merged changes for Nick Mueller <https://github.com/morpheusxaut> from v3 to v4

Tickets:

* https://github.com/DefinitelyTyped/DefinitelyTyped/issues/19014

types/sequelize/index.d.ts

@@ -5,6 +5,7 @@
 //                 Ivan Drinchev <https://github.com/drinchev>
 //                 Brendan Abolivier <https://github.com/babolivier>
 //                 Patsakol Tangjitcharoenchai <https://github.com/kukoo1>
+//                 Nick Mueller <https://github.com/morpheusxaut>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
 // TypeScript Version: 2.3
 
@@ -4868,7 +4869,7 @@ declare namespace sequelize {
          * (field name), `length` (create a prefix index of length chars), `order` (the direction the column
          * should be sorted in), `collate` (the collation (sort order) for the column)
          */
-        fields?: Array<string | { attribute: string, length: number, order: string, collate: string }>;
+        fields?: Array<string | fn | { attribute: string, length: number, order: string, collate: string }>;
 
         /**
          * Method the index should use, for example 'gin' index.

types/sequelize/sequelize-tests.ts

@@ -1627,6 +1627,40 @@ s.define( 'TriggerTest', {
     hasTrigger : true
 } );
 
+s.define('DefineOptionsIndexesTest', {
+    id: {
+        type: Sequelize.INTEGER,
+        autoIncrement: true,
+        primaryKey: true,
+        validate: {
+            min: 1
+        }
+    },
+    email: {
+        allowNull: false,
+        type: Sequelize.STRING(255),
+        set: function (val) {
+            if (typeof val === "string") {
+                val = val.toLowerCase();
+            } else {
+                throw new Error("email must be a string");
+            }
+            this.setDataValue("email", val);
+        }
+    }
+}, {
+        timestamps: false,
+        indexes: [
+            {
+                name: "DefineOptionsIndexesTest_lower_email",
+                unique: true,
+                fields: [
+                    Sequelize.fn("LOWER", Sequelize.col("email"))
+                ]
+            }
+        ]
+} );
+
 //
 //  Transaction
 // ~~~~~~~~~~~~~