[gcc r14-4902] c++: error with bit-fields and scoped enums [PR111895]

[gcc r14-4902] c++: error with bit-fields and scoped enums [PR111895]

[Marek Polacek]

https://gcc.gnu.org/g:6fa7284e2824310bb7204d41b5243c677ecb62d3

commit r14-4902-g6fa7284e2824310bb7204d41b5243c677ecb62d3
Author: Marek Polacek <polacek@redhat.com>
Date:   Mon Oct 23 17:06:45 2023 -0400

    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:
---
 gcc/cp/typeck.cc                           | 7 +++++++
 gcc/testsuite/g++.dg/cpp0x/scoped_enum12.C | 8 ++++++++
 2 files changed, 15 insertions(+)

diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc
index f3dc80c40cf..3b719326d76 100644
--- 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
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); }