[clang] Implement CWG2851: floating-point conversions in converted co…

…nstant expressions

clang/docs/ReleaseNotes.rst

@@ -166,6 +166,9 @@ Resolutions to C++ Defect Reports
 - Clang now diagnoses declarative nested-name-specifiers with pack-index-specifiers.
   (`CWG2858: Declarative nested-name-specifiers and pack-index-specifiers <https://cplusplus.github.io/CWG/issues/2858.html>`_).
 
+- Allow floating-point promotions and conversions in converted constant expressions.
+  (`CWG2851 Allow floating-point conversions in converted constant expressions <https://cplusplus.github.io/CWG/issues/2851.html>`_).
+
 C Language Changes
 ------------------
 

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -85,6 +85,10 @@ def err_expr_not_cce : Error<
   "%select{case value|enumerator value|non-type template argument|"
   "array size|explicit specifier argument|noexcept specifier argument|"
   "call to 'size()'|call to 'data()'}0 is not a constant expression">;
+def err_float_conv_cant_represent : Error<
+  "non-type template argument evaluates to %0 which cannot be "
+  "exactly represented in type %1"
+>;
 def ext_cce_narrowing : ExtWarn<
   "%select{case value|enumerator value|non-type template argument|"
   "array size|explicit specifier argument|noexcept specifier argument|"

clang/lib/Sema/SemaOverload.cpp

@@ -6072,6 +6072,10 @@ static bool CheckConvertedConstantConversions(Sema &S,
   case ICK_Integral_Promotion:
   case ICK_Integral_Conversion: // Narrowing conversions are checked elsewhere.
   case ICK_Zero_Queue_Conversion:
+  // Per CWG2851, floating-point promotions and conversions are allowed.
+  // The value of a conversion is checked afterwards.
+  case ICK_Floating_Promotion:
+  case ICK_Floating_Conversion:
     return true;
 
   case ICK_Boolean_Conversion:
@@ -6091,9 +6095,7 @@ static bool CheckConvertedConstantConversions(Sema &S,
     // only permitted if the source type is std::nullptr_t.
     return SCS.getFromType()->isNullPtrType();
 
-  case ICK_Floating_Promotion:
   case ICK_Complex_Promotion:
-  case ICK_Floating_Conversion:
   case ICK_Complex_Conversion:
   case ICK_Floating_Integral:
   case ICK_Compatible_Conversion:
@@ -6229,7 +6231,37 @@ static ExprResult BuildConvertedConstantExpression(Sema &S, Expr *From,
   if (Result.isInvalid())
     return Result;
 
-  // Check for a narrowing implicit conversion.
+  if (SCS->Second == ICK_Floating_Conversion) {
+    // Unlike with narrowing conversions, the value must fit
+    // exactly even if it is in range
+    assert(CCE == Sema::CCEKind::CCEK_TemplateArg &&
+           "Only non-type template args should use floating-point conversions");
+
+    // Initializer is From, except it is a full-expression
+    const Expr *Initializer =
+        IgnoreNarrowingConversion(S.Context, Result.get());
+
+    // If it's value-dependent, we can't tell whether it will fit
+    if (Initializer->isValueDependent())
+      return Result;
+
+    // Not-constant diagnosed afterwards
+    if (!Initializer->isCXX11ConstantExpr(S.Context, &PreNarrowingValue))
+      return Result;
+
+    llvm::APFloat PostNarrowingValue = PreNarrowingValue.getFloat();
+    bool LosesInfo = true;
+    PostNarrowingValue.convert(S.Context.getFloatTypeSemantics(T),
+                               llvm::APFloat::rmNearestTiesToEven, &LosesInfo);
+
+    if (LosesInfo)
+      S.Diag(From->getBeginLoc(), diag::err_float_conv_cant_represent)
+          << PreNarrowingValue.getAsString(S.Context, From->getType()) << T;
+
+    return Result;
+  }
+
+  // Check for a narrowing integer conversion.
   bool ReturnPreNarrowingValue = false;
   QualType PreNarrowingType;
   switch (SCS->getNarrowingKind(S.Context, Result.get(), PreNarrowingValue,

clang/test/CXX/drs/dr28xx.cpp

@@ -3,6 +3,7 @@
 // RUN: %clang_cc1 -std=c++14 -verify=expected %s
 // RUN: %clang_cc1 -std=c++17 -verify=expected %s
 // RUN: %clang_cc1 -std=c++20 -verify=expected,since-cxx20 %s
+// RUN: %clang_cc1 -std=c++20 -mlong-double-64 -verify=expected,since-cxx20 %s
 // RUN: %clang_cc1 -std=c++23 -verify=expected,since-cxx20,since-cxx23 %s
 // RUN: %clang_cc1 -std=c++2c -verify=expected,since-cxx20,since-cxx23,since-cxx26 %s
 
@@ -67,6 +68,69 @@ void B<int>::g() requires true;
 
 } // namespace cwg2847
 
+namespace cwg2851 { // cwg2851: 19
+
+#if __cplusplus >= 202002L
+template<typename T, T v> struct Val { static constexpr T value = v; };
+
+
+// Floating-point promotions
+
+static_assert(Val<long double, 0.0>::value == 0.0L);
+static_assert(Val<long double, 0.0f>::value == 0.0L);
+static_assert(Val<double, 0.0f>::value == 0.0);
+static_assert(Val<long double, -0.0>::value == -0.0L);
+
+static_assert(!__is_same(Val<long double, -0.0>, Val<long double, 0.0L>));
+static_assert(__is_same(Val<long double, 0.5>, Val<long double, 0.5L>));
+
+static_assert(__is_same(Val<long double, __builtin_inff()>, Val<long double, __builtin_infl()>));
+
+static_assert(__is_same(Val<long double, __builtin_nanf("")>, Val<long double, static_cast<long double>(__builtin_nanf(""))>));
+static_assert(__is_same(Val<long double, __builtin_nansf("")>, Val<long double, static_cast<long double>(__builtin_nansf(""))>));
+static_assert(__is_same(Val<long double, __builtin_nanf("0x1")>, Val<long double, static_cast<long double>(__builtin_nanf("0x1"))>));
+static_assert(__is_same(Val<long double, __builtin_nansf("0x1")>, Val<long double, static_cast<long double>(__builtin_nansf("0x1"))>));
+
+
+// Floating-point conversions where the source value can be represented exactly in the destination type
+
+static_assert(Val<float, 0.0L>::value == 0.0L);
+static_assert(__is_same(Val<float, 0.0>, Val<float, 0.0L>));
+static_assert(__is_same(Val<float, 0.0>, Val<float, 0.0f>));
+static_assert(!__is_same(Val<float, -0.0L>, Val<float, 0.0f>));
+static_assert(__is_same(Val<float, 0.5L>, Val<float, 0.5f>));
+static_assert(__is_same(Val<float, 0.5L>, Val<float, 0.5f>));
+
+static_assert(__is_same(Val<float, double{__FLT_DENORM_MIN__}>, Val<float, __FLT_DENORM_MIN__>));
+Val<float, double{__FLT_DENORM_MIN__} / 2.0> _1;
+// since-cxx20-error-re@-1 {{non-type template argument evaluates to {{.+}} which cannot be exactly represented in type 'float'}}
+Val<float, static_cast<long double>(__FLT_DENORM_MIN__) / 2.0L> _2;
+// since-cxx20-error-re@-1 {{non-type template argument evaluates to {{.+}} which cannot be exactly represented in type 'float'}}
+Val<float, __DBL_MAX__> _3;
+// since-cxx20-error-re@-1 {{non-type template argument evaluates to {{.+}} which cannot be exactly represented in type 'float'}}
+
+static_assert(__is_same(Val<float, __builtin_infl()>, Val<float, __builtin_inff()>));
+
+static_assert(__is_same(Val<float, __builtin_nanl("")>, Val<float, static_cast<float>(__builtin_nanl(""))>));
+static_assert(__is_same(Val<float, __builtin_nansl("")>, Val<float, static_cast<float>(__builtin_nansl(""))>));
+#if __SIZEOF_LONG_DOUBLE__ > 8
+// since-cxx20-error@-2 {{non-type template argument evaluates to nan which cannot be exactly represented in type 'float'}}
+#endif
+// Payload is shifted right so these payloads will be preserved
+static_assert(__is_same(Val<float, __builtin_nan("0xFF00000000")>, Val<float, static_cast<float>(__builtin_nan("0xFF00000000"))>));
+static_assert(__is_same(Val<float, __builtin_nans("0xFF00000000")>, Val<float, static_cast<float>(__builtin_nans("0xFF00000000"))>));
+static_assert(__is_same(Val<float, __builtin_nanl("0x1")>, Val<float, static_cast<float>(__builtin_nanl("0x1"))>));
+// since-cxx20-error@-1 {{non-type template argument evaluates to nan which cannot be exactly represented in type 'float'}}
+static_assert(__is_same(Val<float, __builtin_nansl("0x1")>, Val<float, static_cast<float>(__builtin_nansl("0x1"))>));
+// since-cxx20-error@-1 {{non-type template argument evaluates to nan which cannot be exactly represented in type 'float'}}
+static_assert(__is_same(Val<float, __builtin_nanl("0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF")>, Val<float, static_cast<float>(__builtin_nanl("0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF"))>));
+// since-cxx20-error@-1 {{non-type template argument evaluates to nan which cannot be exactly represented in type 'float'}}
+static_assert(__is_same(Val<float, __builtin_nansl("0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF")>, Val<float, static_cast<float>(__builtin_nansl("0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF"))>));
+// since-cxx20-error@-1 {{non-type template argument evaluates to nan which cannot be exactly represented in type 'float'}}
+#endif
+
+}
+
 namespace cwg2858 { // cwg2858: 19 tentatively ready 2024-04-05
 
 #if __cplusplus > 202302L

clang/www/cxx_dr_status.html

@@ -16915,7 +16915,7 @@ <h2 id="cxxdr">C++ defect report implementation status</h2>
     <td><a href="https://cplusplus.github.io/CWG/issues/2851.html">2851</a></td>
     <td>DR</td>
     <td>Allow floating-point conversions in converted constant expressions</td>
-    <td class="unknown" align="center">Unknown</td>
+    <td class="unreleased" align="center">Clang 19</td>
   </tr>
   <tr class="open" id="2852">
     <td><a href="https://cplusplus.github.io/CWG/issues/2852.html">2852</a></td>