How to reproduce:
1. Open the attached project “IAsyncEnumerableTest.zip“
2. Switch the API Compatibility Level from “.NET Standard” to “.NET Framework 2.1” (Project Settings > Player > Api Compatibility Level)

Expected result: No errors in the Console
Actual result: Errors in the Console

Reproducible with: 2021.3.22f1, 2022.2.15f1, 2023.1.0b11, 2023.2.0a10
Can’t test with: 2020.3.47f1 (“error CS0246: The type or namespace name 'IAsyncEnumerable<>' could not be found (are you missing a using directive or an assembly reference?)“ error)

Reproduced on: macOS 13.0.1 (Intel), Windows 10 (by reporter)

Full error: Assets/TestScript.cs(18,16): error CS0266: Cannot implicitly convert type 'System.Collections.Generic.IAsyncEnumerable<TestScript.Test>' to 'System.Collections.Generic.IAsyncEnumerable<TestScript.ITest>'. An explicit conversion exists (are you missing a cast?)