* [PATCH] c++: error with bit-fields and scoped enums [PR111895]
@ 2023-10-24 16:18 Marek Polacek
2023-10-24 20:46 ` Jason Merrill
0 siblings, 1 reply; 3+ messages in thread
From: Marek Polacek @ 2023-10-24 16:18 UTC (permalink / raw)
To: Jason Merrill, GCC Patches
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
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH] c++: error with bit-fields and scoped enums [PR111895]
2023-10-24 16:18 [PATCH] c++: error with bit-fields and scoped enums [PR111895] Marek Polacek
@ 2023-10-24 20:46 ` Jason Merrill
2023-10-24 20:56 ` Marek Polacek
0 siblings, 1 reply; 3+ messages in thread
From: Jason Merrill @ 2023-10-24 20:46 UTC (permalink / raw)
To: Marek Polacek, GCC Patches
On 10/24/23 12:18, Marek Polacek wrote:
> 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.)
It's possible that we might want some of that more, particularly
mark_rvalue_use; decay_conversion seems like the right answer. OK with
that change.
rvalue() would also make sense, though that seems to be missing a call
to unlowered_expr_type at the moment. In fact, after "otherwise, it's
the lvalue-to-rvalue conversion" in decay_conv should probably just be a
call to rvalue, with missing bits added to the latter function.
Jason
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH] c++: error with bit-fields and scoped enums [PR111895]
2023-10-24 20:46 ` Jason Merrill
@ 2023-10-24 20:56 ` Marek Polacek
0 siblings, 0 replies; 3+ messages in thread
From: Marek Polacek @ 2023-10-24 20:56 UTC (permalink / raw)
To: Jason Merrill; +Cc: GCC Patches
On Tue, Oct 24, 2023 at 04:46:02PM -0400, Jason Merrill wrote:
> On 10/24/23 12:18, Marek Polacek wrote:
> > 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.)
>
> It's possible that we might want some of that more, particularly
> mark_rvalue_use; decay_conversion seems like the right answer. OK with that
> change.
Makes total sense, thank you. (I'd tested the version with decay_conversion
and it worked fine.)
> rvalue() would also make sense, though that seems to be missing a call to
> unlowered_expr_type at the moment. In fact, after "otherwise, it's the
> lvalue-to-rvalue conversion" in decay_conv should probably just be a call to
> rvalue, with missing bits added to the latter function.
Sounds good; I hope I'll get to it next week. I'm not going to make it
part of this patch so that I can backport this one to 13 and leave the
cleanup for trunk only.
Marek
^ permalink raw reply [flat|nested] 3+ messages in thread
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