remove ambiguity for foo.try_reduce(...)?

Author: MusicalNinjaDad

examples/signatures.rs

@@ -1,14 +1,18 @@
 #![feature(try_trait_v2)]
+#![feature(try_trait_v2_residual)]
+#![feature(iterator_try_reduce)]
 #![allow(dead_code)]
 #![allow(clippy::disallowed_names)]
 
-use std::ops::{ControlFlow, FromResidual, Try};
+use std::ops::{ControlFlow, FromResidual, Residual, Try};
 
 trait Foo: Iterator {
     fn try_reduce2<R, U>(&mut self, f: impl FnMut(Self::Item, Self::Item) -> R) -> U
     where
         Self: Sized,
         R: Try<Output = Self::Item>,
+        // U is the *canonical* TryType from R::Residual, without this foo.try_reduce(...)? is ambiguous
+        R::Residual: Residual<Option<Self::Item>, TryType = U>,
         U: Try<Output = Option<Self::Item>> + FromResidual<R::Residual>,
     {
         let first = match self.next() {
@@ -23,4 +27,17 @@ trait Foo: Iterator {
     }
 }
 
+impl<I: Iterator> Foo for I {}
+
+fn immediate_qmark() -> Result<Option<i32>, i32> {
+    let nums: [i32; 3] = [1, 2, 3];
+    let sum_positive = |acc, n| match n {
+        ..0 => Err(n),
+        _ => Ok(acc + n),
+    };
+    let _sum = nums.into_iter().try_reduce(sum_positive)?;
+    let sum2 = nums.into_iter().try_reduce2(sum_positive)?;
+    Ok(sum2)
+}
+
 fn main() {}