Compiler stack overflow on self-referential nullable union with updated-memory-safety-rules

[akoeplinger]

Repro:

public union NullableNat(bool, NullableNat?);

Compile as a library with /langversion:preview /nullable:enable /features:updated-memory-safety-rules and .NET 11 reference assemblies.

Compiler 5.12.0-1.26463.105 succeeds. Compiler 5.12.0-1.26471.108 stack-overflows; the latter succeeds without the feature flag.

The recursive stack goes through SourceMemberContainerTypeSymbol.AddSynthesizedTypeMembersIfNecessary, Binder.BindType, and Binder.ReportDiagnosticsIfUnsafeMemberAccess / checkTypeArgumentWithConstructorConstraint, which accesses InstanceConstructors while union members are being synthesized.

This blocks dotnet/runtime#134394. CI build: https://dev.azure.com/dnceng-public/public/_build/results?buildId=1609539. The constraint-check changes in #84888 appear relevant. The same failure reproduces locally on macOS arm64 compiling System.Text.Json.Tests.

Expected: Successful compilation without disabling updated memory-safety checks.

Note

This issue was generated by GitHub Copilot.

[akoeplinger]

/cc @jjonescz