Allow illegal custom modifiers on generic constraints

Roslyn decided to do a breaking change to the ECMA-335 file format by generating an illegal custom modifier for the `unmanaged` constraint. The purpose of the modifier is to poison such types for old versions of the C# compiler (the modifier otherwise isn't necessary for the feature to work). This also poisons a lot of the .NET ecosystem too (breaking everything ranging from Mono to the C++/CLI compiler).

We need to update our stack to support this and unblock our customers.

[tfs-changeset: 1709243]

2 files changed, 23 insertions, 1 deletions

diff --git a/src/System.Private.Reflection.Core/src/System/Reflection/Runtime/General/MetadataReaderExtensions.NativeFormat.cs b/src/System.Private.Reflection.Core/src/System/Reflection/Runtime/General/MetadataReaderExtensions.NativeFormat.cs
index 901d0f4ba..5896aa812 100644
--- a/src/System.Private.Reflection.Core/src/System/Reflection/Runtime/General/MetadataReaderExtensions.NativeFormat.cs
+++ b/src/System.Private.Reflection.Core/src/System/Reflection/Runtime/General/MetadataReaderExtensions.NativeFormat.cs
@@ -162,6 +162,24 @@ namespace System.Reflection.Runtime.General
             return customModifiers.ToArray();
         }
 
+        public static Handle SkipCustomModifiers(this Handle handle, MetadataReader reader)
+        {
+            HandleType handleType = handle.HandleType;
+            Debug.Assert(handleType == HandleType.TypeDefinition || handleType == HandleType.TypeReference || handleType == HandleType.TypeSpecification || handleType == HandleType.ModifiedType);
+            if (handleType != HandleType.ModifiedType)
+                return handle;
+
+            do
+            {
+                ModifiedType modifiedType = handle.ToModifiedTypeHandle(reader).GetModifiedType(reader);
+                handle = modifiedType.Type;
+                handleType = handle.HandleType;
+            }
+            while (handleType == HandleType.ModifiedType);
+
+            return handle;
+        }
+
         public static MethodSignature ParseMethodSignature(this Handle handle, MetadataReader reader)
         {
             return handle.ToMethodSignatureHandle(reader).GetMethodSignature(reader);
diff --git a/src/System.Private.Reflection.Core/src/System/Reflection/Runtime/TypeInfos/NativeFormat/NativeFormatRuntimeGenericParameterTypeInfo.cs b/src/System.Private.Reflection.Core/src/System/Reflection/Runtime/TypeInfos/NativeFormat/NativeFormatRuntimeGenericParameterTypeInfo.cs
index 678577135..f027f9caa 100644
--- a/src/System.Private.Reflection.Core/src/System/Reflection/Runtime/TypeInfos/NativeFormat/NativeFormatRuntimeGenericParameterTypeInfo.cs
+++ b/src/System.Private.Reflection.Core/src/System/Reflection/Runtime/TypeInfos/NativeFormat/NativeFormatRuntimeGenericParameterTypeInfo.cs
@@ -81,7 +81,11 @@ namespace System.Reflection.Runtime.TypeInfos.NativeFormat
                 LowLevelList<QTypeDefRefOrSpec> constraints = new LowLevelList<QTypeDefRefOrSpec>();
                 foreach (Handle constraintHandle in _genericParameter.Constraints)
                 {
-                    constraints.Add(new QTypeDefRefOrSpec(reader, constraintHandle));
+                    // We're skipping custom modifiers here because Roslyn generates
+                    // a modifier for the "unmanaged" constraint. This doesn't conform to the
+                    // ECMA-335 spec, but we need to deal with it. The modifier is not visible
+                    // to reflection.
+                    constraints.Add(new QTypeDefRefOrSpec(reader, constraintHandle.SkipCustomModifiers(reader)));
                 }
                 return constraints.ToArray();
             }