Fix explicit offset of ByRefLike fields.

docs/coding-guidelines/breaking-change-rules.md

@@ -169,6 +169,8 @@ Breaking Change Rules
 
 * Changing a `struct` type to a `ref struct` type and vice versa
 
+* Adding a new `ref` field to a type
+
 * Changing the underlying type of an enum
 
     This is a compile-time and behavioral breaking change as well as a binary breaking change which can make attribute arguments unparsable.

src/coreclr/tools/Common/TypeSystem/Common/ExplicitLayoutValidator.cs

@@ -81,8 +81,9 @@ protected override bool NeedsRecursiveLayout(TypeDesc fieldType)
                     return false;
                 }
 
-                // Valuetypes with GC references and byref-like types need to be checked for overlaps and alignment
-                return ((MetadataType)fieldType).ContainsGCPointers || fieldType.IsByRefLike;
+                // Valuetypes with GC references or byrefs need to be checked for overlaps and alignment
+                var metadataType = ((MetadataType)fieldType);
+                return metadataType.ContainsGCPointers || metadataType.ContainsByRefs;
             }
 
             private void ThrowFieldLayoutError(int offset)

src/coreclr/tools/Common/TypeSystem/Common/MetadataFieldLayoutAlgorithm.cs

@@ -281,7 +281,7 @@ public override bool ComputeContainsByRefs(DefType type)
 
                 TypeDesc fieldType = field.FieldType;
                 if (fieldType.IsByRef
-                    || ((fieldType is DefType df) && df.ContainsByRefs))
+                    || (fieldType.IsByRefLike && ((DefType)fieldType).ContainsByRefs))
                 {
                     return true;
                 }