c++: error with bit-fields and scoped enums [PR111895]

Here we issue a bogus error: invalid operands of types 'unsigned char:2'
and 'int' to binary 'operator!=' when casting a bit-field of scoped enum
type to bool.

In build_static_cast_1, perform_direct_initialization_if_possible returns
NULL_TREE, because the invented declaration T t(e) fails, which is
correct.  So we go down to ocp_convert, which has code to deal with this
case:
          /* We can't implicitly convert a scoped enum to bool, so convert
             to the underlying type first.  */
          if (SCOPED_ENUM_P (intype) && (convtype & CONV_STATIC))
            e = build_nop (ENUM_UNDERLYING_TYPE (intype), e);
but the SCOPED_ENUM_P is false since intype is <unnamed-unsigned:2>.
This could be fixed by using unlowered_expr_type.  But then
c_common_truthvalue_conversion/CASE_CONVERT has a similar problem, and
unlowered_expr_type is a C++-only function.

Rather than adding a dummy unlowered_expr_type to C, I think we should
follow [expr.static.cast]p3: "the lvalue-to-rvalue conversion is applied
to the bit-field and the resulting prvalue is used as the operand of the
static_cast."  There are no prvalue bit-fields, so the l-to-r conversion
performed in decay_conversion will get us an expression whose type is the
enum.

	PR c++/111895

gcc/cp/ChangeLog:

	* typeck.cc (build_static_cast_1): Call decay_conversion.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/scoped_enum12.C: New test.

diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc
index f3dc80c40cfad7dbc5a037e28d70e0c8c1bc9721..3b719326d76c2fe74111b46354d37af95098919d 100644 (file)
--- a/gcc/cp/typeck.cc
+++ b/gcc/cp/typeck.cc
@@ -8405,6 +8405,13 @@ build_static_cast_1 (location_t loc, tree type, tree expr, bool c_cast_p,
        return expr;
       if (TREE_CODE (expr) == EXCESS_PRECISION_EXPR)
        expr = TREE_OPERAND (expr, 0);
+      /* [expr.static.cast]: "If the value is not a bit-field, the result
+        refers to the object or the specified base class subobject thereof;
+        otherwise, the lvalue-to-rvalue conversion is applied to the
+        bit-field and the resulting prvalue is used as the operand of the
+        static_cast."  There are no prvalue bit-fields; the l-to-r conversion
+        will give us an object of the underlying type of the bit-field.  */
+      expr = decay_conversion (expr, complain);
       return ocp_convert (type, expr, CONV_C_CAST, LOOKUP_NORMAL, complain);
     }
 

diff --git a/gcc/testsuite/g++.dg/cpp0x/scoped_enum12.C b/gcc/testsuite/g++.dg/cpp0x/scoped_enum12.C
new file mode 100644 (file)
index 0000000..1d10431
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/scoped_enum12.C
@@ -0,0 +1,8 @@
+// PR c++/111895
+// { dg-do compile { target c++11 } }
+
+enum class o_field : unsigned char { no, yes, different_from_s };
+struct fields {
+  o_field o : 2;
+};
+bool func(fields f) { return static_cast<bool>(f.o); }