main.zig

const std = @import("std");
const c = @cImport({
    @cInclude("c-header.h");
});

const MyInputType = struct {
    value: u8
};

const MyResultType = struct {
    input: type,

    pub fn doSomethingWithInputType(comptime self: MyResultType) void {
        std.debug.warn("{}\n", @memberName(self.input, 0));
    }
};

pub fn main() void {
    const a = getResultType(MyInputType);
    const b = getResultType(struct { another_input_type: u8 });

    a.doSomethingWithInputType();
    b.doSomethingWithInputType();
}

fn getResultType(comptime input_type: type) MyResultType {
    // As soon as I add the c call, I get 'unable to evaluate constant expression'.
    // If you comment the line below this snippet should run properly.
    c.some_function();

    return MyResultType {
        .input = input_type
    };
}