Address empty selection-set

Replicates graphql/graphql-js@4ef04286

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: Cito

src/graphql/validation/rules/scalar_leafs.py

@@ -39,3 +39,12 @@ def enter_field(self, node: FieldNode, *_args: Any) -> None:
                         node,
                     )
                 )
+            elif not selection_set.selections:
+                field_name = node.name.value
+                self.report_error(
+                    GraphQLError(
+                        f"Field '{field_name}' of type '{type_}'"
+                        " must have at least one field selected.",
+                        node,
+                    )
+                )

tests/validation/test_scalar_leafs.py

@@ -1,8 +1,17 @@
 from functools import partial
 
+from graphql.language import (
+    DocumentNode,
+    FieldNode,
+    NameNode,
+    OperationDefinitionNode,
+    OperationType,
+    SelectionSetNode,
+)
 from graphql.validation import ScalarLeafsRule
+from graphql.validation.validate import validate
 
-from .harness import assert_validation_errors
+from .harness import assert_validation_errors, test_schema
 
 assert_errors = partial(assert_validation_errors, ScalarLeafsRule)
 
@@ -137,3 +146,30 @@ def scalar_selection_not_allowed_with_directives_and_args():
                 },
             ],
         )
+
+    def object_type_having_only_one_selection():
+        # We can't leverage assert_errors since it doesn't support passing in the
+        # document node directly. We have to do this because this is technically
+        # an invalid document.
+        doc = DocumentNode(
+            definitions=[
+                OperationDefinitionNode(
+                    operation=OperationType.QUERY,
+                    selection_set=SelectionSetNode(
+                        selections=[
+                            FieldNode(
+                                name=NameNode(value="human"),
+                                selection_set=SelectionSetNode(selections=[]),
+                            ),
+                        ],
+                    ),
+                ),
+            ],
+        )
+        errors = validate(test_schema, doc, [ScalarLeafsRule])
+        assert errors == [
+            {
+                "message": "Field 'human' of type 'Human'"
+                " must have at least one field selected.",
+            },
+        ]