1 files changed, 91 insertions, 0 deletions
diff --git a/tests/ui/issue-11881.rs b/tests/ui/issue-11881.rs
new file mode 100644
index 000000000..f6360db9b
--- /dev/null
+++ b/tests/ui/issue-11881.rs
@@ -0,0 +1,91 @@
+// run-pass
+
+#![allow(unused_must_use)]
+#![allow(dead_code)]
+#![allow(unused_imports)]
+
+use std::fmt;
+use std::io::prelude::*;
+use std::io::Cursor;
+use std::slice;
+use std::marker::PhantomData;
+
+trait Encoder {
+    type Error;
+}
+
+trait Encodable<S: Encoder> {
+    fn encode(&self, s: &mut S) -> Result<(), S::Error>;
+}
+
+struct JsonEncoder<'a>(PhantomData<&'a mut ()>);
+
+impl Encoder for JsonEncoder<'_> {
+    type Error = ();
+}
+
+struct AsJson<'a, T> {
+    inner: &'a T,
+}
+
+impl<'a, T: for<'r> Encodable<JsonEncoder<'r>>> fmt::Display for AsJson<'a, T> {
+    /// Encodes a json value into a string
+    fn fmt(&self, _f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        Ok(())
+    }
+}
+
+fn as_json<T>(t: &T) -> AsJson<'_, T> {
+    AsJson { inner: t }
+}
+
+struct OpaqueEncoder(Vec<u8>);
+
+impl Encoder for OpaqueEncoder {
+    type Error = ();
+}
+
+
+struct Foo {
+    baz: bool,
+}
+
+impl<S: Encoder> Encodable<S> for Foo {
+    fn encode(&self, _s: &mut S) -> Result<(), S::Error> {
+        Ok(())
+    }
+}
+
+struct Bar {
+    froboz: usize,
+}
+
+impl<S: Encoder> Encodable<S> for Bar {
+    fn encode(&self, _s: &mut S) -> Result<(), S::Error> {
+        Ok(())
+    }
+}
+
+enum WireProtocol {
+    JSON,
+    Opaque,
+    // ...
+}
+
+fn encode_json<T: for<'a> Encodable<JsonEncoder<'a>>>(val: &T, wr: &mut Cursor<Vec<u8>>) {
+    write!(wr, "{}", as_json(val));
+}
+
+fn encode_opaque<T: Encodable<OpaqueEncoder>>(val: &T, wr: Vec<u8>) {
+    let mut encoder = OpaqueEncoder(wr);
+    val.encode(&mut encoder);
+}
+
+pub fn main() {
+    let target = Foo { baz: false };
+    let proto = WireProtocol::JSON;
+    match proto {
+        WireProtocol::JSON => encode_json(&target, &mut Cursor::new(Vec::new())),
+        WireProtocol::Opaque => encode_opaque(&target, Vec::new()),
+    }
+}