Fix interpreter when inlining pure InstanceVar virtual def and only one concrete subclass exists

spec/compiler/interpreter/bugs_spec.cr

@@ -211,6 +211,28 @@ describe Crystal::Repl::Interpreter do
      CRYSTAL
     end
 
+    it "doesn't error if def body is InstanceVar and virtual dispatch has only one subclass (#16278)" do
+      interpret(<<-CRYSTAL).should eq(0)
+        abstract class A
+          abstract def foo : Int32
+
+          def bar
+            foo
+          end
+        end
+
+        class B < A
+          @test = 0
+
+          def foo : Int32
+            @test
+          end
+        end
+
+        B.new.as(A).bar
+      CRYSTAL
+    end
+
     it "handles self in inlined method with arguments (#16210)" do
       interpret(<<-CRYSTAL, prelude: "prelude").should eq(%("hello"))
         class Foo

src/compiler/crystal/interpreter/compiler.cr

@@ -1027,16 +1027,16 @@ class Crystal::Repl::Compiler < Crystal::Visitor
     compile_instance_var(node)
   end
 
-  private def compile_instance_var(node : InstanceVar)
+  private def compile_instance_var(node : InstanceVar, owner = scope)
     return false unless @wants_value
 
     closured_self = lookup_closured_var?("self")
     if closured_self
       ivar_offset, ivar_size = get_closured_self_pointer(closured_self, node.name, node: node)
       pointer_get ivar_size, node: node
     else
-      ivar_offset = ivar_offset(scope, node.name)
-      ivar_size = inner_sizeof_type(scope.lookup_instance_var(node.name))
+      ivar_offset = ivar_offset(owner, node.name)
+      ivar_size = inner_sizeof_type(owner.lookup_instance_var(node.name))
 
       get_self_ivar ivar_offset, ivar_size, node: node
     end
@@ -1859,7 +1859,7 @@ class Crystal::Repl::Compiler < Crystal::Visitor
         if obj
           compile_read_instance_var(node, obj, body.name, owner: target_def.owner)
         else
-          compile_instance_var(body)
+          compile_instance_var(body, owner: target_def.owner)
         end
       end