[clang][modules] Add features for recent C++ versions

[Hahnfeld]

Summary

Add cplusplus20, cplusplus23, and cplusplus26 (but don't document the latter, following the current policy).

[aaron.ballman]

LGTM, but please add a release note when landing.

[Hahnfeld]

LGTM, but please add a release note when landing.

There wasn't a category specific to modules, so I just added two entries for C++20 and C++23 - ok?

[aaron.ballman]

In D150773#4349607, @Hahnfeld wrote:

LGTM, but please add a release note when landing.

There wasn't a category specific to modules, so I just added two entries for C++20 and C++23 - ok?

Sure, that works! We can recategorize as we get close to the release if we feel like consolidating all the modules changes into one section.

clang/docs/Modules.rst

cplusplus11
   C++11 support is available.
 
 cplusplus14
   C++14 support is available.
 
 cplusplus17
   C++17 support is available.
 
+cplusplus20
+  C++20 support is available.
+
+cplusplus23
+  C++23 support is available.
+
 c99
   C99 support is available.
 
 c11
   C11 support is available.
 
 c17
   C17 support is available.

clang/docs/ReleaseNotes.rst

- Downgraded use of a reserved identifier in a module export declaration from
   group. This warning is enabled by default. This addresses `#61446
   <https://github.com/llvm/llvm-project/issues/61446>`_ and allows easier
   building of standard modules. This diagnostic may be strengthened into an
   error again in the future once there is a less fragile way to mark a module
   as being part of the implementation rather than a user module.
 - Clang now implements `[temp.deduct]p9`. Substitution failures inside lambdas from
   unevaluated contexts will be surfaced as errors. They were previously handled as
   SFINAE.
+- Clang now supports `requires cplusplus20` for module maps.
 
 C++23 Feature Support
 ^^^^^^^^^^^^^^^^^^^^^
 
 - Implemented `P2036R3: Change scope of lambda trailing-return-type <https://wg21.link/P2036R3>`_
   and `P2579R0 Mitigation strategies for P2036 <https://wg21.link/P2579R0>`_.
   These proposals modify how variables captured in lambdas can appear in trailing return type
   expressions and how their types are deduced therein, in all C++ language versions.
 - Implemented partial support for `P2448R2: Relaxing some constexpr restrictions <https://wg21.link/p2448r2>`_
   Explicitly defaulted functions no longer have to be constexpr-compatible but merely constexpr suitable.
   We do not support outside of defaulted special memeber functions the change that constexpr functions no
   longer have to be constexpr compatible but rather support a less restricted requirements for constexpr
   functions. Which include allowing non-literal types as return values and parameters, allow calling of
   non-constexpr functions and constructors.
+- Clang now supports `requires cplusplus23` for module maps.
 
 C++2c Feature Support
 ^^^^^^^^^^^^^^^^^^^^^
 - Compiler flags ``-std=c++2c`` and ``-std=gnu++2c`` have been added for experimental C++2c implementation work.
 
 Resolutions to C++ Defect Reports
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 - Implemented `DR2397 <https://wg21.link/CWG2397>`_ which allows ``auto`` specifier for pointers

clang/lib/Basic/Module.cpp

static bool hasFeature(StringRef Feature, const LangOptions &LangOpts,
   bool HasFeature = llvm::StringSwitch<bool>(Feature)
                         .Case("altivec", LangOpts.AltiVec)
                         .Case("blocks", LangOpts.Blocks)
                         .Case("coroutines", LangOpts.Coroutines)
                         .Case("cplusplus", LangOpts.CPlusPlus)
                         .Case("cplusplus11", LangOpts.CPlusPlus11)
                         .Case("cplusplus14", LangOpts.CPlusPlus14)
                         .Case("cplusplus17", LangOpts.CPlusPlus17)
+                        .Case("cplusplus20", LangOpts.CPlusPlus20)
+                        .Case("cplusplus23", LangOpts.CPlusPlus23)
+                        .Case("cplusplus26", LangOpts.CPlusPlus26)
                         .Case("c99", LangOpts.C99)
                         .Case("c11", LangOpts.C11)
                         .Case("c17", LangOpts.C17)
                         .Case("freestanding", LangOpts.Freestanding)
                         .Case("gnuinlineasm", LangOpts.GNUAsm)
                         .Case("objc", LangOpts.ObjC)
                         .Case("objc_arc", LangOpts.ObjCAutoRefCount)
                         .Case("opencl", LangOpts.OpenCL)

clang/test/Modules/Inputs/DependsOnModule.framework/module.map

explicit module CXX11 {
     requires cplusplus11
   }
   explicit module CXX14 {
     requires cplusplus14
   }
   explicit module CXX17 {
     requires cplusplus17
   }
+  explicit module CXX20 {
+    requires cplusplus20
+  }
+  explicit module CXX23 {
+    requires cplusplus23
+  }
+  explicit module CXX26 {
+    requires cplusplus26
+  }
   explicit module C99 {
     requires c99
   }
   explicit module C11 {
     requires c11
   }
   explicit module C17 {
     requires c17
   }
 }

clang/test/Modules/requires.m

 // [e-mail]:* {{module 'RequiresWithMissingHeader.HeaderAfter' requires feature 'missing'}}
 @import RequiresWithMissingHeader.HeaderAfter; // expected-note {{module imported here}}
 // [e-mail]/module.map:40 {{module 'DependsOnModule.CXX11' requires feature 'cplusplus11'}}
 @import DependsOnModule.CXX11; // expected-note {{module imported here}}
 // [e-mail]/module.map:43 {{module 'DependsOnModule.CXX14' requires feature 'cplusplus14'}}
 @import DependsOnModule.CXX14; // expected-note {{module imported here}}
 // [e-mail]/module.map:46 {{module 'DependsOnModule.CXX17' requires feature 'cplusplus17'}}
 @import DependsOnModule.CXX17; // expected-note {{module imported here}}
+// [e-mail]/module.map:49 {{module 'DependsOnModule.CXX20' requires feature 'cplusplus20'}}
+@import DependsOnModule.CXX20; // expected-note {{module imported here}}
+// [e-mail]/module.map:52 {{module 'DependsOnModule.CXX23' requires feature 'cplusplus23'}}
+@import DependsOnModule.CXX23; // expected-note {{module imported here}}
+// [e-mail]/module.map:55 {{module 'DependsOnModule.CXX26' requires feature 'cplusplus26'}}
+@import DependsOnModule.CXX26; // expected-note {{module imported here}}
 #else
-// [e-mail]/module.map:49 {{module 'DependsOnModule.C99' requires feature 'c99'}}
+// [e-mail]/module.map:58 {{module 'DependsOnModule.C99' requires feature 'c99'}}
 @import DependsOnModule.C99; // expected-note {{module imported here}}
-// [e-mail]/module.map:52 {{module 'DependsOnModule.C11' requires feature 'c11'}}
+// [e-mail]/module.map:61 {{module 'DependsOnModule.C11' requires feature 'c11'}}
 @import DependsOnModule.C11; // expected-note {{module imported here}}
-// [e-mail]/module.map:55 {{module 'DependsOnModule.C17' requires feature 'c17'}}
+// [e-mail]/module.map:64 {{module 'DependsOnModule.C17' requires feature 'c17'}}
 @import DependsOnModule.C17; // expected-note {{module imported here}}
 #endif