ICE - rust-lang/rust#79768

ices/79768.rs

@@ -0,0 +1,40 @@
+#![feature(generic_associated_types)]
+
+trait Monad /* : Applicative (for pure/return, doesn't matter for this example) */ {
+    // Self is like the "f a" in haskell
+
+    /// extract the "a" from "f a"
+    type Unplug;
+
+    /// exchange the "a" in "f a" in the type of Self with B
+    type Plug<B>: Monad;
+
+    fn bind<B, F>(self, f: F) -> Self::Plug<B>
+    where
+        F: Fn(Self::Unplug) -> Self::Plug<B>;
+}
+
+impl<A> Monad for Option<A> {
+    type Unplug = A;
+    type Plug<B> = Option<B>;
+    fn bind<B, F>(self, f: F) -> Option<B>
+    where
+        F: Fn(A) -> Option<B>,
+    {
+        self.and_then(f)
+    }
+}
+
+// This function causes the compiler error, specifically, when i added the Plug = P constraint.
+fn stringify<T, M1, P>(m: M1) -> M1::Plug<P>
+where
+    T: core::fmt::Display,
+    M1: Monad<Unplug = T, Plug = P>,
+{
+    m.bind(|x| format!("{}", x))
+}
+
+fn main() {
+    let x = Some(1).bind(|x| Some(x * 2));
+    println!("{:?}", x);
+}