[clang] Implement CWG2627 Bit-fields and narrowing conversions

clang/docs/ReleaseNotes.rst

@@ -237,6 +237,11 @@ Resolutions to C++ Defect Reports
 - P0522 implementation is enabled by default in all language versions, and
   provisional wording for CWG2398 is implemented.
 
+- Casts from a bit-field to an integral type is now not considered narrowing if the
+  width of the bit-field means that all potential values are in the range
+  of the target type, even if the type of the bit-field is larger.
+  (`CWG2627. Bit-fields and narrowing conversions  <https://cplusplus.github.io/CWG/issues/2627.html>`_).
+
 C Language Changes
 ------------------
 

clang/lib/Sema/SemaOverload.cpp

@@ -464,61 +464,97 @@ NarrowingKind StandardConversionSequence::getNarrowingKind(
 
   // -- from an integer type or unscoped enumeration type to an integer type
   //    that cannot represent all the values of the original type, except where
-  //    the source is a constant expression and the actual value after
+  //    (CWG2627) -- the source is a bit-field whose width w is less than that
+  //    of its type (or, for an enumeration type, its underlying type) and the
+  //    target type can represent all the values of a hypothetical extended
+  //    integer type with width w and with the same signedness as the original
+  //    type or
+  //    -- the source is a constant expression and the actual value after
   //    conversion will fit into the target type and will produce the original
   //    value when converted back to the original type.
   case ICK_Integral_Conversion:
   IntegralConversion: {
     assert(FromType->isIntegralOrUnscopedEnumerationType());
     assert(ToType->isIntegralOrUnscopedEnumerationType());
     const bool FromSigned = FromType->isSignedIntegerOrEnumerationType();
-    const unsigned FromWidth = Ctx.getIntWidth(FromType);
+    unsigned FromWidth = Ctx.getIntWidth(FromType);
     const bool ToSigned = ToType->isSignedIntegerOrEnumerationType();
     const unsigned ToWidth = Ctx.getIntWidth(ToType);
 
-    if (FromWidth > ToWidth ||
-        (FromWidth == ToWidth && FromSigned != ToSigned) ||
-        (FromSigned && !ToSigned)) {
-      // Not all values of FromType can be represented in ToType.
-      const Expr *Initializer = IgnoreNarrowingConversion(Ctx, Converted);
+    constexpr auto CanRepresentAll = [](bool FromSigned, unsigned FromWidth,
+                                        bool ToSigned, unsigned ToWidth) {
+      return (FromWidth < ToWidth + (FromSigned == ToSigned)) &&
+             (FromSigned <= ToSigned);
+    };
 
-      // If it's value-dependent, we can't tell whether it's narrowing.
-      if (Initializer->isValueDependent())
-        return NK_Dependent_Narrowing;
+    if (CanRepresentAll(FromSigned, FromWidth, ToSigned, ToWidth))
+      return NK_Not_Narrowing;
 
-      std::optional<llvm::APSInt> OptInitializerValue;
-      if (!(OptInitializerValue = Initializer->getIntegerConstantExpr(Ctx))) {
-        // Such conversions on variables are always narrowing.
-        return NK_Variable_Narrowing;
-      }
-      llvm::APSInt &InitializerValue = *OptInitializerValue;
-      bool Narrowing = false;
-      if (FromWidth < ToWidth) {
-        // Negative -> unsigned is narrowing. Otherwise, more bits is never
-        // narrowing.
-        if (InitializerValue.isSigned() && InitializerValue.isNegative())
-          Narrowing = true;
-      } else {
-        // Add a bit to the InitializerValue so we don't have to worry about
-        // signed vs. unsigned comparisons.
-        InitializerValue = InitializerValue.extend(
-          InitializerValue.getBitWidth() + 1);
-        // Convert the initializer to and from the target width and signed-ness.
-        llvm::APSInt ConvertedValue = InitializerValue;
-        ConvertedValue = ConvertedValue.trunc(ToWidth);
-        ConvertedValue.setIsSigned(ToSigned);
-        ConvertedValue = ConvertedValue.extend(InitializerValue.getBitWidth());
-        ConvertedValue.setIsSigned(InitializerValue.isSigned());
-        // If the result is different, this was a narrowing conversion.
-        if (ConvertedValue != InitializerValue)
-          Narrowing = true;
-      }
-      if (Narrowing) {
-        ConstantType = Initializer->getType();
-        ConstantValue = APValue(InitializerValue);
-        return NK_Constant_Narrowing;
+    // Not all values of FromType can be represented in ToType.
+    const Expr *Initializer = IgnoreNarrowingConversion(Ctx, Converted);
+
+    bool DependentBitField = false;
+    if (const FieldDecl *BitField = Initializer->getSourceBitField()) {
+      if (BitField->getBitWidth()->isValueDependent())
+        DependentBitField = true;
+      else {
+        unsigned BitFieldWidth = BitField->getBitWidthValue(Ctx);
+        if (CanRepresentAll(FromSigned, BitFieldWidth, ToSigned, ToWidth)) {
+          assert(BitFieldWidth < FromWidth &&
+                 "Oversized bit-field can fit in target type but smaller field "
+                 "type couldn't?");
+          return NK_Not_Narrowing;
+        }
+
+        // The initializer will be truncated to the bit-field width
+        FromWidth = BitFieldWidth;
       }
     }
+
+    // If it's value-dependent, we can't tell whether it's narrowing.
+    if (Initializer->isValueDependent())
+      return NK_Dependent_Narrowing;
+
+    std::optional<llvm::APSInt> OptInitializerValue =
+        Initializer->getIntegerConstantExpr(Ctx);
+    if (!OptInitializerValue) {
+      // If the bit-field width was dependent, it might end up being small
+      // enough to fit in the target type (unless the target type is unsigned
+      // and the source type is signed, in which case it will never fit)
+      if (DependentBitField && (FromSigned <= ToSigned))
+        return NK_Dependent_Narrowing;
+
+      // Otherwise, such a conversion is always narrowing
+      return NK_Variable_Narrowing;
+    }
+    llvm::APSInt &InitializerValue = *OptInitializerValue;
+    bool Narrowing = false;
+    if (FromWidth < ToWidth) {
+      // Negative -> unsigned is narrowing. Otherwise, more bits is never
+      // narrowing.
+      if (InitializerValue.isSigned() && InitializerValue.isNegative())
+        Narrowing = true;
+    } else {
+      // Add a bit to the InitializerValue so we don't have to worry about
+      // signed vs. unsigned comparisons.
+      InitializerValue =
+          InitializerValue.extend(InitializerValue.getBitWidth() + 1);
+      // Convert the initializer to and from the target width and signed-ness.
+      llvm::APSInt ConvertedValue = InitializerValue;
+      ConvertedValue = ConvertedValue.trunc(ToWidth);
+      ConvertedValue.setIsSigned(ToSigned);
+      ConvertedValue = ConvertedValue.extend(InitializerValue.getBitWidth());
+      ConvertedValue.setIsSigned(InitializerValue.isSigned());
+      // If the result is different, this was a narrowing conversion.
+      if (ConvertedValue != InitializerValue)
+        Narrowing = true;
+    }
+    if (Narrowing) {
+      ConstantType = Initializer->getType();
+      ConstantValue = APValue(InitializerValue);
+      return NK_Constant_Narrowing;
+    }
+
     return NK_Not_Narrowing;
   }
   case ICK_Complex_Real:

clang/test/CXX/dcl.decl/dcl.init/dcl.init.list/p7-0x.cpp

@@ -203,6 +203,30 @@ void shrink_int() {
   unsigned short usc1 = { c }; // expected-error {{non-constant-expression cannot be narrowed from type 'signed char'}} expected-note {{silence}}
   unsigned short usc2 = { (signed char)'x' }; // OK
   unsigned short usc3 = { (signed char)-1 }; // expected-error {{ -1 which cannot be narrowed}} expected-note {{silence}}
+
+#if __BITINT_MAXWIDTH__ >= 3
+  _BitInt(2) S2 = 0;
+  unsigned _BitInt(2) U2 = 0;
+  _BitInt(3) S3 = 0;
+  unsigned _BitInt(3) U3 = 0;
+
+           _BitInt(2) bi0 = { S2 };
+           _BitInt(2) bi1 = { U2 }; // expected-error {{cannot be narrowed from type 'unsigned _BitInt(2)'}}
+           _BitInt(2) bi2 = { S3 }; // expected-error {{cannot be narrowed from type '_BitInt(3)'}}
+           _BitInt(2) bi3 = { U3 }; // expected-error {{cannot be narrowed from type 'unsigned _BitInt(3)'}}
+  unsigned _BitInt(2) bi4 = { S2 }; // expected-error {{cannot be narrowed from type '_BitInt(2)'}}
+  unsigned _BitInt(2) bi5 = { U2 };
+  unsigned _BitInt(2) bi6 = { S3 }; // expected-error {{cannot be narrowed from type '_BitInt(3)'}}
+  unsigned _BitInt(2) bi7 = { U3 }; // expected-error {{cannot be narrowed from type 'unsigned _BitInt(3)'}}
+           _BitInt(3) bi8 = { S2 };
+           _BitInt(3) bi9 = { U2 };
+           _BitInt(3) bia = { S3 };
+           _BitInt(3) bib = { U3 }; // expected-error {{cannot be narrowed from type 'unsigned _BitInt(3)'}}
+  unsigned _BitInt(3) bic = { S2 }; // expected-error {{cannot be narrowed from type '_BitInt(2)'}}
+  unsigned _BitInt(3) bid = { U2 };
+  unsigned _BitInt(3) bie = { S3 }; // expected-error {{cannot be narrowed from type '_BitInt(3)'}}
+  unsigned _BitInt(3) bif = { U3 };
+#endif
 }
 
 // Be sure that type- and value-dependent expressions in templates get the error

clang/test/CXX/drs/cwg26xx.cpp

@@ -6,6 +6,26 @@
 // RUN: %clang_cc1 -std=c++23 -triple x86_64-unknown-unknown %s -verify=expected,since-cxx11,since-cxx20,since-cxx23
 // RUN: %clang_cc1 -std=c++2c -triple x86_64-unknown-unknown %s -verify=expected,since-cxx11,since-cxx20,since-cxx23
 
+namespace std {
+#if __cplusplus >= 202002L
+  struct strong_ordering {
+    int n;
+    constexpr operator int() const { return n; }
+    static const strong_ordering less, equal, greater;
+  };
+  constexpr strong_ordering strong_ordering::less{-1},
+      strong_ordering::equal{0}, strong_ordering::greater{1};
+#endif
+
+  typedef __INT16_TYPE__ int16_t;
+  typedef __UINT16_TYPE__ uint16_t;
+  typedef __INT32_TYPE__ int32_t;
+  typedef __UINT32_TYPE__ uint32_t;
+  typedef __INT64_TYPE__ int64_t;
+  typedef __UINT64_TYPE__ uint64_t;
+
+  template<typename T> T declval();
+}
 
 namespace cwg2621 { // cwg2621: 16
 #if __cplusplus >= 202002L
@@ -24,6 +44,103 @@ using enum E;
 #endif
 }
 
+namespace cwg2627 { // cwg2627: 19
+#if __cplusplus >= 202002L
+struct C {
+  long long i : 8;
+  friend auto operator<=>(C, C) = default;
+};
+
+void f() {
+  C x{1}, y{2};
+  static_cast<void>(x <=> y);
+  static_cast<void>(x.i <=> y.i);
+}
+
+template<typename T>
+struct CDependent {
+  T i : 8;
+  friend auto operator<=>(CDependent, CDependent) = default;
+};
+
+template<typename T>
+concept three_way_comparable = requires(T t) { { t <=> t }; };
+template<typename T>
+concept bf_three_way_comparable = requires(T t) { { t.i <=> t.i }; };
+static_assert(three_way_comparable<CDependent<long long>>);
+static_assert(bf_three_way_comparable<CDependent<long long>>);
+#endif
+
+#if __cplusplus >= 201103L
+template<int W>
+struct D {
+  __int128 i : W;
+};
+
+template<int W>
+std::int64_t f(D<W> d) {
+    return std::int64_t{ d.i }; // #cwg2627-f
+}
+
+template std::int64_t f(D<63>);
+template std::int64_t f(D<64>);
+template std::int64_t f(D<65>);
+// since-cxx11-error-re@#cwg2627-f {{non-constant-expression cannot be narrowed from type '__int128' to 'std::int64_t' (aka '{{.+}}') in initializer list}}
+//   since-cxx11-note@-2 {{in instantiation of function template specialization 'cwg2627::f<65>' requested here}}
+//   since-cxx11-note@#cwg2627-f {{insert an explicit cast to silence this issue}}
+
+template<typename Target, typename Source>
+Target g(Source x) {
+    return Target{ x.i }; // #cwg2627-g
+}
+
+template<typename T, int N>
+struct E {
+  T i : N;
+};
+
+template std::int16_t g(E<int, 16>);
+template std::int16_t g(E<unsigned, 15>);
+template std::int16_t g(E<unsigned, 16>);
+// since-cxx11-error-re@#cwg2627-g {{non-constant-expression cannot be narrowed from type 'unsigned int' to '{{.+}}' in initializer list}}
+//   since-cxx11-note-re@-2 {{in instantiation of function template specialization 'cwg2627::g<{{.+}}, cwg2627::E<unsigned int, 16>>' requested here}}
+//   since-cxx11-note@#cwg2627-g {{insert an explicit cast to silence this issue}}
+template std::uint16_t g(E<unsigned, 16>);
+template std::uint16_t g(E<int, 1>);
+// since-cxx11-error-re@#cwg2627-g {{non-constant-expression cannot be narrowed from type 'int' to '{{.+}}' in initializer list}}
+//   since-cxx11-note-re@-2 {{in instantiation of function template specialization 'cwg2627::g<{{.+}}, cwg2627::E<int, 1>>' requested here}}
+//   since-cxx11-note@#cwg2627-g {{insert an explicit cast to silence this issue}}
+
+template bool g(E<unsigned, 1>);
+template bool g(E<int, 1>);
+// since-cxx11-error@#cwg2627-g {{non-constant-expression cannot be narrowed from type 'int' to 'bool' in initializer list}}
+//   since-cxx11-note@-2 {{in instantiation of function template specialization 'cwg2627::g<bool, cwg2627::E<int, 1>>' requested here}}
+//   since-cxx11-note@#cwg2627-g {{insert an explicit cast to silence this issue}}
+
+template<typename Target, typename Source>
+constexpr decltype(Target{ std::declval<Source>().i }, false) is_narrowing(int) { return false; }
+template<typename Target, typename Source>
+constexpr bool is_narrowing(long) { return true; }
+
+static_assert(!is_narrowing<std::int16_t, E<int, 16>>(0), "");
+static_assert(!is_narrowing<std::int16_t, E<unsigned, 15>>(0), "");
+static_assert(is_narrowing<std::int16_t, E<unsigned, 16>>(0), "");
+static_assert(!is_narrowing<std::uint16_t, E<unsigned, 16>>(0), "");
+static_assert(is_narrowing<std::uint16_t, E<int, 1>>(0), "");
+static_assert(!is_narrowing<bool, E<unsigned, 1>>(0), "");
+static_assert(is_narrowing<bool, E<int, 1>>(0), "");
+
+template<int N>
+struct F {
+  signed int x : N;
+  decltype(std::int16_t{ x }) dependent_narrowing;
+  decltype(unsigned{ x }) always_narrowing;
+// since-cxx11-error@-1 {{non-constant-expression cannot be narrowed from type 'int' to 'unsigned int' in initializer list}}
+//   since-cxx11-note@-2 {{insert an explicit cast to silence this issue}}
+};
+#endif
+}
+
 namespace cwg2628 { // cwg2628: no
                    // this was reverted for the 16.x release
                    // due to regressions, see the issue for more details:

clang/www/cxx_dr_status.html

@@ -15570,7 +15570,7 @@ <h2 id="cxxdr">C++ defect report implementation status</h2>
     <td><a href="https://cplusplus.github.io/CWG/issues/2627.html">2627</a></td>
     <td>C++23</td>
     <td>Bit-fields and narrowing conversions</td>
-    <td class="unknown" align="center">Unknown</td>
+    <td class="unreleased" align="center">Clang 19</td>
   </tr>
   <tr id="2628">
     <td><a href="https://cplusplus.github.io/CWG/issues/2628.html">2628</a></td>