[PATCH] c++: friend vs inherited guide confusion [PR117855]

[Patrick Palka]

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look
OK for trunk/14?

-- >8 --

We recently started using the lang_decl_fn::context field to track
inheritedness of a deduction guide (for C++23 inherited CTAD).  This
new overloading of the field accidentally made DECL_FRIEND_CONTEXT
return non-NULL for inherited guides, which breaks the below testcase
overload resolution containing an inherited guide.

This patch fixes this by refining DECL_FRIEND_CONTEXT to return NULL
for deduction guides.

        PR c++/117855

gcc/cp/ChangeLog:

        * cp-tree.h (DECL_FRIEND_CONTEXT): Refine to exclude deduction
        guides.

gcc/testsuite/ChangeLog:

        * g++.dg/cpp23/class-deduction-inherited7.C: New test.
---
 gcc/cp/cp-tree.h                                     |  3 ++-
 .../g++.dg/cpp23/class-deduction-inherited7.C        | 12 ++++++++++++
 2 files changed, 14 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp23/class-deduction-inherited7.C

diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index c9128bd3e7b..dc527380921 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -3631,7 +3631,8 @@ struct GTY(()) lang_decl {
    the DECL_FRIEND_CONTEXT for `f' will be `S'.  */
 #define DECL_FRIEND_CONTEXT(NODE)                              \
   ((DECL_DECLARES_FUNCTION_P (NODE) && !DECL_VIRTUAL_P (NODE)  \
-    && !DECL_CONSTRUCTOR_P (NODE))                             \
+    && !DECL_CONSTRUCTOR_P (NODE)                              \
+    && (cxx_dialect < cxx23 || !deduction_guide_p (NODE)))     \
    ? LANG_DECL_FN_CHECK (NODE)->context                                \
    : NULL_TREE)
 
diff --git a/gcc/testsuite/g++.dg/cpp23/class-deduction-inherited7.C 
b/gcc/testsuite/g++.dg/cpp23/class-deduction-inherited7.C
new file mode 100644
index 00000000000..b1d5e89ad02
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp23/class-deduction-inherited7.C
@@ -0,0 +1,12 @@
+// PR c++/117855
+// { dg-do compile { target c++20 } }
+
+template <typename T, int _Extent = -1> struct span { span(T&&);};
+template <typename T> span(T &&) -> span<T>;
+template <typename et, int e = -1>
+struct this_span : span<et, e> {
+  using span<et, e>::span;
+};
+template <typename T> this_span(T &&) -> this_span<T>;
+int vec;
+this_span a = vec;
-- 
2.48.1.91.g5f8f7081f7