[gcc r12-6255] [c++] Adjust mark used member in instantiated class scope

[Nathan Sidwell]

https://gcc.gnu.org/g:b1e701dc4adb11a5ed5f45c2fb31ba4689b718d0

commit r12-6255-gb1e701dc4adb11a5ed5f45c2fb31ba4689b718d0
Author: Nathan Sidwell <nathan@acm.org>
Date:   Tue Jan 4 13:36:44 2022 -0800

    [c++] Adjust mark used member in instantiated class scope
    
    The fix for PR97966 caused a regression with (non-template) member
    functions of template classes.  We need to mark them used in the
    instantiated class's scope, rather than the scope we were in before
    instantiating, as the latter may itself be in template and change the
    behaviour of marking a function as used.
    
            gcc/cp/
            * pt.c (instantiate_class_template_1): Process attribute((used)) set
            in class's context.
            gcc/testsuite/
            * g++.dg/template/attr-used.C: New.

Diff:
---
 gcc/cp/pt.c                               | 13 ++++++++-----
 gcc/testsuite/g++.dg/template/attr-used.C | 16 ++++++++++++++++
 2 files changed, 24 insertions(+), 5 deletions(-)

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index c587966adbe..0fa4a162354 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -12277,6 +12277,14 @@ instantiate_class_template_1 (tree type)
 
   perform_instantiation_time_access_checks (pattern, args);
   perform_deferred_access_checks (tf_warning_or_error);
+
+  /* Now that we've gone through all the members, instantiate those
+     marked with attribute used.  We must do this in the context of
+     the class -- not the context we pushed from, as that might be
+     inside a template and change the behaviour of mark_used.  */
+  for (tree x : used)
+    mark_used (x);
+
   pop_nested_class ();
   maximum_field_alignment = saved_maximum_field_alignment;
   if (!fn_context)
@@ -12290,11 +12298,6 @@ instantiate_class_template_1 (tree type)
   if (TYPE_CONTAINS_VPTR_P (type) && CLASSTYPE_KEY_METHOD (type))
     vec_safe_push (keyed_classes, type);
 
-  /* Now that we've gone through all the members, instantiate those
-     marked with attribute used.  */
-  for (tree x : used)
-    mark_used (x);
-
   return type;
 }
 
diff --git a/gcc/testsuite/g++.dg/template/attr-used.C b/gcc/testsuite/g++.dg/template/attr-used.C
new file mode 100644
index 00000000000..e12bf4caa3f
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/attr-used.C
@@ -0,0 +1,16 @@
+// { dg-do compile }
+
+template<typename T> struct X {
+  void g () {}
+  void f () __attribute__((__used__)) {}
+};
+
+extern X<int> x; // X<int> incomplete here
+
+template <typename T>
+void Frob (T t) {
+  x.g(); // X<int> completed here, X<int>::f's body marked for instantiation
+}
+
+// Make sure X<int>::f is emitted
+// { dg-final { scan-assembler "_ZN1XIiE1fEv:" } }