Re: [PATCH] c++/modules: anon union member of as-base class [PR112580]

[Jason Merrill]

On 2/1/24 12:18, Patrick Palka wrote:

Tested on x86_64-pc-linux-gnu, does this look OK for trunk?

OK.

-- >8 --

Here when streaming in the fields of the as-base version of
_Formatting_scanner<int> we end up clobbering ANON_AGGR_TYPE_FIELD
of the anonymous union type, since it turns out this type is shared
between the original FIELD_DECL and the as-base FIELD_DECL copy (copied
during layout_class_type).  ANON_AGGR_TYPE_FIELD first gets properly set
to the original FIELD_DECL when streaming in the canonical definition of
_Formatting_scanner<int>, and then gets overwritten to the as-base
FIELD_DECL when streaming in the the as-base definition.  This leads to
lookup_anon_field giving the wrong answer when resolving the _M_values
use.

This patch makes us avoid overwriting ANON_AGGR_TYPE_FIELD when streaming
in an as-base class definition, it should already be properly set at that
point.

        PR c++/112580

gcc/cp/ChangeLog:

        * module.cc (trees_in::read_class_def): When streaming in
        an anonymous union field of an as-base class, don't clobber
        ANON_AGGR_TYPE_FIELD.

gcc/testsuite/ChangeLog:

        * g++.dg/modules/anon-3_a.H: New test.
        * g++.dg/modules/anon-3_b.C: New test.
---
  gcc/cp/module.cc                        | 12 ++++++++++--
  gcc/testsuite/g++.dg/modules/anon-3_a.H | 21 +++++++++++++++++++++
  gcc/testsuite/g++.dg/modules/anon-3_b.C |  8 ++++++++
  3 files changed, 39 insertions(+), 2 deletions(-)
  create mode 100644 gcc/testsuite/g++.dg/modules/anon-3_a.H
  create mode 100644 gcc/testsuite/g++.dg/modules/anon-3_b.C

diff --git a/gcc/cp/module.cc b/gcc/cp/module.cc
index 3c2fef0e3f4..d36ba2eeeb3 100644
--- a/gcc/cp/module.cc
+++ b/gcc/cp/module.cc
@@ -12178,8 +12178,16 @@ trees_in::read_class_def (tree defn, tree 
maybe_template)

if (TREE_CODE (decl) == FIELD_DECL

                  && ANON_AGGR_TYPE_P (TREE_TYPE (decl)))
-               ANON_AGGR_TYPE_FIELD
-                 (TYPE_MAIN_VARIANT (TREE_TYPE (decl))) = decl;
+               {
+                 tree anon_type = TYPE_MAIN_VARIANT (TREE_TYPE (decl));
+                 if (DECL_NAME (defn) == as_base_identifier)
+                   /* ANON_AGGR_TYPE_FIELD should already point to the
+                      original FIELD_DECL, don't overwrite it to point
+                      to the as-base FIELD_DECL copy.  */
+                   gcc_checking_assert (ANON_AGGR_TYPE_FIELD (anon_type));
+                 else
+                   ANON_AGGR_TYPE_FIELD (anon_type) = decl;
+               }

if (TREE_CODE (decl) == USING_DECL

                  && TREE_CODE (USING_DECL_SCOPE (decl)) == RECORD_TYPE)
diff --git a/gcc/testsuite/g++.dg/modules/anon-3_a.H 
b/gcc/testsuite/g++.dg/modules/anon-3_a.H
new file mode 100644
index 00000000000..64a6aab51dd
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/anon-3_a.H
@@ -0,0 +1,21 @@
+// PR c++/112580
+// { dg-additional-options "-fmodule-header" }
+// { dg-module-cmi {} }
+
+template <typename _Out>
+struct _Formatting_scanner {
+  union {
+    int _M_values = 42;
+  };
+  virtual int _M_format_arg() { return _M_values; }
+};
+
+template <typename _Tp>
+auto __do_vformat_to() {
+  _Formatting_scanner<int> s;
+  return s;
+}
+
+inline auto vformat() {
+  return __do_vformat_to<int>();
+}
diff --git a/gcc/testsuite/g++.dg/modules/anon-3_b.C 
b/gcc/testsuite/g++.dg/modules/anon-3_b.C
new file mode 100644
index 00000000000..d676fe47536
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/anon-3_b.C
@@ -0,0 +1,8 @@
+// PR c++/112580
+// { dg-additional-options "-fmodules-ts" }
+
+import "anon-3_a.H";
+
+int main() {
+  vformat()._M_format_arg();
+}