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

[Marek Polacek <polacek@redhat.com>]

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

-- >8 --
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
will get us an expression whose type is the enum.  (I thought we didn't
need decay_conversion because that does a whole lot more but using it
would make sense to me too.)

	PR c++/111895

gcc/cp/ChangeLog:

	* typeck.cc (build_static_cast_1): Call
	convert_bitfield_to_declared_type.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/scoped_enum12.C: New test.
---
 gcc/cp/typeck.cc                           | 9 +++++++++
 gcc/testsuite/g++.dg/cpp0x/scoped_enum12.C | 8 ++++++++
 2 files changed, 17 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/scoped_enum12.C

diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc
index f3dc80c40cf..50427090e5d 100644
--- a/gcc/cp/typeck.cc
+++ b/gcc/cp/typeck.cc
@@ -8405,6 +8405,15 @@ 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.  We
+	 can let convert_bitfield_to_declared_type convert EXPR to the desired
+	 type.  */
+      expr = convert_bitfield_to_declared_type (expr);
       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
index 00000000000..1d10431e6dc
--- /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); }

base-commit: 99a6c1065de2db04d0f56f4b2cc89acecf21b72e
-- 
2.41.0