Bug description

Both MessagePackGenerator (CS8785) and MsgPack00xMessagePackAnalyzer (AD0001) throw IndexOutOfRangeException when a [MessagePackObject] type has a member whose type is a closed generic value type defined in a referenced assembly.

Because the generator then "will not contribute to the output", the compiling assembly silently ends up with no generated resolver at all. It is only a warning, so a normal build looks fine, but trimmed and NativeAOT apps fail at runtime. In our case three assemblies of a large app had been shipping without a resolver across two releases before we noticed.

Repro steps

Two projects. lib:

using MessagePack;
namespace Lib;

[MessagePackObject]
public partial record struct Item<T>([property: Key(0)] string Key);

app, with a ProjectReference to lib:

using MessagePack; using Lib;
namespace App;

[MessagePackObject]
public sealed partial record Cmd([property: Key(0)] string Scope)
{
    [Key(1)] public Item<int> Member { get; init; }
}

Then dotnet build on app. Both projects are in the attached mp-repro.zip.

Expected behavior

A clean build, with a generated resolver in app.

Actual behavior

CSC : warning CS8785: Generator 'MessagePackGenerator' failed to generate source. It will not
contribute to the output and compilation errors may occur as a result. Exception was of type
'IndexOutOfRangeException' with message 'Index was outside the bounds of the array.'.
CSC : warning AD0001: Analyzer 'MessagePack.SourceGenerator.Analyzers.MsgPack00xMessagePackAnalyzer'
threw an exception of type 'System.IndexOutOfRangeException' with message 'Index was outside the
bounds of the array.'.

app.dll contains no GeneratedMessagePackResolver.

• Version used: MessagePack 3.1.8
• Runtime: .NET 10 (SDK 10.0.204), Windows 11

Additional context

What triggers it. Item is declared in lib, Cmd in app:

So it needs all three of: generic, value type, from a referenced assembly. Moving the identical Item<T> declaration into app itself makes the crash disappear, which is what makes this specific to metadata symbols.
Stack trace.

System.IndexOutOfRangeException: Index was outside the bounds of the array.
   at TypeCollector.GetObjectInfo(INamedTypeSymbol formattedType, ISymbol callerSymbol)
   at TypeCollector.CollectGeneric(INamedTypeSymbol type, ISymbol callerSymbol) in TypeCollector.cs:line 661
   at TypeCollector.CollectCore(ITypeSymbol typeSymbol, ISymbol callerSymbol) in TypeCollector.cs:line 371
   at TypeCollector.GetObjectInfo(INamedTypeSymbol formattedType, ISymbol callerSymbol) in TypeCollector.cs:line 1062
   at TypeCollector.CollectCore(ITypeSymbol typeSymbol, ISymbol callerSymbol) in TypeCollector.cs:line 386
   at TypeCollector.Collect() in TypeCollector.cs:line 300
   at MsgPack00xMessagePackAnalyzer.AnalyzeSymbol(SymbolAnalysisContext, ReferenceSymbols, AnalyzerOptions) in MsgPack00xMessagePackAnalyzer.cs:line 431

CollectGeneric line 661 is the closed-generic branch that calls GetObjectInfo directly:

else
{
    // NOTE: ... closed generic types such as `Foo<string>` are not registered as a formatter.
    this.GetObjectInfo(type, callerSymbol);

so GetObjectInfo runs against a type that has no syntax. Several diagnostic paths inside it index DeclaringSyntaxReferences[0] directly (lines 735, 1009, 1101) instead of using FirstOrDefault() the way the neighbouring GetIdentifierLocation / GetLocation / GetParameterListLocation helpers already do, and that collection is empty for a metadata symbol. I did not narrow down which of the three fires.

Getting the stack trace. Console output truncates the exception to its message; -p:ErrorLog=out.sarif preserves the full description, including the stack trace and an ISymbol: context line naming the offending type:

dotnet build -p:ErrorLog=out.sarif

Workaround. Putting [MessagePackFormatter(typeof(ItemFormatter<>))] on the generic value type short-circuits CollectCore (line 356) before GetObjectInfo is reached, at the cost of hand-writing the formatter.

Possibly related: #2092 reports CS8785 from the v3 generator, but with a different inner exception and no repro.

mp-repro.zip