diff --git a/src/Error/Error.php b/src/Error/Error.php
index 9491d24ac..e4ad9ac99 100644
--- a/src/Error/Error.php
+++ b/src/Error/Error.php
@@ -38,7 +38,11 @@ class Error extends Exception implements JsonSerializable, ClientAware
     const CATEGORY_GRAPHQL  = 'graphql';
     const CATEGORY_INTERNAL = 'internal';
 
-    /** @var SourceLocation[] */
+    /**
+     * Lazily initialized.
+     *
+     * @var SourceLocation[]
+     */
     private $locations;
 
     /**
@@ -203,10 +207,7 @@ public function getCategory()
         return $this->category;
     }
 
-    /**
-     * @return Source|null
-     */
-    public function getSource()
+    public function getSource() : ?Source
     {
         if ($this->source === null) {
             if (isset($this->nodes[0]) && $this->nodes[0]->loc !== null) {
@@ -258,9 +259,9 @@ static function ($p) : bool {
      *
      * @api
      */
-    public function getLocations()
+    public function getLocations() : array
     {
-        if ($this->locations === null) {
+        if (! isset($this->locations)) {
             $positions = $this->getPositions();
             $source    = $this->getSource();
             $nodes     = $this->nodes;
diff --git a/src/Type/Definition/EnumType.php b/src/Type/Definition/EnumType.php
index 93f6283d8..9451ec74a 100644
--- a/src/Type/Definition/EnumType.php
+++ b/src/Type/Definition/EnumType.php
@@ -24,11 +24,17 @@ class EnumType extends Type implements InputType, OutputType, LeafType, Nullable
     /** @var EnumTypeDefinitionNode|null */
     public $astNode;
 
-    /** @var EnumValueDefinition[] */
+    /**
+     * Lazily initialized.
+     *
+     * @var EnumValueDefinition[]
+     */
     private $values;
 
     /**
-     * Actually a MixedStore<mixed, EnumValueDefinition>, PHPStan won't let us type it that way
+     * Lazily initialized.
+     *
+     * Actually a MixedStore<mixed, EnumValueDefinition>, PHPStan won't let us type it that way.
      *
      * @var MixedStore
      */
@@ -88,9 +94,9 @@ private function getNameLookup() : ArrayObject
     /**
      * @return EnumValueDefinition[]
      */
-    public function getValues()
+    public function getValues() : array
     {
-        if ($this->values === null) {
+        if (! isset($this->values)) {
             $this->values = [];
             $config       = $this->config;
 
@@ -145,7 +151,7 @@ public function serialize($value)
      */
     private function getValueLookup() : MixedStore
     {
-        if ($this->valueLookup === null) {
+        if (! isset($this->valueLookup)) {
             $this->valueLookup = new MixedStore();
 
             foreach ($this->getValues() as $valueName => $value) {
diff --git a/src/Type/Definition/FieldDefinition.php b/src/Type/Definition/FieldDefinition.php
index 8a3e4bb59..c0fd318a6 100644
--- a/src/Type/Definition/FieldDefinition.php
+++ b/src/Type/Definition/FieldDefinition.php
@@ -84,7 +84,12 @@ protected function __construct(array $config)
         $this->complexityFn = $config['complexity'] ?? self::DEFAULT_COMPLEXITY_FN;
     }
 
-    public static function defineFieldMap(Type $type, $fields)
+    /**
+     * @param (callable():mixed[])|mixed[] $fields
+     *
+     * @return array<string, self>
+     */
+    public static function defineFieldMap(Type $type, $fields) : array
     {
         if (is_callable($fields)) {
             $fields = $fields();
diff --git a/src/Type/Definition/InputObjectType.php b/src/Type/Definition/InputObjectType.php
index 7eaa4d1f2..455e2efc8 100644
--- a/src/Type/Definition/InputObjectType.php
+++ b/src/Type/Definition/InputObjectType.php
@@ -4,7 +4,6 @@
 
 namespace GraphQL\Type\Definition;
 
-use Exception;
 use GraphQL\Error\InvariantViolation;
 use GraphQL\Language\AST\InputObjectTypeDefinitionNode;
 use GraphQL\Language\AST\InputObjectTypeExtensionNode;
@@ -21,7 +20,11 @@ class InputObjectType extends Type implements InputType, NullableType, NamedType
     /** @var InputObjectTypeDefinitionNode|null */
     public $astNode;
 
-    /** @var InputObjectField[] */
+    /**
+     * Lazily initialized.
+     *
+     * @var InputObjectField[]
+     */
     private $fields;
 
     /** @var InputObjectTypeExtensionNode[] */
@@ -46,16 +49,12 @@ public function __construct(array $config)
     }
 
     /**
-     * @param string $name
-     *
-     * @return InputObjectField
-     *
-     * @throws Exception
+     * @throws InvariantViolation
      */
-    public function getField($name)
+    public function getField(string $name) : InputObjectField
     {
-        if ($this->fields === null) {
-            $this->getFields();
+        if (! isset($this->fields)) {
+            $this->initializeFields();
         }
         Utils::invariant(isset($this->fields[$name]), "Field '%s' is not defined for type '%s'", $name, $this->name);
 
@@ -65,32 +64,36 @@ public function getField($name)
     /**
      * @return InputObjectField[]
      */
-    public function getFields()
+    public function getFields() : array
     {
-        if ($this->fields === null) {
-            $this->fields = [];
+        if (! isset($this->fields)) {
+            $this->initializeFields();
+        }
 
-            $fields = $this->config['fields'] ?? [];
-            if (is_callable($fields)) {
-                $fields = $fields();
-            }
+        return $this->fields;
+    }
 
-            if (! is_array($fields)) {
-                throw new InvariantViolation(
-                    sprintf('%s fields must be an array or a callable which returns such an array.', $this->name)
-                );
-            }
+    protected function initializeFields() : void
+    {
+        $this->fields = [];
+        $fields       = $this->config['fields'] ?? [];
+        if (is_callable($fields)) {
+            $fields = $fields();
+        }
 
-            foreach ($fields as $name => $field) {
-                if ($field instanceof Type) {
-                    $field = ['type' => $field];
-                }
-                $field                      = new InputObjectField($field + ['name' => $name]);
-                $this->fields[$field->name] = $field;
-            }
+        if (! is_array($fields)) {
+            throw new InvariantViolation(
+                sprintf('%s fields must be an array or a callable which returns such an array.', $this->name)
+            );
         }
 
-        return $this->fields;
+        foreach ($fields as $name => $field) {
+            if ($field instanceof Type) {
+                $field = ['type' => $field];
+            }
+            $field                      = new InputObjectField($field + ['name' => $name]);
+            $this->fields[$field->name] = $field;
+        }
     }
 
     /**
@@ -99,7 +102,7 @@ public function getFields()
      *
      * @throws InvariantViolation
      */
-    public function assertValid()
+    public function assertValid() : void
     {
         parent::assertValid();
 
diff --git a/src/Type/Definition/InterfaceType.php b/src/Type/Definition/InterfaceType.php
index c13d7fb05..e01987653 100644
--- a/src/Type/Definition/InterfaceType.php
+++ b/src/Type/Definition/InterfaceType.php
@@ -20,7 +20,11 @@ class InterfaceType extends Type implements AbstractType, OutputType, CompositeT
     /** @var InterfaceTypeExtensionNode[] */
     public $extensionASTNodes;
 
-    /** @var FieldDefinition[] */
+    /**
+     * Lazily initialized.
+     *
+     * @var FieldDefinition[]
+     */
     private $fields;
 
     /**
@@ -44,9 +48,11 @@ public function __construct(array $config)
     /**
      * @param mixed $type
      *
-     * @return self
+     * @return $this
+     *
+     * @throws InvariantViolation
      */
-    public static function assertInterfaceType($type)
+    public static function assertInterfaceType($type) : self
     {
         Utils::invariant(
             $type instanceof self,
@@ -56,30 +62,20 @@ public static function assertInterfaceType($type)
         return $type;
     }
 
-    /**
-     * @param string $name
-     *
-     * @return FieldDefinition
-     */
-    public function getField($name)
+    public function getField(string $name) : FieldDefinition
     {
-        if ($this->fields === null) {
-            $this->getFields();
+        if (! isset($this->fields)) {
+            $this->initializeFields();
         }
         Utils::invariant(isset($this->fields[$name]), 'Field "%s" is not defined for type "%s"', $name, $this->name);
 
         return $this->fields[$name];
     }
 
-    /**
-     * @param string $name
-     *
-     * @return bool
-     */
-    public function hasField($name)
+    public function hasField(string $name) : bool
     {
-        if ($this->fields === null) {
-            $this->getFields();
+        if (! isset($this->fields)) {
+            $this->initializeFields();
         }
 
         return isset($this->fields[$name]);
@@ -88,21 +84,26 @@ public function hasField($name)
     /**
      * @return FieldDefinition[]
      */
-    public function getFields()
+    public function getFields() : array
     {
-        if ($this->fields === null) {
-            $fields       = $this->config['fields'] ?? [];
-            $this->fields = FieldDefinition::defineFieldMap($this, $fields);
+        if (! isset($this->fields)) {
+            $this->initializeFields();
         }
 
         return $this->fields;
     }
 
+    protected function initializeFields() : void
+    {
+        $fields       = $this->config['fields'] ?? [];
+        $this->fields = FieldDefinition::defineFieldMap($this, $fields);
+    }
+
     /**
      * Resolves concrete ObjectType for given object value
      *
-     * @param object  $objectValue
-     * @param mixed[] $context
+     * @param object $objectValue
+     * @param mixed  $context
      *
      * @return Type|null
      */
@@ -120,7 +121,7 @@ public function resolveType($objectValue, $context, ResolveInfo $info)
     /**
      * @throws InvariantViolation
      */
-    public function assertValid()
+    public function assertValid() : void
     {
         parent::assertValid();
 
diff --git a/src/Type/Definition/ObjectType.php b/src/Type/Definition/ObjectType.php
index e9d069cd1..199d90df1 100644
--- a/src/Type/Definition/ObjectType.php
+++ b/src/Type/Definition/ObjectType.php
@@ -4,7 +4,6 @@
 
 namespace GraphQL\Type\Definition;
 
-use Exception;
 use GraphQL\Deferred;
 use GraphQL\Error\InvariantViolation;
 use GraphQL\Language\AST\ObjectTypeDefinitionNode;
@@ -67,13 +66,25 @@ class ObjectType extends Type implements OutputType, CompositeType, NullableType
     /** @var ?callable */
     public $resolveFieldFn;
 
-    /** @var FieldDefinition[] */
+    /**
+     * Lazily initialized.
+     *
+     * @var FieldDefinition[]
+     */
     private $fields;
 
-    /** @var InterfaceType[] */
+    /**
+     * Lazily initialized.
+     *
+     * @var InterfaceType[]
+     */
     private $interfaces;
 
-    /** @var InterfaceType[]|null */
+    /**
+     * Lazily initialized.
+     *
+     * @var InterfaceType[]
+     */
     private $interfaceMap;
 
     /**
@@ -98,9 +109,11 @@ public function __construct(array $config)
     /**
      * @param mixed $type
      *
-     * @return self
+     * @return $this
+     *
+     * @throws InvariantViolation
      */
-    public static function assertObjectType($type)
+    public static function assertObjectType($type) : self
     {
         Utils::invariant(
             $type instanceof self,
@@ -111,31 +124,22 @@ public static function assertObjectType($type)
     }
 
     /**
-     * @param string $name
-     *
-     * @return FieldDefinition
-     *
-     * @throws Exception
+     * @throws InvariantViolation
      */
-    public function getField($name)
+    public function getField(string $name) : FieldDefinition
     {
-        if ($this->fields === null) {
-            $this->getFields();
+        if (! isset($this->fields)) {
+            $this->initializeFields();
         }
         Utils::invariant(isset($this->fields[$name]), 'Field "%s" is not defined for type "%s"', $name, $this->name);
 
         return $this->fields[$name];
     }
 
-    /**
-     * @param string $name
-     *
-     * @return bool
-     */
-    public function hasField($name)
+    public function hasField(string $name) : bool
     {
-        if ($this->fields === null) {
-            $this->getFields();
+        if (! isset($this->fields)) {
+            $this->initializeFields();
         }
 
         return isset($this->fields[$name]);
@@ -146,47 +150,41 @@ public function hasField($name)
      *
      * @throws InvariantViolation
      */
-    public function getFields()
+    public function getFields() : array
     {
-        if ($this->fields === null) {
-            $fields       = $this->config['fields'] ?? [];
-            $this->fields = FieldDefinition::defineFieldMap($this, $fields);
+        if (! isset($this->fields)) {
+            $this->initializeFields();
         }
 
         return $this->fields;
     }
 
-    /**
-     * @param InterfaceType $iface
-     *
-     * @return bool
-     */
-    public function implementsInterface($iface)
+    protected function initializeFields() : void
     {
-        $map = $this->getInterfaceMap();
-
-        return isset($map[$iface->name]);
+        $fields       = $this->config['fields'] ?? [];
+        $this->fields = FieldDefinition::defineFieldMap($this, $fields);
     }
 
-    private function getInterfaceMap()
+    public function implementsInterface(InterfaceType $interfaceType) : bool
     {
-        if ($this->interfaceMap === null) {
+        if (! isset($this->interfaceMap)) {
             $this->interfaceMap = [];
             foreach ($this->getInterfaces() as $interface) {
+                /** @var Type&InterfaceType $interface */
                 $interface                            = Schema::resolveType($interface);
                 $this->interfaceMap[$interface->name] = $interface;
             }
         }
 
-        return $this->interfaceMap;
+        return isset($this->interfaceMap[$interfaceType->name]);
     }
 
     /**
      * @return InterfaceType[]
      */
-    public function getInterfaces()
+    public function getInterfaces() : array
     {
-        if ($this->interfaces === null) {
+        if (! isset($this->interfaces)) {
             $interfaces = $this->config['interfaces'] ?? [];
             if (is_callable($interfaces)) {
                 $interfaces = $interfaces();
@@ -208,8 +206,8 @@ public function getInterfaces()
     }
 
     /**
-     * @param mixed        $value
-     * @param mixed[]|null $context
+     * @param mixed $value
+     * @param mixed $context
      *
      * @return bool|Deferred|null
      */
@@ -230,7 +228,7 @@ public function isTypeOf($value, $context, ResolveInfo $info)
      *
      * @throws InvariantViolation
      */
-    public function assertValid()
+    public function assertValid() : void
     {
         parent::assertValid();
 
diff --git a/src/Type/Definition/ResolveInfo.php b/src/Type/Definition/ResolveInfo.php
index 6b6dba6d7..6c118c234 100644
--- a/src/Type/Definition/ResolveInfo.php
+++ b/src/Type/Definition/ResolveInfo.php
@@ -108,7 +108,11 @@ class ResolveInfo
      */
     public $variableValues = [];
 
-    /** @var QueryPlan */
+    /**
+     * Lazily initialized.
+     *
+     * @var QueryPlan
+     */
     private $queryPlan;
 
     /**
@@ -203,7 +207,7 @@ public function getFieldSelection($depth = 0)
      */
     public function lookAhead(array $options = []) : QueryPlan
     {
-        if ($this->queryPlan === null) {
+        if (! isset($this->queryPlan)) {
             $this->queryPlan = new QueryPlan(
                 $this->parentType,
                 $this->schema,
diff --git a/src/Type/Definition/UnionType.php b/src/Type/Definition/UnionType.php
index 51d6cb623..6301e60a7 100644
--- a/src/Type/Definition/UnionType.php
+++ b/src/Type/Definition/UnionType.php
@@ -19,10 +19,18 @@ class UnionType extends Type implements AbstractType, OutputType, CompositeType,
     /** @var UnionTypeDefinitionNode */
     public $astNode;
 
-    /** @var ObjectType[] */
+    /**
+     * Lazily initialized.
+     *
+     * @var ObjectType[]
+     */
     private $types;
 
-    /** @var array<string, bool>|null */
+    /**
+     * Lazily initialized.
+     *
+     * @var array<string, bool>
+     */
     private $possibleTypeNames;
 
     /** @var UnionTypeExtensionNode[] */
@@ -57,7 +65,7 @@ public function isPossibleType(Type $type) : bool
             return false;
         }
 
-        if ($this->possibleTypeNames === null) {
+        if (! isset($this->possibleTypeNames)) {
             $this->possibleTypeNames = [];
             foreach ($this->getTypes() as $possibleType) {
                 $this->possibleTypeNames[$possibleType->name] = true;
@@ -69,10 +77,12 @@ public function isPossibleType(Type $type) : bool
 
     /**
      * @return ObjectType[]
+     *
+     * @throws InvariantViolation
      */
-    public function getTypes()
+    public function getTypes() : array
     {
-        if ($this->types === null) {
+        if (! isset($this->types)) {
             $types = $this->config['types'] ?? null;
             if (is_callable($types)) {
                 $types = $types();
@@ -120,7 +130,7 @@ public function resolveType($objectValue, $context, ResolveInfo $info)
     /**
      * @throws InvariantViolation
      */
-    public function assertValid()
+    public function assertValid() : void
     {
         parent::assertValid();
 
diff --git a/src/Type/Schema.php b/src/Type/Schema.php
index 173f340cf..3f0f49bfc 100644
--- a/src/Type/Schema.php
+++ b/src/Type/Schema.php
@@ -57,7 +57,11 @@ class Schema
      */
     private $resolvedTypes = [];
 
-    /** @var array<string, array<string, ObjectType|UnionType>> */
+    /**
+     * Lazily initialized.
+     *
+     * @var array<string, array<string, ObjectType|UnionType>>
+     */
     private $possibleTypeMap;
 
     /**
@@ -402,7 +406,7 @@ public function getPossibleTypes(Type $abstractType) : array
     /**
      * @return array<string, array<string, ObjectType|UnionType>>
      */
-    private function getPossibleTypeMap()
+    private function getPossibleTypeMap() : array
     {
         if (! isset($this->possibleTypeMap)) {
             $this->possibleTypeMap = [];