enums should disallow multiple enumerations with the same value

[tiehuis]

@andrewrk: I re-opened this to propose reversing the decision

---

A C enum allows multiple values. e.g.

enum Foo {
    OPTION1 = 0,
    OPTION2 = 1,
    DEFAULT = OPTION1,
};

However, zig extern enum's do not (note: we cannot declare a member based on another member, another possibly improvement)

const Foo = extern enum {
    Option1 = 0,
    Option2 = 1,
    Default = 0,
};

test "asd" {
    const a = Foo.Default;
}

Fails with compilation

/tmp/t.zig:4:15: error: enum tag value 0 already taken
    Default = 0,
              ^
/tmp/t.zig:2:15: note: other occurrence here
    Option1 = 0,
              ^

We should allow extern enums to have the same tag value for multiple members. The specific use-case here would be for translate-c. Encountered this when trying to process gmp.h which has this enum present:

/* Available random number generation algorithms.  */
typedef enum
{
  GMP_RAND_ALG_DEFAULT = 0,
  GMP_RAND_ALG_LC = GMP_RAND_ALG_DEFAULT /* Linear congruential.  */
} gmp_randalg_t;

Alternatively for translate-c enums could be translated as sets of constants but this feels wrong. There are possibly some questions regarding switch cases and handling of duplicate values there that I haven't considered.

[Hejsil]

I'll add something related to this, that I've hit before.
C users also uses enums for flags, so sometimes you'll see this in headers:

enum flags {
    A = 1 << 0,
    B = 1 << 1,
};
enum flags all = A | B;

translate_c will also fail to translate something like this if all is ever used.

[tgschultz]

It's possible to do this using definitions:

pub const Compression = extern enum(u32)
{
    Uncompressed    =   0,
    RLE8            =   1,  //8bpp only
    RLE4            =   2,  //4bpp only
    Bitfields       =   3,  //Windows, 16bpp/32bpp
    
    //These seem to only exist for passthrough to
    // devices like printers and don't actually
    // exist in .bmp files
    JPEG            =   4,  //Windows v3
    PNG             =   5,  //Windows v3
    
    //This is legit
    AlphaBitfields  =   6,  //Windows v4
    
    //I think these only show up in Windows Metafiles,
    // not actual .bmp files
    CMYK            =   11,
    RLE8CMYK        =   12,
    RLE4CMYK        =   13,
    
    //These only appear in OS/2 files
    pub const Huffman1D = @This().Bitfields; //OS/2, Monochrome only
    pub const RLE24   = @This().JPEG;        //OS/2, 24bpp only
};

However the definitions don't work as enum literals.

[eira-fransham]

I think that C enums should be converted to:

pub const mylib_enumname: type = struct {
    pub const A: comptime_int = 0;
    pub const B: comptime_int = 1;
    // ...
};

Since that matches how C sees them. At the moment there are plenty of C headers that cannot be imported without modification.

[kkartaltepe]

This also shows up in clang-c/libclang https://clang.llvm.org/doxygen/Index_8h_source.html in the CXCursor enumeration.