[red-knot] dedicated error message for all-union-elements not callable (

#13412)

This was mentioned in an earlier review, and seemed easy enough to just
do it. No need to repeat all the types twice when it gives no additional
information.

crates/red_knot_python_semantic/src/types.rs

@@ -817,18 +817,26 @@ impl<'db> CallOutcome<'db> {
                         node,
                         "call-non-callable",
                         format_args!(
-                            "Union element '{}' of type '{}' is not callable.",
+                            "Object of type '{}' is not callable (due to union element '{}').",
+                            called_ty.display(db),
                             elem.display(db),
+                        ),
+                    ),
+                    _ if not_callable.len() == outcomes.len() => builder.add_diagnostic(
+                        node,
+                        "call-non-callable",
+                        format_args!(
+                            "Object of type '{}' is not callable.",
                             called_ty.display(db)
                         ),
                     ),
                     _ => builder.add_diagnostic(
                         node,
                         "call-non-callable",
                         format_args!(
-                            "Union elements {} of type '{}' are not callable.",
+                            "Object of type '{}' is not callable (due to union elements {}).",
+                            called_ty.display(db),
                             not_callable.display(db),
-                            called_ty.display(db)
                         ),
                     ),
                 }

crates/red_knot_python_semantic/src/types/infer.rs

@@ -3486,7 +3486,7 @@ mod tests {
         assert_file_diagnostics(
             &db,
             "src/a.py",
-            &["Union element 'Literal[1]' of type 'Literal[1] | Literal[f]' is not callable."],
+            &["Object of type 'Literal[1] | Literal[f]' is not callable (due to union element 'Literal[1]')."],
         );
         assert_public_ty(&db, "src/a.py", "x", "Unknown | int");
 
@@ -3515,14 +3515,39 @@ mod tests {
             &db,
             "src/a.py",
             &[
-                r#"Union elements Literal[1], Literal["foo"] of type 'Literal[1] | Literal["foo"] | Literal[f]' are not callable."#,
+                r#"Object of type 'Literal[1] | Literal["foo"] | Literal[f]' is not callable (due to union elements Literal[1], Literal["foo"])."#,
             ],
         );
         assert_public_ty(&db, "src/a.py", "x", "Unknown | int");
 
         Ok(())
     }
 
+    #[test]
+    fn call_union_with_all_not_callable() -> anyhow::Result<()> {
+        let mut db = setup_db();
+
+        db.write_dedented(
+            "src/a.py",
+            "
+            if flag:
+                f = 1
+            else:
+                f = 'foo'
+            x = f()
+            ",
+        )?;
+
+        assert_file_diagnostics(
+            &db,
+            "src/a.py",
+            &[r#"Object of type 'Literal[1] | Literal["foo"]' is not callable."#],
+        );
+        assert_public_ty(&db, "src/a.py", "x", "Unknown");
+
+        Ok(())
+    }
+
     #[test]
     fn invalid_callable() {
         let mut db = setup_db();