Throw an error when using a Union or Interface as an argument type

RELEASE.md

@@ -0,0 +1,21 @@
+Release type: patch
+
+This release adds a new exception called `InvalidFieldArgument` which is raised when a Union or Interface is used as an argument type.
+For example this will raise an exception:
+```python
+import strawberry
+
+@strawberry.type
+class Noun:
+    text: str
+
+@strawberry.type
+class Verb:
+    text: str
+
+Word = strawberry.union("Word", types=(Noun, Verb))
+
+@strawberry.field
+def add_word(word: Word) -> bool:
+	...
+```

strawberry/exceptions.py

@@ -169,3 +169,10 @@ def __init__(self):
         message = 'Request data is missing a "query" value'
 
         super().__init__(message)
+
+
+class InvalidFieldArgument(Exception):
+    def __init__(self, field_name: str, argument_name: str, argument_type: str):
+        message = f'Argument "{argument_name}" on field "{field_name}" cannot be of type\
+            "{argument_type}"'
+        super().__init__(message)

strawberry/field.py

@@ -18,8 +18,10 @@
 
 from strawberry.annotation import StrawberryAnnotation
 from strawberry.arguments import UNSET, StrawberryArgument
+from strawberry.exceptions import InvalidFieldArgument
 from strawberry.type import StrawberryType
 from strawberry.types.info import Info
+from strawberry.union import StrawberryUnion
 from strawberry.utils.mixins import GraphQLNameMixin
 
 from .permission import BasePermission
@@ -102,6 +104,23 @@ def __call__(self, resolver: _RESOLVER_TYPE) -> "StrawberryField":
         if not isinstance(resolver, StrawberryResolver):
             resolver = StrawberryResolver(resolver)
 
+        for argument in resolver.arguments:
+            if isinstance(argument.type_annotation.annotation, str):
+                continue
+            elif isinstance(argument.type, StrawberryUnion):
+                raise InvalidFieldArgument(
+                    self.python_name,
+                    argument.python_name,
+                    "Union",
+                )
+            elif getattr(argument.type, "_type_definition", False):
+                if argument.type._type_definition.is_interface:
+                    raise InvalidFieldArgument(
+                        self.python_name,
+                        argument.python_name,
+                        "Interface",
+                    )
+
         self.base_resolver = resolver
 
         return self

tests/fields/test_arguments.py

@@ -7,7 +7,7 @@
 
 import strawberry
 from strawberry.arguments import UNSET
-from strawberry.exceptions import MultipleStrawberryArgumentsError
+from strawberry.exceptions import InvalidFieldArgument, MultipleStrawberryArgumentsError
 from strawberry.type import StrawberryList, StrawberryOptional
 
 
@@ -420,3 +420,48 @@ def name(  # type: ignore
     assert argument.type == str
     assert argument.description == "This is a description"
     assert argument.type is str
+
+
+def test_union_as_an_argument_type():
+    with pytest.raises(InvalidFieldArgument):
+
+        @strawberry.type
+        class Noun:
+            text: str
+
+        @strawberry.type
+        class Verb:
+            text: str
+
+        Word = strawberry.union("Word", types=(Noun, Verb))
+
+        @strawberry.field
+        def add_word(word: Word) -> bool:
+            return True
+
+
+def test_interface_as_an_argument_type():
+    with pytest.raises(InvalidFieldArgument):
+
+        @strawberry.interface
+        class Adjective:
+            text: str
+
+        @strawberry.field
+        def add_adjective(adjective: Adjective) -> bool:
+            return True
+
+
+def test_resolver_with_invalid_field_argument_type():
+    with pytest.raises(InvalidFieldArgument):
+
+        @strawberry.interface
+        class Adjective:
+            text: str
+
+        def add_adjective_resolver(adjective: Adjective) -> bool:
+            return True
+
+        @strawberry.type
+        class Mutation:
+            add_adjective: bool = strawberry.field(resolver=add_adjective_resolver)