add regression test for #91489

Author: SNCPlay42

src/test/ui/issues/issue-91489.rs

@@ -0,0 +1,40 @@
+// check-pass
+
+// regression test for #91489
+
+use std::borrow::Borrow;
+use std::borrow::Cow;
+
+pub struct VariantType {}
+pub struct VariantTy {}
+
+impl Borrow<VariantTy> for VariantType {
+    fn borrow(&self) -> &VariantTy {
+        unimplemented!()
+    }
+}
+
+impl ToOwned for VariantTy {
+    type Owned = VariantType;
+    fn to_owned(&self) -> VariantType {
+        unimplemented!()
+    }
+}
+
+impl VariantTy {
+    pub fn as_str(&self) -> () {}
+}
+
+// the presence of this was causing all attempts to call `as_str` on
+// `Cow<'_, VariantTy>, including in itself, to not find the method
+static _TYP: () = {
+    let _ = || {
+        // should be found
+        Cow::Borrowed(&VariantTy {}).as_str();
+    };
+};
+
+fn main() {
+    // should be found
+    Cow::Borrowed(&VariantTy {}).as_str()
+}