default method of T that calls generic fn<S:T>(&S) on self does not work

[pnkfelix]

If you try to implement a default method of trait T by calling out to a generic helper function with a type-parameter bounded by T, rustc claims that it cannot find an implementation of trait T for Self. (Of course the default method is itself within trait T, so any possible choice for Self should indeed implement T.)

Test case:

#[allow(default_methods)]
trait Speak {
    fn say(&self, s:&str);

    #[cfg(not(work_around_the_bug))]
    fn hi(&self) { hello(self); }

    // To see things "working", we can remove
    // the offending default method and
    // cut-and-paste its implementation
    // instead (see below).
    #[cfg(work_around_the_bug)] fn hi(&self);
}

fn hello<S:Speak>(s:&S) {
    s.say("hello");
}

impl Speak for int {
    fn say(&self, s:&str) { println(s); }

    #[cfg(work_around_the_bug)] fn hi(&self) { hello(self); }
}

fn main() {
    3.hi();
}

Transcript of rustc invocation:

% RUST_LOG=rustc=1,::rt::backtrace  x86_64-apple-darwin/stage2/bin/rustc ~/Dev/Rust/default-calls-generic.rs
/Users/pnkfelix/Dev/Rust/default-calls-generic.rs:6:19: 6:24 error: failed to find an implementation of trait Speak for Self
/Users/pnkfelix/Dev/Rust/default-calls-generic.rs:6     fn hi(&self) { hello(self); }
                                                                       ^~~~~
rust: task failed at 'explicit failure', /Users/pnkfelix/Dev/Mozilla/rust.git/src/libsyntax/diagnostic.rs:72
rust: task failed at 'explicit failure', /Users/pnkfelix/Dev/Mozilla/rust.git/src/librustc/rustc.rc:398
rust: domain main @0x7fd9bb015810 root task failed

Discovered while prototyping the visit.rs refactoring (#7081); probably blocks it.

[pnkfelix]

Part of #2794 as well.

[pnkfelix]

Note that this simpler test case should also work (when compiled via rustc --lib), but does not, and may help one avoid false strategies for resolving this:

#[allow(default_methods)]
trait Speak { fn say(&self, s:&str); fn hi(&self) { hello(self); } }
fn hello<S:Speak>(s:&S) { s.say("hello"); }

Its the same as the original code from the description, with the impl and main removed; it yields the same error, but since there are no impls, it won't run any of the loop over all the impls in the "lookup_vtable sty not ty_param" case.

[msullivan]

I have a fix for this that I will land soon. There will still be problems with self and supertraits, though.