Fix walking base hierarchy when computing dictionary slots (#2193)

When the generic ready to run lookup helper needs to know the generic
dictionary vtable slot, it passes the canonical type to the helper that
determines the slot. `GetNumberOfBaseSlots` needs to be able to deal
with this, and the fact that the slots might be lazily determined.

Converting to canon fixes this due to how we ensure canonically
equivalent types have the same vtable in #2096. The other option would
be to back out that change and completely revamp how we do lazy slots
for generic types.

I found this when I ran the reflection invoke test with shared generics
enabled.

1 files changed, 37 insertions, 0 deletions

diff --git a/tests/src/Simple/Generics/Generics.cs b/tests/src/Simple/Generics/Generics.cs
index 033c1d28c..b88239695 100644
--- a/tests/src/Simple/Generics/Generics.cs
+++ b/tests/src/Simple/Generics/Generics.cs
@@ -13,6 +13,7 @@ class Program
         TestInitThisClass.Run();
         TestDelegateFatFunctionPointers.Run();
         TestVirtualMethodUseTracking.Run();
+        TestSlotsInHierarchy.Run();
         TestNameManglingCollisionRegression.Run();
 
         return 100;
@@ -226,6 +227,42 @@ class Program
         }
     }
 
+    /// <summary>
+    /// Makes sure that during the base slot computation for types such as
+    /// Derived&lt;__Canon&gt; (where the base type ends up being Base&lt;__Canon, string&gt;),
+    /// the lazy vtable slot computation works.
+    /// </summary>
+    class TestSlotsInHierarchy
+    {
+        class Base<T, U>
+        {
+            public virtual int Do()
+            {
+                return 42;
+            }
+        }
+
+        class Derived<T> : Base<T, string> where T : class
+        {
+            public T Cast(object v)
+            {
+                return v as T;
+            }
+        }
+
+        public static void Run()
+        {
+            var derived = new Derived<string>();
+            var derivedAsBase = (Base<string, string>)derived;
+
+            if (derivedAsBase.Do() != 42)
+                throw new Exception();
+
+            if (derived.Cast("Hello") != "Hello")
+                throw new Exception();
+        }
+    }
+
     //
     // Regression test for issue https://github.com/dotnet/corert/issues/1964
     //