 14.5 (M3 Pro)