ensure that nullable is not set for required parameters (#2079)

nullable was enforced for optional parameters, but not forbidden for
required parameters. This tests and enforces the latter case.

fixes #2030

Co-authored-by: Evgeny Poberezkin <2769109+epoberezkin@users.noreply.github.com>

lib/types/json-schema.ts

@@ -180,6 +180,7 @@ type Nullable<T> = undefined extends T
       default?: T | null
     }
   : {
+      nullable?: false
       const?: T
       enum?: Readonly<T[]>
       default?: T

spec/types/json-schema.spec.ts

@@ -306,6 +306,23 @@ describe("JSONSchemaType type and validation as a type guard", () => {
       // eslint-disable-next-line no-void
       void optionalSchema
     })
+
+    it("won't accept nullable for non-null types", () => {
+      // @ts-expect-error can't set nullable
+      const nonNullSchema: JSONSchemaType<{a: number}> = {
+        type: "object",
+        properties: {
+          a: {
+            type: "number",
+            nullable: true,
+          },
+        },
+        required: [],
+      }
+
+      // eslint-disable-next-line no-void
+      void nonNullSchema
+    })
   })
 
   describe("schema works for primitives", () => {