[gcc r14-277] c++: unique friend shenanigans [PR69836]

[Jason Merrill <jason@gcc.gnu.org>]

https://gcc.gnu.org/g:1e27e7e0985e055b3d4ec92e93976b709fdbe425

commit r14-277-g1e27e7e0985e055b3d4ec92e93976b709fdbe425
Author: Jason Merrill <jason@redhat.com>
Date:   Wed Feb 1 17:00:48 2023 -0500

    c++: unique friend shenanigans [PR69836]
    
    Normally we re-instantiate a function declaration when we start to
    instantiate the body in case of multiple declarations.  In this wacky
    testcase, this causes a problem because the type of the w_counter parameter
    depends on its declaration not being in scope yet, so the name lookup only
    finds the previous declaration.  This isn't a problem for member functions,
    since they aren't subject to argument-dependent lookup.  So let's just skip
    the regeneration for hidden friends.
    
            PR c++/69836
    
    gcc/cp/ChangeLog:
    
            * pt.cc (regenerate_decl_from_template): Skip unique friends.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/template/friend76.C: New test.

Diff:
---
 gcc/cp/pt.cc                             |  6 ++++++
 gcc/testsuite/g++.dg/template/friend76.C | 36 ++++++++++++++++++++++++++++++++
 2 files changed, 42 insertions(+)

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 678cb7930e3..e1c272b9817 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -26458,6 +26458,11 @@ regenerate_decl_from_template (tree decl, tree tmpl, tree args)
       int args_depth;
       int parms_depth;
 
+      /* Don't bother with this for unique friends that can't be redeclared and
+	 might change type if regenerated (PR69836).  */
+      if (DECL_UNIQUE_FRIEND_P (decl))
+	goto done;
+
       /* Use the source location of the definition.  */
       DECL_SOURCE_LOCATION (decl) = DECL_SOURCE_LOCATION (tmpl);
 
@@ -26528,6 +26533,7 @@ regenerate_decl_from_template (tree decl, tree tmpl, tree args)
   else
     gcc_unreachable ();
 
+ done:
   pop_access_scope (decl);
 }
 
diff --git a/gcc/testsuite/g++.dg/template/friend76.C b/gcc/testsuite/g++.dg/template/friend76.C
new file mode 100644
index 00000000000..ce3ed201dda
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/friend76.C
@@ -0,0 +1,36 @@
+// PR c++/69836
+// { dg-do compile { target c++11 } }
+
+template<int N = 25> struct number : public number<N - 1> {
+    static constexpr int value = N;
+    static constexpr number<N-1> prev() { return {}; }
+};
+template<> struct number<0> { static constexpr int value = 0; };
+
+template<int N> struct S { enum { value = N }; };
+
+template<int X> constexpr S<X+1> increment(S<X>) { return {}; }
+
+#define RETURN(R) -> decltype(R) { return R; }
+
+#define INIT(TYPE) \
+        using W_ThisType = TYPE;  \
+        friend constexpr S<0> state(number<0>, W_ThisType**) { return {}; }
+
+#define STUFF \
+    friend constexpr auto state(number<decltype(state(number<>{}, static_cast<W_ThisType**>(nullptr)))::value+1> w_counter, \
+                                W_ThisType **w_this) \
+        RETURN(increment(state(w_counter.prev(), w_this)))
+
+
+template <typename T> struct TemplateObject   {
+    INIT(TemplateObject)
+    STUFF
+    STUFF
+};
+
+int main() {
+  TemplateObject<int> t;
+    constexpr auto s = state(number<>{}, static_cast<TemplateObject<int>**>(nullptr)) ;
+    static_assert(s.value == 2, "");
+}