[Clang] Implement P2738R1 - constexpr cast from void*

[cor3ntin]

Rebase and update feature macro tests.

[erichkeane]

clang/lib/AST/ExprConstant.cpp

8920

Does this diagnostic work? In the case of the !C++26, the arity of the message is different, right? Do you have a test for this in not c++26 mode that you can post? I Am not sure we do 'fill ins' for discarded 'select' branches.

[cor3ntin]

Add tests for C++23

clang/lib/AST/ExprConstant.cpp

8920

Yes, we do that in a few places. I added tests.

[shafik]

clang/test/CXX/expr/expr.const/p5-26.cpp

14

Why no diagnostic on this line and the next?

[cor3ntin]

clang/test/CXX/expr/expr.const/p5-26.cpp

14

Why do you expect one?
you can add const/volatile (but not remove const) in a static_cast https://godbolt.org/z/qEqoEWohc
In c++23, no diag because we stop evaluating the function at that point.

clang/docs/ReleaseNotes.rst

- Implemented partial support for `P2448R2: Relaxing some constexpr restrictions <https://wg21.link/p2448r2>`_
   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.
 - Implemented `P2564R3: consteval needs to propagate up <https://wg21.link/P2564R3>`_.
 
 C++2c Feature Support
 ^^^^^^^^^^^^^^^^^^^^^
 - Compiler flags ``-std=c++2c`` and ``-std=gnu++2c`` have been added for experimental C++2c implementation work.
+- Implemented `P2738R1: constexpr cast from void* <https://wg21.link/P2738R1>`_.
 
 Resolutions to C++ Defect Reports
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 - Implemented `DR2397 <https://wg21.link/CWG2397>`_ which allows ``auto`` specifier for pointers
   and reference to arrays.
 
 C Language Changes
 ------------------

clang/include/clang/Basic/DiagnosticASTKinds.td

 
 // Constant expression diagnostics. These (and their users) belong in Sema.
 def note_expr_divide_by_zero : Note<"division by zero">;
 def note_constexpr_invalid_cast : Note<
   "%select{reinterpret_cast|dynamic_cast|%select{this conversion|cast that"
   " performs the conversions of a reinterpret_cast}1|cast from %1}0"
   " is not allowed in a constant expression"
   "%select{| in C++ standards before C++20||}0">;
+def note_constexpr_invalid_void_star_cast : Note<
+  "cast from %0 is not allowed in a constant expression "
+  "%select{in C++ standards before C++2c|because the pointed object "
+  "type %2 is not similar to the target type %3}1">;
 def note_constexpr_invalid_downcast : Note<
   "cannot cast object of dynamic type %0 to type %1">;
 def note_constexpr_overflow : Note<
   "value %0 is outside the range of representable values of type %1">;
 def note_constexpr_negative_shift : Note<"negative shift count %0">;
 def note_constexpr_large_shift : Note<
   "shift count %0 >= width of type %1 (%2 bit%s2)">;
 def note_constexpr_lshift_of_negative : Note<"left shift of negative value %0">;

clang/lib/AST/ExprConstant.cpp

case CK_AddressSpaceConversion:
     if (!Visit(SubExpr))
       return false;
     // Bitcasts to cv void* are static_casts, not reinterpret_casts, so are
     // permitted in constant expressions in C++11. Bitcasts from cv void* are
     // also static_casts, but we disallow them as a resolution to DR1312.
     if (!E->getType()->isVoidPointerType()) {
       // In some circumstances, we permit casting from void* to cv1 T*, when the
       // actual pointee object is actually a cv2 T.
+      bool HasValidResult = !Result.InvalidBase && !Result.Designator.Invalid &&
+                            !Result.IsNullPtr;
       bool VoidPtrCastMaybeOK =
-          !Result.InvalidBase && !Result.Designator.Invalid &&
-          !Result.IsNullPtr &&
+          HasValidResult &&
           Info.Ctx.hasSameUnqualifiedType(Result.Designator.getType(Info.Ctx),
                                           E->getType()->getPointeeType());
       // 1. We'll allow it in std::allocator::allocate, and anything which that
       //    calls.
       // 2. HACK 2022-03-28: Work around an issue with libstdc++'s
       //    <source_location> header. Fixed in GCC 12 and later (2022-04-??).
       //    We'll allow it in the body of std::source_location::current.  GCC's
       //    implementation had a parameter of type `void*`, and casts from
       //    that back to `const __impl*` in its body.
       if (VoidPtrCastMaybeOK &&
           (Info.getStdAllocatorCaller("allocate") ||
-           IsDeclSourceLocationCurrent(Info.CurrentCall->Callee))) {
+           IsDeclSourceLocationCurrent(Info.CurrentCall->Callee) ||
+           Info.getLangOpts().CPlusPlus26)) {
         // Permitted.
       } else {
-        Result.Designator.setInvalid();
-        if (SubExpr->getType()->isVoidPointerType())
+        if (SubExpr->getType()->isVoidPointerType()) {
+          if (HasValidResult)
+            CCEDiag(E, diag::note_constexpr_invalid_void_star_cast)
+                << SubExpr->getType() << Info.getLangOpts().CPlusPlus26
+                << Result.Designator.getType(Info.Ctx).getCanonicalType()
+                << E->getType()->getPointeeType();
+          else
             CCEDiag(E, diag::note_constexpr_invalid_cast)
                 << 3 << SubExpr->getType();
-        else
+        } else
           CCEDiag(E, diag::note_constexpr_invalid_cast)
               << 2 << Info.Ctx.getLangOpts().CPlusPlus;
+        Result.Designator.setInvalid();
       }
     }
     if (E->getCastKind() == CK_AddressSpaceConversion && Result.IsNullPtr)
       ZeroInitialization(E);
     return true;
 
   case CK_DerivedToBase:
   case CK_UncheckedDerivedToBase:

clang/lib/Frontend/InitPreprocessor.cpp

static void InitializeCPlusPlusFeatureTestMacros(const LangOptions &LangOpts,
 
   // C++11 features.
   if (LangOpts.CPlusPlus11) {
     Builder.defineMacro("__cpp_unicode_characters", "200704L");
     Builder.defineMacro("__cpp_raw_strings", "200710L");
     Builder.defineMacro("__cpp_unicode_literals", "200710L");
     Builder.defineMacro("__cpp_user_defined_literals", "200809L");
     Builder.defineMacro("__cpp_lambdas", "200907L");
-    Builder.defineMacro("__cpp_constexpr", LangOpts.CPlusPlus23   ? "202211L"
+    Builder.defineMacro("__cpp_constexpr", LangOpts.CPlusPlus26   ? "202306L"
+                                           : LangOpts.CPlusPlus23 ? "202211L"
                                            : LangOpts.CPlusPlus20 ? "201907L"
                                            : LangOpts.CPlusPlus17 ? "201603L"
                                            : LangOpts.CPlusPlus14 ? "201304L"
                                                                   : "200704");
     Builder.defineMacro("__cpp_constexpr_in_decltype", "201711L");
     Builder.defineMacro("__cpp_range_based_for",
                         LangOpts.CPlusPlus17 ? "201603L" : "200907");
     Builder.defineMacro("__cpp_static_assert",

clang/test/CXX/expr/expr.const/p5-26.cpp

+// RUN: %clang_cc1 -fsyntax-only -std=c++2c -verify=expected,cxx26 %s
+// RUN: %clang_cc1 -fsyntax-only -std=c++2b -verify=expected=cxx23 %s
+
+
+struct S {};
+struct T : S {} t;
+
+consteval void test() { // cxx23-error{{consteval function never produces a constant expression}}
+    void* a = &t;
+    const void* b = &t;
+    volatile void* c = &t;
+    (void)static_cast<T*>(a); //cxx23-note {{cast from 'void *' is not allowed in a constant expression in C++ standards before C++2c}}
+    (void)static_cast<const T*>(a);
+    (void)static_cast<volatile T*>(a);
+
+    (void)(T*)(a);
+    (void)(const T*)(a);
+    (void)(volatile T*)(a);
+
+    (void)static_cast<T*>(b); // expected-error {{static_cast from 'const void *' to 'T *' casts away qualifiers}}
+    (void)static_cast<volatile T*>(b); // expected-error {{static_cast from 'const void *' to 'volatile T *' casts away qualifiers}}
+    (void)static_cast<const T*>(b);
+    (void)static_cast<volatile const T*>(b);
+
+    (void)static_cast<T*>(c); // expected-error{{static_cast from 'volatile void *' to 'T *' casts away qualifiers}}
+    (void)static_cast<volatile T*>(c);
+    (void)static_cast<const T*>(b);
+    (void)static_cast<volatile const T*>(b);
+}
+
+void err() {
+    constexpr void* a = &t;
+    constexpr auto err1 = static_cast<int*>(a); // expected-error{{constexpr variable 'err1' must be initialized by a constant expression}} \
+                                                // cxx23-note {{cast from 'void *' is not allowed in a constant expression in C++ standards before C++2c}} \
+                                                // cxx26-note {{cast from 'void *' is not allowed in a constant expression because the pointed object type 'T' is not similar to the target type 'int'}}
+    constexpr auto err2 = static_cast<S*>(a);   // expected-error{{constexpr variable 'err2' must be initialized by a constant expression}} \
+                                                // cxx23-note {{cast from 'void *' is not allowed in a constant expression in C++ standards before C++2c}} \
+                                                // cxx26-note {{cast from 'void *' is not allowed in a constant expression because the pointed object type 'T' is not similar to the target type 'S'}}
+}

clang/test/Lexer/cxx-features.cpp

#if defined(NO_THREADSAFE_STATICS) ? check(threadsafe_static_init, 0, 0, 0, 0, 0, 0, 0) : \
                                      check(threadsafe_static_init, 200806, 200806, 200806, 200806, 200806, 200806, 200806)
 #error "wrong value for __cpp_threadsafe_static_init"
 #endif
 
 #if check(lambdas, 0, 200907, 200907, 200907, 200907, 200907, 200907)
 #error "wrong value for __cpp_lambdas"
 #endif
 
-#if check(constexpr, 0, 200704, 201304, 201603, 201907, 202211, 202211)
+#if check(constexpr, 0, 200704, 201304, 201603, 201907, 202211, 202306)
 #error "wrong value for __cpp_constexpr"
 #endif
 
 #if check(range_based_for, 0, 200907, 200907, 201603, 201603, 201603, 201603)
 #error "wrong value for __cpp_range_based_for"
 #endif
 
 #if check(static_assert, 0, 200410, 200410, 201411, 201411, 201411, 201411)

clang/www/cxx_status.html

  <tr>
   <td>Add @, $, and ` to the basic character set</td>
   <td><a href="https://wg21.link/P2558R2">P2558R2</a></td>
   <td class="full" align="center">Yes</td>
  </tr>
  <tr>
   <td>constexpr cast from <tt>void*</tt></td>
   <td><a href="https://wg21.link/P2738R1">P2738R1</a></td>
-  <td class="none" align="center">No</td>
+  <td class="unreleased" align="center">Clang 17</td>
  </tr>
  <tr>
   <td>On the ignorability of standard attributes</td>
   <td><a href="https://wg21.link/P2552R3">P2552R3</a> (<a href="#dr">DR</a>)</td>
   <td class="none" align="center">No</td>
  </tr>
  <tr>
   <td>Static storage for braced initializers</td>