Prevent None and All members in flags enums

Fixes #1251

spec/compiler/semantic/enum_spec.cr

@@ -183,6 +183,37 @@ describe "Semantic: enum" do
       )) { int32 }
   end
 
+  it "disallows None value when defined with @[Flags]" do
+    assert_error %(
+      @[Flags]
+      enum Foo
+        None
+      end
+      ),
+      "flags enum can't contain None or All members"
+  end
+
+  it "disallows All value when defined with @[Flags]" do
+    assert_error %(
+      @[Flags]
+      enum Foo
+        All = 50
+      end
+      ),
+      "flags enum can't contain None or All members"
+  end
+
+  it "doesn't error when defining a non-flags enum with None or All" do
+    assert_type(%(
+      enum Foo
+        None
+        All = 50
+      end
+
+      Foo::None.value
+      )) { int32 }
+  end
+
   it "doesn't error when defining a method for an enum with flags" do
     assert_type(%(
       @[Flags]

src/compiler/crystal/semantic/top_level_visitor.cr

@@ -550,6 +550,10 @@ class Crystal::TopLevelVisitor < Crystal::SemanticVisitor
         node.raise "can't reopen enum and add more constants to it"
       end
 
+      if is_flags && {"None", "All"}.includes?(member.name)
+        member.raise "flags enum can't contain None or All members"
+      end
+
       if default_value = member.default_value
         counter = interpret_enum_value(default_value, base_type)
       end