Support generic type parameter when created with TypeBuilder

src/libraries/System.Private.CoreLib/src/System/Reflection/SignatureConstructedGenericType.cs

@@ -50,6 +50,7 @@ public sealed override bool ContainsGenericParameters
             }
         }
 
+        protected sealed override bool IsValueTypeImpl() => _genericTypeDefinition.IsValueType;
         internal sealed override SignatureType? ElementType => null;
         public sealed override int GetArrayRank() => throw new ArgumentException(SR.Argument_HasToBeArrayClass);
         public sealed override Type GetGenericTypeDefinition() => _genericTypeDefinition;

src/libraries/System.Private.CoreLib/src/System/Reflection/SignatureGenericParameterType.cs

@@ -26,6 +26,7 @@ protected SignatureGenericParameterType(int position)
         public sealed override bool IsGenericParameter => true;
         public abstract override bool IsGenericMethodParameter { get; }
         public sealed override bool ContainsGenericParameters => true;
+        protected sealed override bool IsValueTypeImpl() => false;
 
         internal sealed override SignatureType? ElementType => null;
         public sealed override int GetArrayRank() => throw new ArgumentException(SR.Argument_HasToBeArrayClass);

src/libraries/System.Private.CoreLib/src/System/Reflection/SignatureHasElementType.cs

@@ -27,6 +27,7 @@ protected SignatureHasElementType(SignatureType elementType)
         public sealed override bool IsGenericTypeParameter => false;
         public sealed override bool IsGenericMethodParameter => false;
         public sealed override bool ContainsGenericParameters => _elementType.ContainsGenericParameters;
+        protected sealed override bool IsValueTypeImpl() => _elementType.IsValueType;
 
         internal sealed override SignatureType? ElementType => _elementType;
         public abstract override int GetArrayRank();

src/libraries/System.Private.CoreLib/src/System/Reflection/SignatureType.cs

@@ -198,7 +198,7 @@ public sealed override Type MakeArrayType(int rank)
         [Obsolete(Obsoletions.LegacyFormatterMessage, DiagnosticId = Obsoletions.LegacyFormatterDiagId, UrlFormat = Obsoletions.SharedUrlFormat)]
         public sealed override bool IsSerializable => throw new NotSupportedException(SR.NotSupported_SignatureType);
         public sealed override bool IsSubclassOf(Type c) => throw new NotSupportedException(SR.NotSupported_SignatureType);
-        protected sealed override bool IsValueTypeImpl() => throw new NotSupportedException(SR.NotSupported_SignatureType);
+        protected abstract override bool IsValueTypeImpl();
 
         public sealed override StructLayoutAttribute StructLayoutAttribute => throw new NotSupportedException(SR.NotSupported_SignatureType);
 

src/libraries/System.Reflection.Emit/tests/PersistedAssemblyBuilder/AssemblySaveTypeBuilderTests.cs

@@ -733,8 +733,6 @@ public void MethodBuilderGetParametersReturnParameterTest()
             Assert.True(method3.ReturnParameter.IsRetval);
         }
 
-        public class BaseType<T> { }
-
         [Fact]
         public void GenericTypeWithTypeBuilderGenericParameter_UsedAsParent()
         {
@@ -749,9 +747,53 @@ public void GenericTypeWithTypeBuilderGenericParameter_UsedAsParent()
 
             Assert.NotNull(type.GetConstructor(Type.EmptyTypes)); // Default constructor created
         }
+
+        [Fact]
+        public void CreateGenericTypeFromMetadataLoadContextSignatureTypes()
+        {
+            using TempFile file = TempFile.Create();
+
+            PersistedAssemblyBuilder ab = AssemblySaveTools.PopulateAssemblyAndModule(out ModuleBuilder module);
+
+            TypeBuilder childType = module.DefineType("Child");
+            TypeBuilder parentType = module.DefineType("Parent");
+
+            // Get List<T> from MLC and make both reference and value type fields from that.
+            using MetadataLoadContext mlc = new MetadataLoadContext(new CoreMetadataAssemblyResolver());
+            Type listOfTType = mlc.CoreAssembly.GetType(typeof(List<>).FullName!);
+
+            // Currently MakeGenericSignatureType() must be used instead of MakeGenericType() for
+            // generic type parameters created with TypeBuilder.
+            Assert.Throws<ArgumentException>(() => listOfTType.MakeGenericType(childType));
+            Type listOfReferenceTypes = Type.MakeGenericSignatureType(listOfTType, childType);
+            parentType.DefineField("ReferenceTypeChildren", listOfReferenceTypes, FieldAttributes.Public);
+
+            // Pre-existing types can use MakeGenericType().
+            Type int32Type = mlc.CoreAssembly.GetType(typeof(int).FullName);
+            Type listOfValueTypes = listOfTType.MakeGenericType(int32Type);
+            parentType.DefineField("ValueTypeChildren", listOfValueTypes, FieldAttributes.Public);
+
+            parentType.CreateType();
+            childType.CreateType();
+
+            // Save and load the dynamically created assembly.
+            ab.Save(file.Path);
+            Module mlcModule = mlc.LoadFromAssemblyPath(file.Path).Modules.First();
+
+            Assert.Equal("Child", mlcModule.GetTypes()[0].Name);
+            Assert.Equal("Parent", mlcModule.GetTypes()[1].Name);
+
+            FieldInfo[] fields = mlcModule.GetTypes()[1].GetFields(BindingFlags.Public | BindingFlags.Instance);
+            Assert.Equal("ReferenceTypeChildren", fields[0].Name);
+            Assert.False(fields[0].FieldType.GetGenericArguments()[0].IsValueType);
+            Assert.Equal("ValueTypeChildren", fields[1].Name);
+            Assert.True(fields[1].FieldType.GetGenericArguments()[0].IsValueType);
+        }
     }
 
     // Test Types
+    public class BaseType<T> { }
+
     public interface INoMethod
     {
     }