[ILVerify] Fix stind to allow storing a string to ref string (#4412)

* Added test case for assigning a string to a ref string.

* Modified store indirect to correctly check types of value and address.

* Added additional test case for assigning string to string ref as object.

* Changed stind.ref to not replace typeToken with Object but address type.

2 files changed, 29 insertions, 6 deletions

diff --git a/src/ILVerify/src/ILImporter.Verify.cs b/src/ILVerify/src/ILImporter.Verify.cs
index 8ee832cf1..cc7adc402 100644
--- a/src/ILVerify/src/ILImporter.Verify.cs
+++ b/src/ILVerify/src/ILImporter.Verify.cs
@@ -1357,9 +1357,6 @@ namespace Internal.IL
             ClearPendingPrefix(Prefix.Unaligned);
             ClearPendingPrefix(Prefix.Volatile);
 
-            if (type == null)
-                type = GetWellKnownType(WellKnownType.Object);
-
             var value = Pop();
             var address = Pop();
 
@@ -1367,11 +1364,13 @@ namespace Internal.IL
 
             CheckIsByRef(address);
 
+            if (type == null)
+                type = address.Type;
+
             var typeVal = StackValue.CreateFromType(type);
             var addressVal = StackValue.CreateFromType(address.Type);
-            if (!value.IsNullReference)
-                CheckIsAssignable(typeVal, addressVal);
 
+            CheckIsAssignable(typeVal, addressVal);
             CheckIsAssignable(value, typeVal);
         }
 
@@ -1382,7 +1381,7 @@ namespace Internal.IL
             if (value.Kind != StackValueKind.ObjRef)
             {
                 VerificationError(VerifierError.StackObjRef);
-            }            
+            }
         }
 
         void ImportLoadString(int token)
diff --git a/src/ILVerify/tests/ILTests/LoadStoreIndirectTest.il b/src/ILVerify/tests/ILTests/LoadStoreIndirectTest.il
index 3a9cc4cd5..eb7343a54 100644
--- a/src/ILVerify/tests/ILTests/LoadStoreIndirectTest.il
+++ b/src/ILVerify/tests/ILTests/LoadStoreIndirectTest.il
@@ -99,6 +99,30 @@
         ret
     }
 
+    .method static public hidebysig void StoreIndirect.AssignStringToRefString_Valid(string&) cil managed
+    {
+        // ref string x;
+        // string y = "a";
+        // x = y;
+
+        ldarg.0
+        ldstr   "a"
+        stind.ref
+        ret
+    }
+
+    .method static public hidebysig void StoreObject.AssignStringToRefStringAsObject_Invalid_StackUnexpected(string&) cil managed
+    {
+        // ref string x;
+        // string y = "a";
+        // x = y;
+
+        ldarg.0
+        ldstr   "a"
+        stobj   [System.Runtime]System.Object
+        ret
+    }
+
     .method static public hidebysig void StoreIndirect.AssignByteToBoolRef_Valid(bool&) cil managed
     {
         ldarg.0