TypeInfo: use explicit type for getFieldDef arg (#3125)

Author: IvanGoncharov

src/utilities/TypeInfo.d.ts

@@ -26,7 +26,7 @@ export class TypeInfo {
      */
     initialType?: GraphQLType,
     /** @deprecated will be removed in 17.0.0 */
-    getFieldDefFn?: getFieldDef,
+    getFieldDefFn?: GetFieldDefFn,
   );
   getType(): Maybe<GraphQLOutputType>;
   getParentType(): Maybe<GraphQLCompositeType>;
@@ -40,7 +40,7 @@ export class TypeInfo {
   enter(node: ASTNode): any;
   leave(node: ASTNode): any;
 }
-type getFieldDef = (
+type GetFieldDefFn = (
   schema: GraphQLSchema,
   parentType: GraphQLType,
   fieldNode: FieldNode,

src/utilities/TypeInfo.js

@@ -51,7 +51,7 @@ export class TypeInfo {
   _directive: ?GraphQLDirective;
   _argument: ?GraphQLArgument;
   _enumValue: ?GraphQLEnumValue;
-  _getFieldDef: typeof getFieldDef;
+  _getFieldDef: GetFieldDefFn;
 
   constructor(
     schema: GraphQLSchema,
@@ -62,7 +62,7 @@ export class TypeInfo {
     initialType?: ?GraphQLType,
 
     /** @deprecated will be removed in 17.0.0 */
-    getFieldDefFn?: typeof getFieldDef,
+    getFieldDefFn?: GetFieldDefFn,
   ) {
     this._schema = schema;
     this._typeStack = [];
@@ -292,6 +292,12 @@ export class TypeInfo {
   }
 }
 
+type GetFieldDefFn = (
+  schema: GraphQLSchema,
+  parentType: GraphQLType,
+  fieldNode: FieldNode,
+) => ?GraphQLField<mixed, mixed>;
+
 /**
  * Not exactly the same as the executor's definition of getFieldDef, in this
  * statically evaluated environment we do not always have an Object type,