Fix oneOf and add tests

Author: michael-georgiadis

src/Directives/BuiltIn/OneOf.php

@@ -16,7 +16,8 @@
 /**
  * Binds `#[OneOf]` to webonyx's built-in `@oneOf` directive. Putting it on an `#[Input]` class sets
  * the input object's `isOneOf` flag, which makes validation require one field and gets webonyx to
- * print `@oneOf` in the SDL.
+ * print `@oneOf` in the SDL. The flag also tells {@see MutableInputObjectType::getFields()} to drop
+ * field default values, which OneOf inputs must not have (they break variable coercion otherwise).
  *
  * webonyx already defines `@oneOf`, so we don't register our own definition for it
  * ({@see DirectiveDefinition::$builtIn} is `true`).

src/Types/MutableInputObjectType.php

@@ -105,6 +105,13 @@ public function getFields(): array
             if (empty($this->finalFields)) {
                 throw NoFieldsException::create($this->name);
             }
+
+            if ($this->isOneOf) {
+                foreach ($this->finalFields as $field) {
+                    unset($field->config['defaultValue']);
+                    $field->defaultValue = null;
+                }
+            }
         }
 
         return $this->finalFields;

tests/Integration/DirectiveLayerEndToEndTest.php

@@ -109,6 +109,26 @@ public function testOneOfAcceptsExactlyOneField(): void
         $this->assertSame(['data' => ['lookup' => 'abc']], $result);
     }
 
+    public function testOneOfResolvesTheOtherBranch(): void
+    {
+        $result = $this->execute('query { lookup(lookup: {id: 1}) }');
+
+        $this->assertSame(['data' => ['lookup' => '1']], $result);
+    }
+
+    public function testOneOfAcceptsInputPassedThroughAVariable(): void
+    {
+        $result = GraphQL::executeQuery(
+            $this->schema,
+            'query ($lookup: LookupInput!) { lookup(lookup: $lookup) }',
+            null,
+            new Context(),
+            ['lookup' => ['sku' => 'abc']],
+        )->toArray(DebugFlag::INCLUDE_DEBUG_MESSAGE);
+
+        $this->assertSame(['data' => ['lookup' => 'abc']], $result);
+    }
+
     public function testOneOfRejectsMoreThanOneField(): void
     {
         $result = $this->execute('query { lookup(lookup: {sku: "abc", id: 1}) }');
@@ -130,4 +150,19 @@ public function testOneOfRejectsZeroFields(): void
             $result['errors'][0]['message'],
         );
     }
+
+    /**
+     * Passing a single field explicitly set to null is distinct from passing no fields: the one field
+     * is present but its value is null, which a OneOf input still forbids.
+     */
+    public function testOneOfRejectsAnExplicitNullField(): void
+    {
+        $result = $this->execute('query { lookup(lookup: {sku: null}) }');
+
+        $this->assertArrayNotHasKey('data', $result);
+        $this->assertStringContainsString(
+            "OneOf input object 'LookupInput' field 'sku' must be non-null",
+            $result['errors'][0]['message'],
+        );
+    }
 }