Mention requirements in FieldDecl::getBitwidthValue()

Author: tbaederr

clang/include/clang/AST/Decl.h

@@ -3143,6 +3143,8 @@ class FieldDecl : public DeclaratorDecl, public Mergeable<FieldDecl> {
 
   /// Computes the bit width of this field, if this is a bit field.
   /// May not be called on non-bitfields.
+  /// Note that in order to successfully use this function, the bitwidth
+  /// expression must be a ConstantExpr with a valid integer result set.
   unsigned getBitWidthValue() const;
 
   /// Set the bit-field width for this member.

clang/lib/AST/ByteCode/InterpBuiltinBitCast.cpp

@@ -414,7 +414,7 @@ bool clang::interp::DoBitCastPtr(InterpState &S, CodePtr OpPC,
 
         Bits BitWidth;
         if (const FieldDecl *FD = P.getField(); FD && FD->isBitField())
-          BitWidth = Bits(std::min(FD->getBitWidthValue(ASTCtx),
+          BitWidth = Bits(std::min(FD->getBitWidthValue(),
                                    (unsigned)FullBitWidth.getQuantity()));
         else if (T == PT_Bool && PackedBools)
           BitWidth = Bits(1);

clang/lib/AST/Decl.cpp

@@ -4603,6 +4603,7 @@ unsigned FieldDecl::getBitWidthValue() const {
   assert(isBitField() && "not a bitfield");
   assert(isa<ConstantExpr>(getBitWidth()));
   assert(cast<ConstantExpr>(getBitWidth())->hasAPValueResult());
+  assert(cast<ConstantExpr>(getBitWidth())->getAPValueResult().isInt());
   return cast<ConstantExpr>(getBitWidth())
       ->getAPValueResult()
       .getInt()