Generator incorrectly prefixes relative namespaced types with `global::`

[chsienki]

See dotnet/aspnetcore#47540

There is an alias @using WeatherForecast = Pages.GridRendering.WeatherForecast where the Pages namespace is relative to the containing namespace (I.e., not at the root)

We're incorrectly code-genning global::Pages.GridRendering.WeatherForecast which is not valid; we should either leave it relative or add in the full namespace of the type.

[JamesNK]

Please prioritize investigating this. This issue is blocking dotnet/aspnetcore from updating the SDK (it's been blocked on various issues for over 2 weeks)

[jjonescz]

The issue only appears if the using alias is for a type nested in another component. I'm not sure how that would ever work, but will investigate more. The following test reproduces the issue:

[Fact, WorkItem("https://github.com/dotnet/razor/issues/8610")]
public async Task SourceGenerator_RazorFiles_UsingAlias()
{
    // Arrange
    var project = CreateTestProject(new()
    {
        ["Pages/Index.razor"] = """
            @code {
                public class MyModel { }
            }
            """,
        ["Shared/MyComponent.razor"] = """
            @using MyAlias = Pages.Index.MyModel;

            <MyComponent Data="@Data" />

            @code {
                [Parameter]
                public MyAlias Data { get; set; }
            }
            """,
    });
    var compilation = await project.GetCompilationAsync();
    var driver = await GetDriverAsync(project);

    // Act
    var result = RunGenerator(compilation!, ref driver);

    // Assert
    Assert.Empty(result.Diagnostics);
    Assert.Equal(2, result.GeneratedSources.Length);
}

[jjonescz]

OK. This regression has been introduced by #8212. Before, when creating tag helper descriptor for the component, the compilation contained declaration syntax trees of all components, i.e., while inspecting type of property Shared.MyComponent.Data, the nested type Pages.Index+MyModel was available in the compilation and hence resolved properly. After the PR, each component has its descriptor computed in isolation, therefore type of property Shared.MyComponent.Data cannot see the nested type Pages.Index+MyModel.

This leaves the property with an error type which is just emitted as-is. The error has nothing to do with global:: prefixing logic.