add test for unsupported coercion

datafusion/optimizer/src/type_coercion.rs

@@ -239,6 +239,32 @@ mod test {
         Ok(())
     }
 
+    #[test]
+    fn scalar_udf_invalid_input() -> Result<()> {
+        let empty = empty();
+        let return_type: ReturnTypeFunction =
+            Arc::new(move |_| Ok(Arc::new(DataType::Utf8)));
+        let fun: ScalarFunctionImplementation = Arc::new(move |_| unimplemented!());
+        let udf = Expr::ScalarUDF {
+            fun: Arc::new(ScalarUDF::new(
+                "TestScalarUDF",
+                &Signature::uniform(1, vec![DataType::Int32], Volatility::Stable),
+                &return_type,
+                &fun,
+            )),
+            args: vec![lit("Apple")],
+        };
+        let plan = LogicalPlan::Projection(Projection::try_new(vec![udf], empty, None)?);
+        let rule = TypeCoercion::new();
+        let mut config = OptimizerConfig::default();
+        let plan = rule.optimize(&plan, &mut config).err().unwrap();
+        assert_eq!(
+            "Plan(\"Coercion from [Utf8] to the signature Uniform(1, [Int32]) failed.\")",
+            &format!("{:?}", plan)
+        );
+        Ok(())
+    }
+
     fn empty() -> Arc<LogicalPlan> {
         let empty = Arc::new(LogicalPlan::EmptyRelation(EmptyRelation {
             produce_one_row: false,