fix(lint): useless assignment for type definition

closes #342

spec/ameba/rule/lint/useless_assign_spec.cr

@@ -475,6 +475,32 @@ module Ameba::Rule::Lint
         issue.location.to_s.should eq "source.cr:1:1"
         issue.message.should eq "Useless assignment to variable `foo`"
       end
+
+      it "doesn't report if top level variable defined inside class is referenced" do
+        s = Source.new %(
+          class A
+            foo : String? = "foo"
+          end
+
+          puts foo
+        )
+        subject.catch(s).should be_valid
+      end
+
+      it "doesn't report if top level variable assigned inside class and referenced" do
+        s = Source.new %(
+          class A
+            foo : String? = "foo"
+
+            bar do
+              foo = "bar"
+            end
+          end
+
+          puts foo
+        )
+        subject.catch(s).should be_valid
+      end
     end
 
     context "branching" do

src/ameba/ast/scope.cr

@@ -161,7 +161,7 @@ module Ameba::AST
 
     # Returns `true` if this scope is a top level scope, `false` otherwise.
     def top_level?
-      outer_scope.nil?
+      outer_scope.nil? || type_definition?
     end
 
     # Returns `true` if var is an argument in current scope, `false` otherwise.

src/ameba/ast/visitors/scope_visitor.cr

@@ -102,10 +102,18 @@ module Ameba::AST
 
     # :nodoc:
     def visit(node : Crystal::TypeDeclaration)
-      return if @current_scope.type_definition?
-      return if !(var = node.var).is_a?(Crystal::Var)
+      return unless (var = node.var).is_a?(Crystal::Var)
 
       @current_scope.add_variable(var)
+      @current_assign = node.value unless node.value.nil?
+    end
+
+    def end_visit(node : Crystal::TypeDeclaration)
+      return unless (var = node.var).is_a?(Crystal::Var)
+
+      on_assign_end(node.var, node)
+      @current_assign = nil
+      on_scope_end(node) if @current_scope.eql?(node)
     end
 
     # :nodoc: