There are a number of similar questions on SO, and looking at those it always seems the asker has missed a path. But this one has got me stumped. In its simplest form:

public Object Test() {
    string testStr = "test";
    if (testStr != null) {
        return null;
    }
}

It’s fine if the string is a const

public Object Test() {
    const string testStr = "test";
    if (testStr != null) {
        return null;
    }
}

Why does the compiler suggest that the value of testStr may be null (the value may change between the explicit setting and the condition check)?

(VS2022, .NET8, Nullable disabled)