Fix struct layout error when nullable enabled: (#34128)

* Fix struct layout error when nullable enabled:
- Change type comparison to ignore nullable
- Add a test
- Fix an existing test

src/Compilers/CSharp/Portable/Symbols/Source/SourceMemberContainerSymbol.cs

@@ -2095,7 +2095,7 @@ private static bool InfiniteFlatteningGraph(SourceMemberContainerTypeSymbol top,
             if (instanceMap.TryGetValue(tOriginal, out oldInstance))
             {
                 // short circuit when we find a cycle, but only return true when the cycle contains the top struct
-                return (!TypeSymbol.Equals(oldInstance, t, TypeCompareKind.ConsiderEverything2)) && ReferenceEquals(tOriginal, top);
+                return (!TypeSymbol.Equals(oldInstance, t, TypeCompareKind.AllNullableIgnoreOptions)) && ReferenceEquals(tOriginal, top);
             }
             else
             {

src/Compilers/CSharp/Test/Semantic/Semantics/NullableReferenceTypesTests.cs

@@ -53767,9 +53767,6 @@ static void F<T>() where T : class, new()
 }";
             var comp = CreateCompilation(source, options: WithNonNullTypesTrue());
             comp.VerifyDiagnostics(
-                // (5,26): error CS0523: Struct member 'S<T>.Instance' of type 'S<T>' causes a cycle in the struct layout
-                //     internal static S<T> Instance = new S<T>();
-                Diagnostic(ErrorCode.ERR_StructLayoutCycle, "Instance").WithArguments("S<T>.Instance", "S<T>").WithLocation(5, 26),
                 // (11,27): warning CS8625: Cannot convert null literal to non-nullable reference or unconstrained type parameter.
                 //         S<T>.Instance.F = null; // 1
                 Diagnostic(ErrorCode.WRN_NullAsNonNullable, "null").WithLocation(11, 27),
@@ -85823,5 +85820,20 @@ static void MN2()
                 //         node.ToString(); // 8
                 Diagnostic(ErrorCode.WRN_NullReferenceReceiver, "node").WithLocation(118, 9));
         }
+
+        [Fact, WorkItem(32934, "https://github.com/dotnet/roslyn/issues/32934")]
+        public void NoCycleInStructLayout()
+        {
+            var source =
+@"
+#nullable enable
+public struct Foo<T>
+{
+    public static Foo<T> Bar;
+}
+";
+            var comp = CreateCompilation(new[] { source }, options: WithNonNullTypesTrue());
+            comp.VerifyDiagnostics();
+        }
     }
 }