Compiler: correct to handle instance variable assignment inside block…

… on global (#4324)

* Compiler: correct to handle instance variable assignment inside block on global
Fix #4323 (comment)

spec/compiler/semantic/instance_var_spec.cr

@@ -4685,4 +4685,39 @@ describe "Semantic: instance var" do
       {Foo.new.@never_nil, Bar.new.@never_nil}
     )) { tuple_of([int32, int32]) }
   end
+
+  it "errors if assigning instance variable at top level block" do
+    assert_error %(
+      def foo
+        yield
+      end
+
+      foo do
+        @foo = 1
+      end
+      ),
+      "can't use instance variables at the top level"
+  end
+
+  it "errors when assigning instance variable at top level block" do
+    assert_error %(
+      def foo
+        yield
+      end
+
+      foo do
+        @foo = 1
+      end
+      ),
+      "can't use instance variables at the top level"
+  end
+
+  it "errors when assigning instance variable at top level control block" do
+    assert_error %(
+    if true
+      @foo = 1
+    end
+    ),
+      "can't use instance variables at the top level"
+  end
 end

src/compiler/crystal/semantic/main_visitor.cr

@@ -779,6 +779,13 @@ module Crystal
     def type_assign(target : InstanceVar, value, node)
       # Check if this is an instance variable initializer
       unless @scope
+        # `InstanceVar` assignment appered in block is not checked
+        # by `Crystal::InstanceVarsInitializerVisitor` because this block
+        # may be passed to a macro. So, it checks here.
+        if current_type.is_a?(Program) || current_type.is_a?(FileModule)
+          node.raise "can't use instance variables at the top level"
+        end
+
         # Already handled by InstanceVarsInitializerVisitor
         return
       end