[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

1 files changed, 28 insertions, 0 deletions

diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index 9d92c84..0f15859 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -13519,6 +13519,34 @@ bool IntExprEvaluator::VisitCastExpr(const CastExpr *E) {
       return Info.Ctx.getTypeSize(DestType) == Info.Ctx.getTypeSize(SrcType);
     }
 
+    if (const EnumType *ET = dyn_cast<EnumType>(DestType)) {
+      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()) || Min.sgt(Result.getInt())))
+          CCEDiag(E, diag::note_constexpr_unscoped_enum_out_of_range)
+              << Result.getInt() << Min.getSExtValue() << Max.getSExtValue();
+        else if (!ED->getNumNegativeBits() && Max.ule(Result.getInt()))
+          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);
   }