Add regression test and comment

Author: ouz-a

Diff for: compiler/rustc_const_eval/src/interpret/operator.rs

@@ -328,7 +328,8 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                 self.binary_int_op(bin_op, l, left.layout, r, right.layout)
             }
             _ if left.layout.ty.is_any_ptr() => {
-                // The RHS type must be the same *or an integer type* (for `Offset`).
+                // The RHS type must be a `pointer` *or an integer type* (for `Offset`).
+                // (This is workaround for the issue #91636)
                 assert!(
                     right.layout.ty.is_any_ptr() || right.layout.ty.is_integral(),
                     "Unexpected types for BinOp: {:?} {:?} {:?}",

Diff for: src/test/ui/binop/binary-op-on-fn-ptr-eq.rs

@@ -0,0 +1,8 @@
+// Tests equality between supertype and subtype of a function
+// See the issue #91636
+fn foo(_a: &str) {}
+
+fn main() {
+    let x = foo as fn(&'static str);
+    let _ = x == foo;
+}