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test/static/not.ts

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+import { Expect } from './assert'
+import { Type } from '@sinclair/typebox'
+
+{
+  // -------------------------------------------------------------------------
+  // Issue: type T = number extends not number ? true : false // true
+  //        type T = number extends unknown ? true : false    // true
+  //
+  // TypeScript does not support type negation. The best TypeBox can do is
+  // treat "not" as "unknown". From this standpoint, the extends assignability
+  // check needs to return true for the following case to keep TypeBox aligned
+  // with TypeScript static inference.
+  // -------------------------------------------------------------------------
+  const A = Type.Number()
+  const B = Type.Not(Type.Number())
+  const T = Type.Extends(A, B, Type.Literal(true), Type.Literal(false))
+  Expect(T).ToStatic<true>()
+}
+{
+  const T = Type.Not(Type.Number())
+  Expect(T).ToStatic<unknown>()
+}
+{
+  const T = Type.Not(Type.Not(Type.Number()))
+  Expect(T).ToStatic<number>()
+}