Fix non-capturing closure return type coercion

compiler/rustc_typeck/src/check/coercion.rs

@@ -941,6 +941,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             exprs.len()
         );
 
+        if prev_ty == new_ty {
+            return Ok(prev_ty);
+        }
+
         // Special-case that coercion alone cannot handle:
         // Function items or non-capturing closures of differing IDs or InternalSubsts.
         let (a_sig, b_sig) = {