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diff --git a/src/test/ui/associated-types/issue-28871.rs b/src/test/ui/associated-types/issue-28871.rs
new file mode 100644
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+++ b/src/test/ui/associated-types/issue-28871.rs
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+// check-pass
+// Regression test for #28871. The problem is that rustc encountered
+// two ways to project, one from a where clause and one from the where
+// clauses on the trait definition. (In fact, in this case, the where
+// clauses originated from the trait definition as well.) The true
+// cause of the error is that the trait definition where clauses are
+// not being normalized, and hence the two sources are considered in
+// conflict, and not a duplicate. Hacky solution is to prefer where
+// clauses over the data found in the trait definition.
+
+trait T {
+    type T;
+}
+
+struct S;
+impl T for S {
+    type T = S;
+}
+
+trait T2 {
+    type T: Iterator<Item=<S as T>::T>;
+}
+
+fn main() { }