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+The associated type used was not defined in the trait.
+
+Erroneous code example:
+
+```compile_fail,E0220
+trait T1 {
+    type Bar;
+}
+
+type Foo = T1<F=i32>; // error: associated type `F` not found for `T1`
+
+// or:
+
+trait T2 {
+    type Bar;
+
+    // error: Baz is used but not declared
+    fn return_bool(&self, _: &Self::Bar, _: &Self::Baz) -> bool;
+}
+```
+
+Make sure that you have defined the associated type in the trait body.
+Also, verify that you used the right trait or you didn't misspell the
+associated type name. Example:
+
+```
+trait T1 {
+    type Bar;
+}
+
+type Foo = T1<Bar=i32>; // ok!
+
+// or:
+
+trait T2 {
+    type Bar;
+    type Baz; // we declare `Baz` in our trait.
+
+    // and now we can use it here:
+    fn return_bool(&self, _: &Self::Bar, _: &Self::Baz) -> bool;
+}
+```