Add a regression test for issue-54108

Author: JohnTitor

src/test/ui/associated-types/issue-54108.rs

@@ -0,0 +1,41 @@
+use std::ops::Add;
+
+pub trait Encoder {
+    type Size: Add<Output = Self::Size>;
+
+    fn foo(&self) -> Self::Size;
+}
+
+pub trait SubEncoder: Encoder {
+    type ActualSize;
+
+    fn bar(&self) -> Self::Size;
+}
+
+impl<T> Encoder for T
+where
+    T: SubEncoder,
+{
+    type Size = <Self as SubEncoder>::ActualSize;
+    //~^ ERROR: cannot add `<T as SubEncoder>::ActualSize` to `<T as SubEncoder>::ActualSize`
+
+    fn foo(&self) -> Self::Size {
+        self.bar() + self.bar()
+    }
+}
+
+pub struct UnitEncoder;
+
+impl SubEncoder for UnitEncoder {
+    type ActualSize = ();
+
+    fn bar(&self) {}
+}
+
+pub fn fun<R: Encoder>(encoder: &R) {
+    encoder.foo();
+}
+
+fn main() {
+    fun(&UnitEncoder {});
+}

src/test/ui/associated-types/issue-54108.stderr

@@ -0,0 +1,18 @@
+error[E0277]: cannot add `<T as SubEncoder>::ActualSize` to `<T as SubEncoder>::ActualSize`
+  --> $DIR/issue-54108.rs:19:5
+   |
+LL |     type Size: Add<Output = Self::Size>;
+   |                ------------------------ required by this bound in `Encoder::Size`
+...
+LL |     type Size = <Self as SubEncoder>::ActualSize;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ no implementation for `<T as SubEncoder>::ActualSize + <T as SubEncoder>::ActualSize`
+   |
+   = help: the trait `Add` is not implemented for `<T as SubEncoder>::ActualSize`
+help: consider further restricting the associated type
+   |
+LL |     T: SubEncoder, <T as SubEncoder>::ActualSize: Add
+   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.