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[clang][sema] Add support and documentation for __has_extension(c_fixed_enum) #117507

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Merged
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Dec 3, 2024
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[clang][sema] Add support and documentation for __has_extension(c_fixed_enum) #117507

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a6c9b5c
Update LanguageExtensions.rst
AidanGoldfarb Nov 23, 2024
d9e0615
Updated Features.def to include c_fixed_enum. Updated enum.c test to …
Nov 24, 2024
9fd4a25
Added c_fixed_enum as an extension. Cleaned up enum.c
Nov 24, 2024
53ba18d
formatting changes
Nov 24, 2024
8722c53
fixed enum.c typed enum logic
Nov 25, 2024
3a10186
reverted to using pre-c23-warning
Nov 25, 2024
7925096
Added `__has_feature(c_fixed_enum)` test to `enum.c`. Reoganized `Fea…
Nov 26, 2024
e06f739
Remove extra space in Features.def
Nov 27, 2024
4a467b9
Updated enum.c to use static assertions. Updated Features.def to defi…
Nov 28, 2024
728a2d9
Update clang/docs/LanguageExtensions.rst
AidanGoldfarb Dec 3, 2024
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5 changes: 4 additions & 1 deletion clang/docs/LanguageExtensions.rst
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Expand Up @@ -1986,7 +1986,7 @@ Enumerations with a fixed underlying type
-----------------------------------------

Clang provides support for C++11 enumerations with a fixed underlying type
within Objective-C. For example, one can write an enumeration type as:
within Objective-C and C `prior to C23 <https://open-std.org/JTC1/SC22/WG14/www/docs/n3030.htm>`_. For example, one can write an enumeration type as:

.. code-block:: c++

Expand All @@ -1998,6 +1998,9 @@ value, is ``unsigned char``.
Use ``__has_feature(objc_fixed_enum)`` to determine whether support for fixed
underlying types is available in Objective-C.

Use ``__has_extension(c_fixed_enum)`` to determine whether support for fixed
underlying types is available in C.
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@AaronBallman AaronBallman Nov 27, 2024

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We should probably update the wording here to mention __has_feature and language version behavior.


Interoperability with C++11 lambdas
-----------------------------------

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3 changes: 3 additions & 0 deletions clang/include/clang/Basic/Features.def
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Expand Up @@ -163,6 +163,8 @@ FEATURE(c_atomic, LangOpts.C11)
FEATURE(c_generic_selections, LangOpts.C11)
FEATURE(c_static_assert, LangOpts.C11)
FEATURE(c_thread_local, LangOpts.C11 &&PP.getTargetInfo().isTLSSupported())
// C23 features
FEATURE(c_fixed_enum, true)
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@AaronBallman AaronBallman Nov 27, 2024

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Suggested change
FEATURE(c_fixed_enum, true)
FEATURE(c_fixed_enum, LangOpts.C23)

This is only a feature of C23, it's an extension in other modes.

// C++11 features
FEATURE(cxx_access_control_sfinae, LangOpts.CPlusPlus11)
FEATURE(cxx_alias_templates, LangOpts.CPlusPlus11)
Expand Down Expand Up @@ -269,6 +271,7 @@ EXTENSION(c_static_assert, true)
EXTENSION(c_thread_local, PP.getTargetInfo().isTLSSupported())
// C23 features supported by other languages as extensions
EXTENSION(c_attributes, true)
EXTENSION(c_fixed_enum, true)
// C++11 features supported by other languages as extensions.
EXTENSION(cxx_atomic, LangOpts.CPlusPlus)
EXTENSION(cxx_default_function_template_args, LangOpts.CPlusPlus)
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10 changes: 10 additions & 0 deletions clang/test/Sema/enum.c
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Expand Up @@ -121,6 +121,16 @@ int NegativeShortTest[NegativeShort == -1 ? 1 : -1];
enum Color { Red, Green, Blue }; // expected-note{{previous use is here}}
typedef struct Color NewColor; // expected-error {{use of 'Color' with tag type that does not match previous declaration}}

// Enumerations with a fixed underlying type.
// https://github.com/llvm/llvm-project/issues/116880
#if __STDC_VERSION__ >= 202311L && !__has_feature(c_fixed_enum)
#error c_fixed_enum should be set a feature in C23 mode
#elif __STDC_VERSION__ < 202311L && !__has_extension(c_fixed_enum)
#error c_fixed_enum should be a language extension in <C23 mode
#else
typedef enum : unsigned char { Pink, Black, Cyan } Color; // pre-c23-warning {{enumeration types with a fixed underlying type are a C23 extension}}
#endif
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@AaronBallman AaronBallman Nov 27, 2024

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How about something along the lines of:

#if __STDC_VERSION__ >= 202311L
static_assert(__has_feature(c_fixed_enum));
static_assert(__has_extension(c_fixed_enum)); // Matches behavior for c_alignas, etc
#else
_Static_assert(__has_extension(c_fixed_enum), "");
_Static_assert(!__has_feature(c_fixed_enum), "");
#endif

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@AidanGoldfarb AidanGoldfarb Nov 28, 2024

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Updated. I chose to include
typedef enum : unsigned char { Pink, Black, Cyan } Color; // pre-c23-warning {{enumeration types with a fixed underlying type are a C23 extension}}
in enum.c, as I think we want to test for both the behavior of __has_feature() and __has_extension() as well as the actual definition of a typed enum. Please let me know if this is not a good idea!


// PR28903
// In C it is valid to define tags inside enums.
struct PR28903 {
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