859874 - mfbt/TypedEnum.h on non-C++11 compilers disables all arithmetic operators

Status: RESOLVED INVALID

(Core :: MFBT, defect)

Description

[Benoit Jacob [:bjacob] (mostly away)]

I think it shouldn't. Don't C++11 typed enums have arithmetic operators?

The idea is that people should be able to do

  for(MyEnumType n = 0; n < SomeBigEnumValue; ++n)

instead of having to use a plain integer as loop index. That would further cement the strong typing around such code, instead of forcing people to use plain integers and cast them.

Use case in bug 852734.

Comment 1

[(no longer active)]

Do you have a test case?

Comment 2

[Benoit Jacob [:bjacob] (mostly away)]

Attachment: testcase

Tested with G++4.6. Compiling in non-C++11 mode, I get:

$ g++ ehsan.cpp -I .
ehsan.cpp: In function ‘int main()’:
ehsan.cpp:14:9: error: no match for ‘operator++’ in ‘++n’
ehsan.cpp:14:9: note: candidates are:
ehsan.cpp:8:1: note: int& operator++(MyEnumType::Enum&)
ehsan.cpp:8:1: note:   no known conversion for argument 1 from ‘MyEnumType’ to ‘MyEnumType::Enum&’
ehsan.cpp:8:1: note: int operator++(MyEnumType::Enum&, int)
ehsan.cpp:8:1: note:   candidate expects 2 arguments, 1 provided

That's what I'm referring to.

On a separate note, compiling in C++11 mode, I get errors that suggest that something is wrong either in G++ 4.6 or in our C++11 codepath there... I don't know C++11 enums so I can't tell.

$ g++ ehsan.cpp -I . --std=c++0x
ehsan.cpp:3:1: warning: elaborated-type-specifier for a scoped enum must not use the ‘class’ keyword [enabled by default]
ehsan.cpp:3:1: error: use of enum ‘MyEnumType’ without previous declaration
ehsan.cpp:3:1: error: expected unqualified-id before ‘:’ token
ehsan.cpp: In function ‘int main()’:
ehsan.cpp:12:7: error: ‘MyEnumType’ was not declared in this scope
ehsan.cpp:12:18: error: expected ‘;’ before ‘n’
ehsan.cpp:13:7: error: ‘n’ was not declared in this scope
ehsan.cpp:13:11: error: ‘MyEnumType’ is not a class, namespace, or enumeration

Comment 3

[Benoit Jacob [:bjacob] (mostly away)]

Erm. Replacing the typed enum by a plain C++98 enum there, I get "error: no match for ‘operator++’ in ‘++n’".

So I guess that ++n for an enum was never allowed. -> INVALID.

Comment 4

[(no longer active)]

Yep, which is why I asked for a test case.  ;-)