Why is the Visual Studio C++ Compiler rejecting an enum as a template parameter?

Question

I'm using the Microsoft Visual Studio 2019 compiler (cl.exe), and it is rejecting some code accepted by both Clang and GCC, related to using enums as template parameters, where the templates are specialized for particular enum values.

enum Foo {
    Bar,
    Baz
};

template<enum Foo = Bar> class Clazz {

};

template<> class Clazz<Baz> {

};

The VC++ compiler reports several errors on the template specialization:

<source>(10): error C2440: 'specialization': cannot convert from 'Foo' to 'Foo'
<source>(10): note: Conversion to enumeration type requires an explicit cast (static_cast, C-style cast or function-style cast)

This code is accepted without errors by both Clang and GCC. Is this a bug with VC++?

Replacing the 'enum Foo' in the template declaration with just 'int' causes the errors to go away. However, this is not an acceptable answer, as I'm trying to port a large code base over to VC++.

Have you try template<Foo = Bar> class Clazz {? Maybe Msvc introduces another enum named Foo. — Jarod42
– Jarod42, Commented Aug 22, 2019 at 14:58

Blastfurnace · Accepted Answer · 2019-08-22 15:00:35Z

4

Your code will compile if you use the Standards Conformance Mode compiler option /permissive- to specify standards-conforming compiler behavior.

You can add that option on the command line or in the "Project property page -> C/C++ -> Language -> Conformance mode".

answered Aug 22, 2019 at 15:00

Blastfurnace

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3 Comments

Harry Wagstaff Over a year ago

This causes the compiler to accept the code, but it looks like internally it's still treating all enums s equivalent. For example, it rejects this snippet: godbolt.org/z/XxUjDI

Blastfurnace Over a year ago

@HarryWagstaff You might have found a real compiler bug. If you have time you should file a report to the VS Dev Community.

김선달 Over a year ago

It still happens even if /permissive- is set if you're using C++14 in VS2022

Jeffrey · Accepted Answer · 2019-08-22 15:27:51Z

0

You are missing a T:

template<enum Foo T = Bar> class Clazz {

Or have an extra enum:

template<Foo = Bar> class Clazz {

This second one is much nicer, thanks François.

edited Aug 22, 2019 at 15:27

answered Aug 22, 2019 at 14:58

Jeffrey

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2 Comments

Jarod42 Over a year ago

I wouldn't use T as value in template though :-)

François Andrieux Over a year ago

template<Foo = Bar> class Clazz also fixes the problem. Template arguments don't need names, so I'm not sure what is happening here.

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