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

Here when streaming in the fields of the as-base version of
_Formatting_scanner<int> we end up overwriting 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 later giving the wrong answer when resolving the
_M_values use at instantiation time.

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 overwrite
	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.

Reviewed-by: Jason Merrill <jason@redhat.com>

3 files changed, 39 insertions, 2 deletions

diff --git a/gcc/cp/module.cc b/gcc/cp/module.cc
index 3c2fef0e..560d8f3 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 0000000..64a6aab
--- /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 0000000..d676fe4
--- /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();
+}