Adding test and returning a null if the open generic service fails to…

… close because of a generic constraint. See aspnet#471 for discussion.

src/DI.Specification.Tests/Fakes/ConstrainedFakeOpenGenericService.cs

@@ -0,0 +1,16 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+namespace Microsoft.Extensions.DependencyInjection.Specification.Fakes
+{
+    public class ConstrainedFakeOpenGenericService<TVal> : IFakeOpenGenericService<TVal>
+        where TVal : PocoClass
+    {
+        public ConstrainedFakeOpenGenericService(TVal value)
+        {
+            Value = value;
+        }
+
+        public TVal Value { get; }
+    }
+}

src/DI/ServiceLookup/CallSiteFactory.cs

@@ -197,7 +197,16 @@ private IServiceCallSite TryCreateOpenGeneric(ServiceDescriptor descriptor, Type
             {
                 Debug.Assert(descriptor.ImplementationType != null, "descriptor.ImplementationType != null");
 
-                var closedType = descriptor.ImplementationType.MakeGenericType(serviceType.GenericTypeArguments);
+                Type closedType;
+                try
+                {
+                    closedType = descriptor.ImplementationType.MakeGenericType(serviceType.GenericTypeArguments);
+                }
+                catch
+                {
+                    // This is the only way to reliably test generic constraints. See https://stackoverflow.com/questions/4864496/checking-if-an-object-meets-a-generic-parameter-constraint/4864565#4864565
+                    return null;
+                }
                 var constructorCallSite = CreateConstructorCallSite(serviceType, closedType, callSiteChain);
 
                 return ApplyLifetime(constructorCallSite, Tuple.Create(descriptor, serviceType), descriptor.Lifetime);