[PATCH v2] c++: CWG 2229, cv-qualified unnamed bit-fields [PR123935]

[Marek Polacek]

On Tue, Feb 03, 2026 at 02:16:34PM -0800, Andrew Pinski wrote:
> On Tue, Feb 3, 2026 at 2:12 PM Marek Polacek <pola...@redhat.com> wrote:
> >
> > Bootstrapped/regtested on ppc64le-pc-linux-gnu, ok for 17.0?
> >
> > -- >8 --
> > This implements [class.bit]/2: An unnamed bit-field shall not be
> > declared with a cv-qualified type.  This was clarified in DR 2229.
> >
> >         DR 2229
> >         PR c++/123935
> >
> > gcc/cp/ChangeLog:
> >
> >         * decl2.cc (grokbitfield): Disallow cv-qualified unnamed bit-fields.
> >
> > gcc/testsuite/ChangeLog:
> >
> >         * g++.dg/DRs/dr2229.C: New test.
> > ---
> >  gcc/cp/decl2.cc                   |  9 +++++++++
> >  gcc/testsuite/g++.dg/DRs/dr2229.C | 12 ++++++++++++
> >  2 files changed, 21 insertions(+)
> >  create mode 100644 gcc/testsuite/g++.dg/DRs/dr2229.C
> >
> > diff --git a/gcc/cp/decl2.cc b/gcc/cp/decl2.cc
> > index 20ee662eea2..eb2294fffd0 100644
> > --- a/gcc/cp/decl2.cc
> > +++ b/gcc/cp/decl2.cc
> > @@ -1635,6 +1635,15 @@ grokbitfield (const cp_declarator *declarator,
> >        return NULL_TREE;
> >      }
> >
> > +  /* [class.bit]/2 "An unnamed bit-field shall not be declared with
> > +     a cv-qualified type."  */
> > +  if (!DECL_NAME (value) && TYPE_QUALS (type) != TYPE_UNQUALIFIED)
> > +    {
> > +      error_at (DECL_SOURCE_LOCATION (value),
> > +               "unnamed bit-field cannot be cv-qualified");
> > +      return NULL_TREE;
> > +    }
> > +
> >    int flags = LOOKUP_IMPLICIT;
> >    if (init && DIRECT_LIST_INIT_P (init))
> >      flags = LOOKUP_NORMAL;
> > diff --git a/gcc/testsuite/g++.dg/DRs/dr2229.C 
> > b/gcc/testsuite/g++.dg/DRs/dr2229.C
> > new file mode 100644
> > index 00000000000..32548071090
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/DRs/dr2229.C
> > @@ -0,0 +1,12 @@
> > +// DR 2229, Volatile unnamed bit-fields
> > +// PR c++/123935
> > +
> > +struct S {
> > +  const unsigned : 1; // { dg-error "unnamed bit-field cannot be 
> > cv-qualified" }
> > +  volatile unsigned : 1; // { dg-error "unnamed bit-field cannot be 
> > cv-qualified" }
> > +  const volatile unsigned : 1; // { dg-error "unnamed bit-field cannot be 
> > cv-qualified" }
> > +  unsigned : 1;
> > +  const unsigned i1 : 1;
> > +  volatile unsigned i2 : 1;
> > +  const volatile unsigned i3 : 1;
> > +};
> 
> Maybe one more test would be a typedef to a qualifier type.
> Like:
> ```
> typedef const int ci;
> struct S1 {
>   ci : 1; // { dg-error "unnamed bit-field cannot be cv-qualified" }
>   ci ci1 : 1;
> };

Sure, why not.

-- >8 --
This implements [class.bit]/2: An unnamed bit-field shall not be
declared with a cv-qualified type.  This was clarified in DR 2229.

        DR 2229
        PR c++/123935

gcc/cp/ChangeLog:

        * decl2.cc (grokbitfield): Disallow cv-qualified unnamed bit-fields.

gcc/testsuite/ChangeLog:

        * g++.dg/DRs/dr2229.C: New test.
---
 gcc/cp/decl2.cc                   |  9 +++++++++
 gcc/testsuite/g++.dg/DRs/dr2229.C | 23 +++++++++++++++++++++++
 2 files changed, 32 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/DRs/dr2229.C

diff --git a/gcc/cp/decl2.cc b/gcc/cp/decl2.cc
index 20ee662eea2..eb2294fffd0 100644
--- a/gcc/cp/decl2.cc
+++ b/gcc/cp/decl2.cc
@@ -1635,6 +1635,15 @@ grokbitfield (const cp_declarator *declarator,
       return NULL_TREE;
     }
 
+  /* [class.bit]/2 "An unnamed bit-field shall not be declared with
+     a cv-qualified type."  */
+  if (!DECL_NAME (value) && TYPE_QUALS (type) != TYPE_UNQUALIFIED)
+    {
+      error_at (DECL_SOURCE_LOCATION (value),
+               "unnamed bit-field cannot be cv-qualified");
+      return NULL_TREE;
+    }
+
   int flags = LOOKUP_IMPLICIT;
   if (init && DIRECT_LIST_INIT_P (init))
     flags = LOOKUP_NORMAL;
diff --git a/gcc/testsuite/g++.dg/DRs/dr2229.C 
b/gcc/testsuite/g++.dg/DRs/dr2229.C
new file mode 100644
index 00000000000..a55604bf8a9
--- /dev/null
+++ b/gcc/testsuite/g++.dg/DRs/dr2229.C
@@ -0,0 +1,23 @@
+// DR 2229, Volatile unnamed bit-fields
+// PR c++/123935
+
+typedef const unsigned cu;
+typedef volatile unsigned vu;
+typedef const volatile unsigned cvu;
+
+struct S {
+  const unsigned : 1; // { dg-error "unnamed bit-field cannot be cv-qualified" 
}
+  volatile unsigned : 1; // { dg-error "unnamed bit-field cannot be 
cv-qualified" }
+  const volatile unsigned : 1; // { dg-error "unnamed bit-field cannot be 
cv-qualified" }
+  unsigned : 1;
+  const unsigned i1 : 1;
+  volatile unsigned i2 : 1;
+  const volatile unsigned i3 : 1;
+
+  cu : 1;   // { dg-error "unnamed bit-field cannot be cv-qualified" }
+  vu : 1;   // { dg-error "unnamed bit-field cannot be cv-qualified" }
+  cvu : 1;  // { dg-error "unnamed bit-field cannot be cv-qualified" }
+  cu i4: 1;
+  vu i5 : 1;
+  cvu i6 : 1;
+};

base-commit: 9d9663ea1533942e041e184947b19f94eb63b934
-- 
2.52.0