Intellisense triggers ArgumentException when a variable is a different type depending on #if when nullable is enabled

[vsfeedback]

This issue has been moved from a ticket on Developer Community.

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So, let's say I'm targeting both .Net Standard 2.0 and .Net Standard 2.1 in a particular project so that I can use spans in the latter. I am implementing a particular method like this:

#if NETSTANDARD2_1
    ReadOnlySpan<char> value;
#else
    string value;
#endif

#nullable enable
    value = whatever();
    for (int i = 0; i < value.Length; i++)
    {
        ...
    }

IntelliSense crashes with an ArgumentException if I move the mouse over value or value.Length when all of the conditions are true:

1. It's outside of the #if block (that is, it's in a scope in which there's any ambiguity as to whether it's a string or a ReadOnlySpan<char>,
2. the current project is set to one in which value is a span (or Memory, or possibly other ref structs, etc), and
3. nullable reference types are enabled.

This also happens with other type parameters, i.e. byte[]/ReadOnlySpan<byte> and suchlike.

(Also, if I access value. Length when either condition 2 or 3 is not all true, the IntelliSense hover reports value as a string and value. Length as ReadOnlySpan<char>. Length.)

Two obvious workarounds: either 1. disable #nullable in these sorts of scopes, or 2. switch the current project to one where it's not a ref struct.

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Original Comments

Visual Studio Feedback System on 6/28/2020, 01:08 AM:

We have directed your feedback to the appropriate engineering team for further evaluation. The team will review the feedback and notify you about the next steps.

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Original Solutions

(no solutions)

[sharwell]

Here's the stack trace of the exception:

Syntax node is not within syntax tree
   at Microsoft.CodeAnalysis.CSharp.CSharpSemanticModel.CheckSyntaxNode(CSharpSyntaxNode syntax)
   at Microsoft.CodeAnalysis.CSharp.CSharpSemanticModel.GetSymbolInfo(ExpressionSyntax expression, CancellationToken cancellationToken)
   at Microsoft.CodeAnalysis.CSharp.CSharpSemanticModel.GetSymbolInfoFromNode(SyntaxNode node, CancellationToken cancellationToken)
   at Microsoft.CodeAnalysis.CSharp.CSharpSemanticModel.GetSymbolInfoCore(SyntaxNode node, CancellationToken cancellationToken)
   at Microsoft.CodeAnalysis.CSharp.QuickInfo.CSharpSemanticQuickInfoProvider.TryGetNullabilityAnalysis(Workspace workspace, SemanticModel semanticModel, SyntaxToken token, CancellationToken cancellationToken)
   at Microsoft.CodeAnalysis.QuickInfo.CommonSemanticQuickInfoProvider.<CreateContentAsync>d__5.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.CodeAnalysis.QuickInfo.CommonSemanticQuickInfoProvider.<BuildQuickInfoAsync>d__0.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.CodeAnalysis.QuickInfo.CommonQuickInfoProvider.<GetQuickInfoAsync>d__2.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.CodeAnalysis.QuickInfo.CommonQuickInfoProvider.<GetQuickInfoAsync>d__0.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.CodeAnalysis.QuickInfo.QuickInfoServiceWithProviders.<GetQuickInfoAsync>d__5.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at Microsoft.VisualStudio.Telemetry.WindowsErrorReporting.WatsonReport.GetClrWatsonExceptionInfo(Exception exceptionObject)

[DizzyHSlightlyVoided]

Workaround 3 is of course to add the System.Memory NuGet package, which includes spans for pre-.Net Standard 2.0 ...

[333fred]

I have not been able to reproduce the crash in either 16.10 internal preview or in 16.9 P3. However, I can reproduce the issue where quickinfo shows information from the wrong TFM, and if the syntax node from the wrong TFM is sent to a later stage in the compiler pipeline I would expect a failure like the one in the OP, so I doubt that this is actually fixed. Assigning this to the IDE to investigate why wrong TFM is informing quick info.

My repro sample:

using System;

namespace ConsoleApp2
{
    class Program
    {
        static void Main(string[] args)
        {
#if NETCOREAPP3_1
            ReadOnlySpan<char> value;
#else
            string value;
#endif

#nullable enable
            value = whatever();

            Console.WriteLine("Hello World!");

            for (int i = 0; i < value.Length; i++)
            {

            }
        }

        static
#if NETCOREAPP3_1
            ReadOnlySpan<char>
#else
            string
#endif
            whatever() => throw null!;
    }
}

<Project Sdk="Microsoft.NET.Sdk">

    <PropertyGroup>
        <OutputType>Exe</OutputType>
        <TargetFrameworks>netcoreapp3.1;netstandard2.0</TargetFrameworks>
        <LangVersion>8.0</LangVersion>
    </PropertyGroup>

</Project>

[333fred]

(And note that, while it's in the OP, #nullable enable has nothing to do with the bug AFAICT).

[CyrusNajmabadi]

Unable to repro this either.