Fix variable shadowing bug in interpreter

spec/compiler/interpreter/bugs_spec.cr

@@ -237,5 +237,26 @@ describe Crystal::Repl::Interpreter do
         typeof(foo) == Foo
       CRYSTAL
     end
+
+    it "looks up local vars in parent scopes after looking up local vars in current scope and closured scope (#15489)" do
+      interpret(<<-CRYSTAL).should eq("parser")
+        def capture(&block)
+          block
+        end
+
+        def scoped(&)
+          yield 1
+        end
+
+        scoped do |parser|
+          capture do
+            parser
+          end
+          parser # Error: BUG: missing downcast_distinct from String to Int32 (Crystal::NonGenericClassType to Crystal::IntegerType)
+        end
+
+        parser = "parser"
+      CRYSTAL
+    end
   end
 end

src/compiler/crystal/interpreter/compiler.cr

@@ -791,28 +791,28 @@ class Crystal::Repl::Compiler < Crystal::Visitor
   end
 
   def lookup_local_var_or_closured_var(name : String) : LocalVar | ClosuredVar
-    lookup_local_var?(name) ||
+    lookup_local_var?(name, at: @block_level) ||
       lookup_closured_var?(name) ||
+      lookup_local_var?(name, from: @block_level - 1) ||
       raise("BUG: can't find closured var or local var #{name}")
   end
 
   def lookup_local_var(name : String) : LocalVar
     lookup_local_var?(name) || raise("BUG: can't find local var #{name}")
   end
 
-  def lookup_local_var?(name : String) : LocalVar?
-    block_level = @block_level
-    while block_level >= 0
-      index = @local_vars.name_to_index?(name, block_level)
-      if index
-        type = @local_vars.type(name, block_level)
-        return LocalVar.new(index, type)
+  def lookup_local_var?(name : String, *, from : Int32 = @block_level) : LocalVar?
+    from.downto(0) do |block_level|
+      if local_var = lookup_local_var?(name, at: block_level)
+        return local_var
       end
-
-      block_level -= 1
     end
+  end
 
-    nil
+  def lookup_local_var?(name : String, *, at block_level : Int32) : LocalVar?
+    if index = @local_vars.name_to_index?(name, block_level)
+      LocalVar.new(index, @local_vars.type(name, block_level))
+    end
   end
 
   def lookup_closured_var(name : String) : ClosuredVar