Ensure we validate for using nullable variables in oneOf input fields (#4363)

In the spec we explicitly disallow `nullable` variables to be passed to
the fields of a oneOf input object. This is present in v17 but seems
missing from v16.

Author: JoviDeCroock

src/validation/ValidationContext.ts

@@ -33,6 +33,7 @@ interface VariableUsage {
   readonly node: VariableNode;
   readonly type: Maybe<GraphQLInputType>;
   readonly defaultValue: Maybe<unknown>;
+  readonly parentType: Maybe<GraphQLInputType>;
 }
 
 /**
@@ -209,6 +210,7 @@ export class ValidationContext extends ASTValidationContext {
               node: variable,
               type: typeInfo.getInputType(),
               defaultValue: typeInfo.getDefaultValue(),
+              parentType: typeInfo.getParentInputType(),
             });
           },
         }),

src/validation/__tests__/VariablesInAllowedPositionRule-test.ts

@@ -358,3 +358,34 @@ describe('Validate: Variables are in allowed positions', () => {
     });
   });
 });
+
+describe('Validates OneOf Input Objects', () => {
+  it('Allows exactly one non-nullable variable', () => {
+    expectValid(`
+      query ($string: String!) {
+        complicatedArgs {
+          oneOfArgField(oneOfArg: { stringField: $string })
+        }
+      }
+    `);
+  });
+
+  it('Forbids one nullable variable', () => {
+    expectErrors(`
+      query ($string: String) {
+        complicatedArgs {
+          oneOfArgField(oneOfArg: { stringField: $string })
+        }
+      }
+    `).toDeepEqual([
+      {
+        message:
+          'Variable "$string" is of type "String" but must be non-nullable to be used for OneOf Input Object "OneOfInput".',
+        locations: [
+          { line: 2, column: 14 },
+          { line: 4, column: 50 },
+        ],
+      },
+    ]);
+  });
+});

src/validation/rules/VariablesInAllowedPositionRule.ts

@@ -8,7 +8,11 @@ import { Kind } from '../../language/kinds';
 import type { ASTVisitor } from '../../language/visitor';
 
 import type { GraphQLType } from '../../type/definition';
-import { isNonNullType } from '../../type/definition';
+import {
+  isInputObjectType,
+  isNonNullType,
+  isNullableType,
+} from '../../type/definition';
 import type { GraphQLSchema } from '../../type/schema';
 
 import { isTypeSubTypeOf } from '../../utilities/typeComparators';
@@ -36,7 +40,7 @@ export function VariablesInAllowedPositionRule(
       leave(operation) {
         const usages = context.getRecursiveVariableUsages(operation);
 
-        for (const { node, type, defaultValue } of usages) {
+        for (const { node, type, defaultValue, parentType } of usages) {
           const varName = node.name.value;
           const varDef = varDefMap[varName];
           if (varDef && type) {
@@ -66,6 +70,19 @@ export function VariablesInAllowedPositionRule(
                 ),
               );
             }
+
+            if (
+              isInputObjectType(parentType) &&
+              parentType.isOneOf &&
+              isNullableType(varType)
+            ) {
+              context.reportError(
+                new GraphQLError(
+                  `Variable "$${varName}" is of type "${varType}" but must be non-nullable to be used for OneOf Input Object "${parentType}".`,
+                  { nodes: [varDef, node] },
+                ),
+              );
+            }
           }
         }
       },