From a2f8020e72eff390379fbf60dd4fff592f3c1f3a Mon Sep 17 00:00:00 2001
From: "Felix S. Klock II" <pnkfelix@pnkfx.org>
Date: Thu, 13 Apr 2017 12:15:50 +0200
Subject: [PATCH] Add section pointing out how associated item ambiguity is
 handled.

---
 text/0000-trait-alias.md | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/text/0000-trait-alias.md b/text/0000-trait-alias.md
index e289dd2310e..58a244a21cb 100644
--- a/text/0000-trait-alias.md
+++ b/text/0000-trait-alias.md
@@ -179,6 +179,28 @@ fn bar4(x: Box<IntIterator + Sink + 'static>) { ... } // ok (*)
 The lines marked with `(*)` assume that [#24010](https://github.com/rust-lang/rust/issues/24010) is
 fixed.
 
+### Ambiguous constraints
+
+If there are multiple associated types with the same name in a trait alias,
+then it is a static error ("ambiguous associated type") to attempt to
+constrain that associated type via the trait alias. For example:
+
+```rust
+trait Foo { type Assoc; }
+trait Bar { type Assoc; } // same name!
+
+// This works:
+trait FooBar1 = Foo<Assoc = String> + Bar<Assoc = i32>;
+
+// This does not work:
+trait FooBar2 = Foo + Bar;
+fn badness<T: FooBar2<Assoc = String>>() { } // ERROR: ambiguous associated type
+
+// Here are ways to workaround the above error:
+fn better1<T: FooBar2 + Foo<Assoc = String>>() { } // (leaves Bar::Assoc unconstrained)
+fn better2<T: FooBar2 + Foo<Assoc = String> + Bar<Assoc = i32>>() { } // constrains both
+```
+
 # Teaching
 [teaching]: #teaching