Add regression test for #86351

Signed-off-by: Yuki Okushi <jtitor@2k36.org>

tests/ui/const_prop/issue-86351.rs

@@ -0,0 +1,22 @@
+// compile-flags: --crate-type=lib -Zmir-opt-level=2
+// build-pass
+// ^-- Must be build-pass, because check-pass will not run const prop.
+
+pub trait TestTrait {
+    type MyType;
+    fn func() -> Option<Self>
+    where
+        Self: Sized;
+}
+
+impl<T> dyn TestTrait<MyType = T>
+where
+    Self: Sized,
+{
+    pub fn other_func() -> Option<Self> {
+        match Self::func() {
+            Some(me) => Some(me),
+            None => None,
+        }
+    }
+}