[BUG] cannot implicitly convert 'K' value to 'K' in assignment in condicional conformance

[msaelices]

Bug description

I think the Mojo compiler is not able to infer the correct types when there is a trait inheritance, and we use conditional conformance with two versions of the same method, like the __init__() below.

This is making a potential keyed-string x2 speed-up to fail: #3436

Steps to reproduce

Test code for reproducing the issue:

trait SizedHashable(Hashable, Sized):
    pass

@value
struct HashedInt(Hashable):
    var x: Int

    fn __hash__(self) -> UInt:
        return self.x

@value
struct DummyInt(SizedHashable):  # dummy example with minimum code

    fn __hash__(self) -> UInt:
        return 10 * len(self)

    fn __len__(self) -> Int:
        return 2

fn sized_hash[T: SizedHashable](x: T) -> Int:
    return hash(x) * len(x)

struct HashedKey[K: Hashable]:
    var key: K
    var hash: Int

    fn __init__[K: Hashable](inout self, key: K):
        self.key = key
        self.hash = hash(key)

    fn __init__[K: SizedHashable](inout self, key: K):
        self.key = key
        self.hash = sized_hash(key)

fn main() raises:
    var x = DummyInt()
    print(HashedKey(x).hash)

This is the error when compiling:

❯ mojo parametrized_traits.mojo 
/home/msaelices/src/mojo-examples/parametrized_traits.mojo:37:20: error: no matching function in initialization
    print(HashedKey(x).hash)
          ~~~~~~~~~^~~
/home/msaelices/src/mojo-examples/parametrized_traits.mojo:27:8: note: candidate not viable: could not deduce parameter 'K' of parent struct 'HashedKey'
    fn __init__[K: Hashable](inout self, key: K):
       ^
/home/msaelices/src/mojo-examples/parametrized_traits.mojo:23:8: note:  struct declared here
struct HashedKey[K: Hashable]:
       ^
/home/msaelices/src/mojo-examples/parametrized_traits.mojo:37:20: note: failed to infer parameter #0, parameter isn't used in any argument
    print(HashedKey(x).hash)
          ~~~~~~~~~^~~
/home/msaelices/src/mojo-examples/parametrized_traits.mojo:31:8: note: candidate not viable: could not deduce parameter 'K' of parent struct 'HashedKey'
    fn __init__[K: SizedHashable](inout self, key: K):
       ^
/home/msaelices/src/mojo-examples/parametrized_traits.mojo:23:8: note:  struct declared here
struct HashedKey[K: Hashable]:
       ^
/home/msaelices/src/mojo-examples/parametrized_traits.mojo:37:20: note: failed to infer parameter #0, parameter isn't used in any argument
    print(HashedKey(x).hash)
          ~~~~~~~~~^~~
/home/msaelices/src/mojo-examples/parametrized_traits.mojo:28:20: error: cannot implicitly convert 'K' value to 'K' in assignment
        self.key = key
                   ^~~
/home/msaelices/src/mojo-examples/parametrized_traits.mojo:32:20: error: cannot implicitly convert 'K' value to 'K' in assignment
        self.key = key
                   ^~~
mojo: error: failed to parse the provided Mojo source module

It's funny to receive a cannot implicitly convert 'K' value to 'K' in assignment error :)

Thanks to @helehex , I know that it can be fixed by explicitly type the self object, like so:

    fn __init__[K: SizedHashable](inout self: HashedKey[K], key: K):
        self.key = key
        self.hash = sized_hash(key)

To me, this should not be needed and be inferred by the compiler.

System information

❯ cat /etc/lsb-release 
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=22.04
DISTRIB_CODENAME=jammy
DISTRIB_DESCRIPTION="Ubuntu 22.04.4 LTS"

❯ mojo -v
mojo 2024.8.3005 (12110d9c)

❯ modular -v
modular 0.8.0 (39a426b5)

[Mogball]

trait SizedHashable(Sized, Hashable, Copyable):
    pass


@value
struct DummyInt(SizedHashable):  # dummy example with minimum code
    fn __hash__(self) -> UInt:
        return 10 * len(self)

    fn __len__(self) -> Int:
        return 2


fn sized_hash[T: SizedHashable](x: T) -> Int:
    return hash(x) * len(x)


struct HashedKey[K: Hashable]:
    var key: K
    var hash: Int

    fn __init__(inout self, key: K):
        self.key = key
        self.hash = hash(key)

    fn __init__[U: SizedHashable](inout self: HashedKey[U], key: U):
        self.key = key
        self.hash = sized_hash(key)


fn main() raises:
    var x = DummyInt()
    print(HashedKey(x).hash)

this works