[Clang] Diagnose ill-formed constant expression when setting a non fixed enum to a value outside the range of the enumeration values

DR2338 clarified that it was undefined behavior to set the value outside the
range of the enumerations values for an enum without a fixed underlying type.

We should diagnose this with a constant expression context.

Differential Revision: https://reviews.llvm.org/D130058

clang/docs/ReleaseNotes.rst

@@ -56,6 +56,10 @@ Bug Fixes
 
 Improvements to Clang's diagnostics
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+- Clang will now correctly diagnose as ill-formed a constant expression where an
+  enum without a fixed underlying type is set to a value outside the range of
+  of the enumerations values. Fixes
+  `Issue 50055: <https://github.com/llvm/llvm-project/issues/50055>`_.
 
 Non-comprehensive list of changes in this release
 -------------------------------------------------

clang/include/clang/AST/Decl.h

@@ -3842,6 +3842,11 @@ public:
   ///                     -101     1001011                     8
   unsigned getNumNegativeBits() const { return EnumDeclBits.NumNegativeBits; }
 
+  /// Calculates the [Min,Max) values the enum can store based on the
+  /// NumPositiveBits and NumNegativeBits. This matters for enums that do not
+  /// have a fixed underlying type.
+  void getValueRange(llvm::APInt &Max, llvm::APInt &Min) const;
+
   /// Returns true if this is a C++11 scoped enumeration.
   bool isScoped() const { return EnumDeclBits.IsScoped; }
 

clang/include/clang/Basic/DiagnosticASTKinds.td

@@ -366,6 +366,9 @@ def note_constexpr_unsupported_layout : Note<
   "type %0 has unexpected layout">;
 def note_constexpr_unsupported_flexible_array : Note<
   "flexible array initialization is not yet supported">;
+def note_constexpr_unscoped_enum_out_of_range : Note<
+  "integer value %0 is outside the valid range of values [%1, %2) for this "
+  "enumeration type">;
 def err_experimental_clang_interp_failed : Error<
   "the experimental clang interpreter failed to evaluate an expression">;
 

clang/lib/AST/Decl.cpp

@@ -4621,6 +4621,21 @@ SourceRange EnumDecl::getSourceRange() const {
   return Res;
 }
 
+void EnumDecl::getValueRange(llvm::APInt &Max, llvm::APInt &Min) const {
+  unsigned Bitwidth = getASTContext().getIntWidth(getIntegerType());
+  unsigned NumNegativeBits = getNumNegativeBits();
+  unsigned NumPositiveBits = getNumPositiveBits();
+
+  if (NumNegativeBits) {
+    unsigned NumBits = std::max(NumNegativeBits, NumPositiveBits + 1);
+    Max = llvm::APInt(Bitwidth, 1) << (NumBits - 1);
+    Min = -Max;
+  } else {
+    Max = llvm::APInt(Bitwidth, 1) << NumPositiveBits;
+    Min = llvm::APInt::getZero(Bitwidth);
+  }
+}
+
 //===----------------------------------------------------------------------===//
 // RecordDecl Implementation
 //===----------------------------------------------------------------------===//

clang/lib/AST/ExprConstant.cpp

@@ -13519,6 +13519,37 @@ bool IntExprEvaluator::VisitCastExpr(const CastExpr *E) {
       return Info.Ctx.getTypeSize(DestType) == Info.Ctx.getTypeSize(SrcType);
     }
 
+    if (DestType->isEnumeralType()) {
+      const EnumType *ET = dyn_cast<EnumType>(DestType.getCanonicalType());
+      const EnumDecl *ED = ET->getDecl();
+      // Check that the value is within the range of the enumeration values.
+      //
+      // This corressponds to [expr.static.cast]p10 which says:
+      // A value of integral or enumeration type can be explicitly converted
+      // to a complete enumeration type ... If the enumeration type does not
+      // have a fixed underlying type, the value is unchanged if the original
+      // value is within the range of the enumeration values ([dcl.enum]), and
+      // otherwise, the behavior is undefined.
+      //
+      // This was resolved as part of DR2338 which has CD5 status.
+      if (!ED->isFixed()) {
+        llvm::APInt Min;
+        llvm::APInt Max;
+
+        ED->getValueRange(Max, Min);
+
+        if (ED->getNumNegativeBits() &&
+            (Max.sle(Result.getInt().getSExtValue()) ||
+             Min.sgt(Result.getInt().getSExtValue())))
+          CCEDiag(E, diag::note_constexpr_unscoped_enum_out_of_range)
+              << Result.getInt() << Min.getSExtValue() << Max.getSExtValue();
+        else if (!ED->getNumNegativeBits() &&
+                 Max.ule(Result.getInt().getZExtValue()))
+          CCEDiag(E, diag::note_constexpr_unscoped_enum_out_of_range)
+              << Result.getInt() << Min.getZExtValue() << Max.getZExtValue();
+      }
+    }
+
     return Success(HandleIntToIntCast(Info, E, DestType, SrcType,
                                       Result.getInt()), E);
   }

clang/lib/CodeGen/CGExpr.cpp

@@ -1661,21 +1661,7 @@ static bool getRangeForType(CodeGenFunction &CGF, QualType Ty,
     End = llvm::APInt(CGF.getContext().getTypeSize(Ty), 2);
   } else {
     const EnumDecl *ED = ET->getDecl();
-    llvm::Type *LTy = CGF.ConvertTypeForMem(ED->getIntegerType());
-    unsigned Bitwidth = LTy->getScalarSizeInBits();
-    unsigned NumNegativeBits = ED->getNumNegativeBits();
-    unsigned NumPositiveBits = ED->getNumPositiveBits();
-
-    if (NumNegativeBits) {
-      unsigned NumBits = std::max(NumNegativeBits, NumPositiveBits + 1);
-      assert(NumBits <= Bitwidth);
-      End = llvm::APInt(Bitwidth, 1) << (NumBits - 1);
-      Min = -End;
-    } else {
-      assert(NumPositiveBits <= Bitwidth);
-      End = llvm::APInt(Bitwidth, 1) << NumPositiveBits;
-      Min = llvm::APInt::getZero(Bitwidth);
-    }
+    ED->getValueRange(End, Min);
   }
   return true;
 }

clang/test/Analysis/cfg.cpp

@@ -223,7 +223,7 @@ namespace NoReturnSingleSuccessor {
 // CHECK-NEXT:    Succs (1): B1
 // CHECK:  [B0 (EXIT)]
 // CHECK-NEXT:    Preds (1): B1
-enum MyEnum { A, B, C };
+enum MyEnum : int { A, B, C };
 static const enum MyEnum D = (enum MyEnum) 32;
 
 int test_enum_with_extension(enum MyEnum value) {

clang/test/Sema/aarch64-sve-intrinsics/acle_sve_imm.cpp

@@ -206,19 +206,19 @@ void test_range_0_13(svbool_t pg, const void *const_void_ptr)
 {
   // expected-error-re@+1 {{argument value {{[0-9]+}} is outside the valid range [0, 13]}}
   svprfb(pg, const_void_ptr, svprfop(14));
-  // expected-error-re@+1 {{argument value {{[0-9]+}} is outside the valid range [0, 13]}}
+  // expected-error-re@+1 {{must be a constant integer}}
   svprfb_vnum(pg, const_void_ptr, 0, svprfop(-1));
   // expected-error-re@+1 {{argument value {{[0-9]+}} is outside the valid range [0, 13]}}
   svprfd(pg, const_void_ptr, svprfop(14));
-  // expected-error-re@+1 {{argument value {{[0-9]+}} is outside the valid range [0, 13]}}
+  // expected-error-re@+1 {{must be a constant integer}}
   svprfd_vnum(pg, const_void_ptr, 0, svprfop(-1));
   // expected-error-re@+1 {{argument value {{[0-9]+}} is outside the valid range [0, 13]}}
   svprfh(pg, const_void_ptr, svprfop(14));
-  // expected-error-re@+1 {{argument value {{[0-9]+}} is outside the valid range [0, 13]}}
+  // expected-error-re@+1 {{must be a constant integer}}
   svprfh_vnum(pg, const_void_ptr, 0, svprfop(-1));
   // expected-error-re@+1 {{argument value {{[0-9]+}} is outside the valid range [0, 13]}}
   svprfw(pg, const_void_ptr, svprfop(14));
-  // expected-error-re@+1 {{argument value {{[0-9]+}} is outside the valid range [0, 13]}}
+  // expected-error-re@+1 {{must be a constant integer}}
   svprfw_vnum(pg, const_void_ptr, 0, svprfop(-1));
 }
 

clang/test/SemaCXX/constant-expression-cxx11.cpp

@@ -2399,3 +2399,50 @@ void local_constexpr_var() {
   constexpr int a = 0; // expected-note {{address of non-static constexpr variable 'a' may differ on each invocation of the enclosing function; add 'static' to give it a constant address}}
   constexpr const int *p = &a; // expected-error {{constant expression}} expected-note {{pointer to 'a' is not a constant expression}}
 }
+
+namespace GH50055 {
+// Enums without fixed underlying type
+enum E1 {e11=-4, e12=4};
+enum E2 {e21=0, e22=4};
+enum E3 {e31=-4, e32=1024};
+enum E4 {e41=0};
+// Empty but as-if it had a single enumerator with value 0
+enum EEmpty {};
+
+// Enum with fixed underlying type because the underlying type is explicitly specified
+enum EFixed : int {efixed1=-4, efixed2=4};
+// Enum with fixed underlying type because it is scoped
+enum class EScoped {escoped1=-4, escoped2=4};
+
+void testValueInRangeOfEnumerationValues() {
+  constexpr E1 x1 = static_cast<E1>(-8);
+  constexpr E1 x2 = static_cast<E1>(8); // expected-error {{must be initialized by a constant expression}}
+  // expected-note@-1 {{integer value 8 is outside the valid range of values [-8, 8) for this enumeration type}}
+
+  constexpr E2 x3 = static_cast<E2>(-8); // expected-error {{must be initialized by a constant expression}}
+  // expected-note@-1 {{integer value -8 is outside the valid range of values [0, 8) for this enumeration type}}
+  constexpr E2 x4 = static_cast<E2>(0);
+  constexpr E2 x5 = static_cast<E2>(8); // expected-error {{must be initialized by a constant expression}}
+  // expected-note@-1 {{integer value 8 is outside the valid range of values [0, 8) for this enumeration type}}
+
+  constexpr E3 x6 = static_cast<E3>(-2048);
+  constexpr E3 x7 = static_cast<E3>(-8);
+  constexpr E3 x8 = static_cast<E3>(0);
+  constexpr E3 x9 = static_cast<E3>(8);
+  constexpr E3 x10 = static_cast<E3>(2048); // expected-error {{must be initialized by a constant expression}}
+  // expected-note@-1 {{integer value 2048 is outside the valid range of values [-2048, 2048) for this enumeration type}}
+
+  constexpr E4 x11 = static_cast<E4>(0);
+  constexpr E4 x12 = static_cast<E4>(1);
+  constexpr E4 x13 = static_cast<E4>(2); // expected-error {{must be initialized by a constant expression}}
+  // expected-note@-1 {{integer value 2 is outside the valid range of values [0, 2) for this enumeration type}}
+
+  constexpr EEmpty x14 = static_cast<EEmpty>(0);
+  constexpr EEmpty x15 = static_cast<EEmpty>(1);
+  constexpr EEmpty x16 = static_cast<EEmpty>(2); // expected-error {{must be initialized by a constant expression}}
+  // expected-note@-1 {{integer value 2 is outside the valid range of values [0, 2) for this enumeration type}}
+
+  constexpr EFixed x17 = static_cast<EFixed>(100);
+  constexpr EScoped x18 = static_cast<EScoped>(100);
+}
+}

clang/test/SemaCXX/enum-scoped.cpp

@@ -327,7 +327,7 @@ namespace test11 {
 }
 
 namespace PR35586 {
-  enum C { R, G, B };
+  enum C { R=-1, G, B };
   enum B { F = (enum C) -1, T}; // this should compile cleanly, it used to assert.
 };
 

clang/test/SemaTemplate/temp_arg_enum_printing.cpp

@@ -3,7 +3,7 @@
 namespace NamedEnumNS
 {
   
-enum NamedEnum
+enum class NamedEnum
 {
   Val0,
   Val1
@@ -13,9 +13,9 @@ template <NamedEnum E>
 void foo();
   
 void test() {
-  // CHECK: template<> void foo<NamedEnumNS::Val0>()
-  NamedEnumNS::foo<Val0>();
-  // CHECK: template<> void foo<NamedEnumNS::Val1>()
+  // CHECK: template<> void foo<NamedEnumNS::NamedEnum::Val0>()
+  NamedEnumNS::foo<NamedEnum::Val0>();
+  // CHECK: template<> void foo<NamedEnumNS::NamedEnum::Val1>()
   NamedEnumNS::foo<(NamedEnum)1>();
   // CHECK: template<> void foo<(NamedEnumNS::NamedEnum)2>()
   NamedEnumNS::foo<(NamedEnum)2>();

clang/www/cxx_dr_status.html

@@ -13842,7 +13842,7 @@ and <I>POD class</I></td>
     <td><a href="https://wg21.link/cwg2338">2338</a></td>
     <td>CD5</td>
     <td>Undefined behavior converting to short enums with fixed underlying types</td>
-    <td class="full" align="center">Clang 12</td>
+    <td class="full" align="center">Clang 16</td>
   </tr>
   <tr id="2339">
     <td><a href="https://wg21.link/cwg2339">2339</a></td>

cross-project-tests/debuginfo-tests/clang_llvm_roundtrip/simplified_template_names.cpp

@@ -25,9 +25,9 @@ struct udt { };
 }
 template<template<typename> class T>
 void ttp_user() { }
-enum Enumeration { Enumerator1, Enumerator2, Enumerator3 = 1 };
+enum Enumeration : int { Enumerator1, Enumerator2, Enumerator3 = 1 };
 enum class EnumerationClass { Enumerator1, Enumerator2, Enumerator3 = 1 };
-enum { AnonEnum1, AnonEnum2, AnonEnum3 = 1 };
+enum : int { AnonEnum1, AnonEnum2, AnonEnum3 = 1 };
 enum EnumerationSmall : unsigned char { kNeg = 0xff };
 }
 template <typename... Ts>