Add test for rust-lang#100246.

Author: luqmana

src/test/ui/typeck/issue-100246.rs

@@ -0,0 +1,30 @@
+#![recursion_limit = "5"] // To reduce noise
+
+//expect incompatible type error when ambiguous traits are in scope
+//and not an overflow error on the span in the main function.
+
+struct Ratio<T>(T);
+
+pub trait Pow {
+    fn pow(self) -> Self;
+}
+
+impl<'a, T> Pow for &'a Ratio<T>
+where
+    &'a T: Pow,
+{
+    fn pow(self) -> Self {
+        self
+    }
+}
+
+fn downcast<'a, W: ?Sized>() -> std::io::Result<&'a W> {
+    todo!()
+}
+
+struct Other;
+
+fn main() -> std::io::Result<()> {
+    let other: Other = downcast()?;//~ERROR 28:24: 28:35: `?` operator has incompatible types
+    Ok(())
+}

src/test/ui/typeck/issue-100246.stderr

@@ -0,0 +1,13 @@
+error[E0308]: `?` operator has incompatible types
+  --> $DIR/issue-100246.rs:28:24
+   |
+LL |     let other: Other = downcast()?;
+   |                        ^^^^^^^^^^^ expected struct `Other`, found reference
+   |
+   = note: `?` operator cannot convert from `&_` to `Other`
+   = note: expected struct `Other`
+           found reference `&_`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.