Incorrect error CS8602: Dereference of a possibly null reference on a guarded string value

[jozefizso]

Version Used: C# 8.0

Steps to Reproduce:

1. Create project targeting .NET Framework 4.8 and .NET 8, enable C# 8 and treat nullable warnings as compilation errors
2. Selectively enable nullables for a portion of a codebase
3. A nullable string variable is guarded in a static method, but the code cannot be compiled because accessing the variable after the guard will cause the CS8602 error.

A minimal repro: https://gist.github.com/jozefizso/ff7e0ce119366b1129cb6a0d5fb0edee

Diagnostic Id: CS8602: Dereference of a possibly null reference

Expected Behavior: The code should compile.

Actual Behavior: Compiler fails on s.Length expression while the s variable is reported as not null.

[CyrusNajmabadi]

.Net framework likely doesn't have annotations for string.IsNullOrEmpty. I'd recommend you just do nullable testing for your .net8 target.

[huoyaoyuan]

See also #73823