<!-- 🚨 STOP 🚨 𝗦𝗧𝗢𝗣 🚨 𝑺𝑻𝑶𝑷 🚨 Half of all issues filed here are duplicates, answered in the FAQ, or not appropriate for the bug tracker. Even if you think you've found a *bug*, please read the FAQ first, especially the Common "Bugs" That Aren't Bugs section! Please help us by doing the following steps before logging an issue: * Search: https://github.com/Microsoft/TypeScript/search?type=Issues * Read the FAQ: https://github.com/Microsoft/TypeScript/wiki/FAQ Please fill in the *entire* template below. --> <!-- Please try to reproduce the issue with `typescript@next`. It may have already been fixed. --> **TypeScript Version:** 3.5.1, 3.6.3, Nightly <!-- Search terms you tried before logging this (so others can find this issue more easily) --> **Search Terms:** generic type parameter, generic type constraint, variance **Code** ```ts declare const x0 : <InT extends boolean|null>(arg : InT) => boolean; //Expected: Error //Actual : Error const x1 : <InT extends string>(arg : InT) => boolean = x0; type Y<In, Out> = <InT extends In>(arg : InT) => Out; declare const y0 : Y<boolean|null, boolean>; //Expected: Error //Actual : OK const y1 : Y<string, boolean> = y0; ``` **Expected behavior:** `y1 = y0` assignment should fail **Actual behavior:** `y1 = y0` assignment passes **Playground Link:** [3.5.1 Playground](http://www.typescriptlang.org/play/?ts=3.5.1#code/CYUwxgNghgTiAEYD2A7AzgF3gDwAzwC54AeASRQBV4RsMQVg14AjJJCEKFAHxQFcIEAHwAKWAHNC8chQCU8ALxCWbDlwDcAKAD02gKLYADuDrAiemDCQwd2gIJgMfKBHhSLVm8nRZsARikySmpaekZ4TBgASxRxUQkpGXklFXZOFEUcXC1NDABPY3gATSCAGngAeT4MZQUSGRC6BiZyeJhJIiTFZSqMLVBIWARvTHg8-CIS1jSuXgEIcum1FCEtXQNjRxAzeA9rWwcnFzciCoBpTRGsPIDJ4kiY8UXVdNqx7M0gA) [3.6.3 Playground](http://www.typescriptlang.org/play/#code/CYUwxgNghgTiAEYD2A7AzgF3gDwAzwC54AeASRQBV4RsMQVg14AjJJCEKFAHxQFcIEAHwAKWAHNC8chQCU8ALxCWbDlwDcAKAD02gKLYADuDrAiemDCQwd2gIJgMfKBHhSLVm8nRZsARikySmpaekZ4TBgASxRxUQkpGXklFXZOFEUcXC1NDABPY3gATSCAGngAeT4MZQUSGRC6BiZyeJhJIiTFZSqMLVBIWARvTHg8-CIS1jSuXgEIcum1FCEtXQNjRxAzeA9rWwcnFzciCoBpTRGsPIDJ4kiY8UXVdNqx7M0gA) [Nightly Playground](http://www.typescriptlang.org/play/?ts=Nightly#code/CYUwxgNghgTiAEYD2A7AzgF3gDwAzwC54AeASRQBV4RsMQVg14AjJJCEKFAHxQFcIEAHwAKWAHNC8chQCU8ALxCWbDlwDcAKAD02gKLYADuDrAiemDCQwd2gIJgMfKBHhSLVm8nRZsARikySmpaekZ4TBgASxRxUQkpGXklFXZOFEUcXC1NDABPY3gATSCAGngAeT4MZQUSGRC6BiZyeJhJIiTFZSqMLVBIWARvTHg8-CIS1jSuXgEIcum1FCEtXQNjRxAzeA9rWwcnFzciCoBpTRGsPIDJ4kiY8UXVdNqx7M0gA) **Related Issues:** <!-- Did you find other bugs that looked similar? -->