initial impl

Author: rawls238

graphql/core/validation/rules.py

@@ -1,6 +1,7 @@
 from ..utils import type_from_ast, is_valid_literal_value
 from ..error import GraphQLError
-from ..type.definition import is_composite_type, is_input_type, is_leaf_type, GraphQLNonNull
+from ..type.definition import is_composite_type, is_input_type, is_leaf_type, GraphQLNonNull, GraphQLObjectType,
+  GraphQLInterfaceType, GraphQLUnionType
 from ..language import ast
 from ..language.visitor import Visitor, visit
 from ..language.printer import print_ast
@@ -234,7 +235,38 @@ def unused_fragment_message(fragment_name):
 
 
 class PossibleFragmentSpreads(ValidationRule):
-    pass
+    def enter_InlineFragment(self, node, *args):
+        frag_type = self.context.get_type()
+        parent_type = self.context.get_parent_type()
+        if frag_type and parent_type and not self.do_types_overlap(frag_type, parent_type):
+            return GraphQLError(
+                self.type_incompatible_anon_spread_message(parent_type, frag_type),
+                [node]
+            )
+
+    def enter_FragmentSpread(self, node, *args):
+        frag_name = node.name.value
+        frag_type = self.get_fragment_type(context, frag_name)
+        parent_type = self.context.get_parent_type()
+        if frag_type and parent_type and not self.do_types_overlap(frag_type, parent_type):
+            return GraphQLError(
+                self.type_incompatible_spread_message(frag_name, parent_type, frag_type),
+                [node]
+            )
+
+    @staticmethod
+    def get_fragment_type(context, name):
+        frag = context.get_fragment(name)
+        return frag and type_from_ast(context.get_schema(), frag.type_condition)
+
+
+    @staticmethod
+    def type_incompatible_spread_message(frag_name, parent_type, frag_type):
+        return 'Fragment {} cannot be spread here as objects of type {} can never be of type {}'.format(frag_name, parent_type, frag_type)
+
+    @staticmethod
+    def type_incompatible_anon_spread_message(parent_type, frag_type):
+        return 'Fragment cannot be spread here as objects of type {} can never be of type {}'.format(parent_type, frag_type)
 
 
 class NoFragmentCycles(ValidationRule):