remove OneOf-specific rule in favor of update to VariablesInAllowedPositions

for simplicity, this PR retains the same problems for variables with defaults that are fixed by strict All Variable Usages Are Allowed

Author: yaacovCR

spec/Section 5 -- Validation.md

@@ -1452,103 +1452,6 @@ arguments, an input object may have required fields. An input field is required
 if it has a non-null type and does not have a default value. Otherwise, the
 input object field is optional.
 
-### OneOf Input Objects Have Exactly One Field
-
-**Formal Specification**
-
-- For each {operation} in {document}:
-  - Let {oneofInputObjects} be all OneOf Input Objects transitively included in
-    the {operation}.
-  - For each {oneofInputObject} in {oneofInputObjects}:
-    - Let {fields} be the fields provided by {oneofInputObject}.
-    - {fields} must contain exactly one entry.
-    - Let {field} be the sole entry in {fields}.
-    - Let {value} be the value of {field}.
-    - {value} must not be the {null} literal.
-    - If {value} is a variable:
-      - Let {variableName} be the name of {variable}.
-      - Let {variableDefinition} be the {VariableDefinition} named
-        {variableName} defined within {operation}.
-      - Let {variableType} be the expected type of {variableDefinition}.
-      - {variableType} must be a non-null type.
-
-**Explanatory Text**
-
-OneOf Input Objects require that exactly one field must be supplied and that
-field must not be {null}.
-
-An empty OneOf Input Object is invalid.
-
-```graphql counter-example
-mutation addPet {
-  addPet(pet: {}) {
-    name
-  }
-}
-```
-
-Multiple fields are not allowed.
-
-```graphql counter-example
-mutation addPet($cat: CatInput, $dog: DogInput) {
-  addPet(pet: { cat: $cat, dog: $dog }) {
-    name
-  }
-}
-```
-
-```graphql counter-example
-mutation addPet($dog: DogInput) {
-  addPet(pet: { cat: { name: "Brontie" }, dog: $dog }) {
-    name
-  }
-}
-```
-
-```graphql counter-example
-mutation addPet {
-  addPet(pet: { cat: { name: "Brontie" }, dog: null }) {
-    name
-  }
-}
-```
-
-Variables used for OneOf Input Object fields must be non-nullable.
-
-```graphql example
-mutation addPet($cat: CatInput!) {
-  addPet(pet: { cat: $cat }) {
-    name
-  }
-}
-```
-
-```graphql counter-example
-mutation addPet($cat: CatInput) {
-  addPet(pet: { cat: $cat }) {
-    name
-  }
-}
-```
-
-If a field with a literal value is present then the value must not be {null}.
-
-```graphql example
-mutation addPet {
-  addPet(pet: { cat: { name: "Brontie" } }) {
-    name
-  }
-}
-```
-
-```graphql counter-example
-mutation addPet {
-  addPet(pet: { cat: null }) {
-    name
-  }
-}
-```
-
 ## Directives
 
 ### Directives Are Defined
@@ -1989,8 +1892,8 @@ IsVariableUsageAllowed(variableDefinition, variableUsage):
 - Let {variableType} be the expected type of {variableDefinition}.
 - Let {locationType} be the expected type of the {Argument}, {ObjectField}, or
   {ListValue} entry where {variableUsage} is located.
-- If {locationType} is a non-null type AND {variableType} is NOT a non-null
-  type:
+- If {IsNonNullPosition(locationType, variableUsage)} AND {variableType} is NOT
+  a non-null type:
   - Let {hasNonNullVariableDefaultValue} be {true} if a default value exists for
     {variableDefinition} and is not the value {null}.
   - Let {hasLocationDefaultValue} be {true} if a default value exists for the
@@ -2001,6 +1904,15 @@ IsVariableUsageAllowed(variableDefinition, variableUsage):
   - Return {AreTypesCompatible(variableType, nullableLocationType)}.
 - Return {AreTypesCompatible(variableType, locationType)}.
 
+IsNonNullPosition(locationType, variableUsage):
+
+- Let {isOneOfField} be {true} if {variableUsage} is located within an
+  {ObjectField} entry and the parent type of {ObjectField} is a OneOf Input
+  Object; otherwise {false}.
+- If {isOneOfField} is {true} or {locationType} is a non-null type, return
+  {true}.
+- Return {false}.
+
 AreTypesCompatible(variableType, locationType):
 
 - If {locationType} is a non-null type:
@@ -2089,6 +2001,34 @@ query listToNonNullList($booleanList: [Boolean]) {
 This would fail validation because a `[T]` cannot be passed to a `[T]!`.
 Similarly a `[T]` cannot be passed to a `[T!]`.
 
+Variables used for OneOf Input Object fields must be non-nullable.
+
+```graphql example
+mutation addPet($cat: CatInput!) {
+  addPet(pet: { cat: $cat }) {
+    name
+  }
+}
+```
+
+```graphql counter-example
+mutation addPet($cat: CatInput) {
+  addPet(pet: { cat: $cat }) {
+    name
+  }
+}
+```
+
+Variables used for OneOf Input Object fields cannot have default values.
+
+```graphql counter-example
+mutation addPet($cat: CatInput = { name: "Kitty" }) {
+  addPet(pet: { cat: $cat }) {
+    name
+  }
+}
+```
+
 **Allowing Optional Variables When Default Values Exist**
 
 A notable exception to typical variable type compatibility is allowing a