[PATCH v3] c++: CWG 2229, cv-qualified unnamed bit-fields [PR123935]

[Marek Polacek]

On Wed, Feb 04, 2026 at 08:06:18PM +0900, Jason Merrill wrote:
> On 2/4/26 6:11 AM, Marek Polacek 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),
> 
> This seems like a pedwarn rather than an error.
> 
> > +           "unnamed bit-field cannot be cv-qualified");
> > +      return NULL_TREE;
> 
> ...and then continue.

clang++ gives an error, but any diagnostic will do.  Thus:

dg.exp passed on x86_64-pc-linux-gnu.

-- >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): Add pedwarn for cv-qualified unnamed
        bit-fields.

gcc/testsuite/ChangeLog:

        * g++.dg/DRs/dr2229.C: New test.
---
 gcc/cp/decl2.cc                   |  6 ++++++
 gcc/testsuite/g++.dg/DRs/dr2229.C | 23 +++++++++++++++++++++++
 2 files changed, 29 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..694c23d851e 100644
--- a/gcc/cp/decl2.cc
+++ b/gcc/cp/decl2.cc
@@ -1635,6 +1635,12 @@ 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)
+    pedwarn (DECL_SOURCE_LOCATION (value), 0,
+            "unnamed bit-field cannot be cv-qualified");
+
   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: f6838e731633d8b5b11f15ce3e227e18df2fe46e
-- 
2.52.0