diagnostic: fix AssignTypeMismatch

Author: xuhuanzy

crates/emmylua_code_analysis/src/diagnostic/test/assign_type_mismatch_test.rs

@@ -42,6 +42,28 @@ mod tests {
         ));
     }
 
+    #[test]
+    fn test_3() {
+        let mut ws = VirtualWorkspace::new();
+        assert!(ws.check_code_for_namespace(
+            DiagnosticCode::AssignTypeMismatch,
+            r#"
+                ---@param s    string
+                ---@param i?   integer
+                ---@param j?   integer
+                ---@param lax? boolean
+                ---@return integer?
+                ---@return integer? errpos
+                ---@nodiscard
+                local function get_len(s, i, j, lax) end
+
+                local len = 0
+                ---@diagnostic disable-next-line: need-check-nil
+                len = len + get_len("", 1, 1, true)
+            "#
+        ));
+    }
+
     #[test]
     fn test_issue_193() {
         let mut ws = VirtualWorkspace::new();

crates/emmylua_code_analysis/src/semantic/infer/infer_binary.rs

@@ -162,6 +162,11 @@ fn infer_binary_expr_add(db: &DbIndex, left: LuaType, right: LuaType) -> InferRe
             }
         };
     }
+    match (left.is_nil(), right.is_nil()) {
+        (true, false) => return Some(right),
+        (false, true) => return Some(left),
+        _ => {}
+    }
 
     infer_binary_custom_operator(db, &left, &right, LuaOperatorMetaMethod::Add)
 }