[gcc r12-8441] c++: deduction from auto fn [PR105623]

[Jason Merrill]

https://gcc.gnu.org/g:d90576952356735a2152c318ef1d60221b958b15

commit r12-8441-gd90576952356735a2152c318ef1d60221b958b15
Author: Jason Merrill <jason@redhat.com>
Date:   Tue May 24 17:37:58 2022 -0400

    c++: deduction from auto fn [PR105623]
    
    Since my patch for PR90451, we defer mark_used of single functions as late
    as possible.  And since my r12-1273, we keep BASELINK from lookup around
    rather than reconstruct it later.  These both made us try to instantiate g
    with a function type that still had 'auto' as its return type.
    
            PR c++/105623
    
    gcc/cp/ChangeLog:
    
            * decl2.cc (mark_used): Copy type from fn to BASELINK.
            * pt.cc (unify_one_argument): Call mark_single_function.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/cpp1y/auto-fn62.C: New test.

Diff:
---
 gcc/cp/decl2.cc                        | 11 ++++++++---
 gcc/cp/pt.cc                           |  4 ++++
 gcc/testsuite/g++.dg/cpp1y/auto-fn62.C | 14 ++++++++++++++
 3 files changed, 26 insertions(+), 3 deletions(-)

diff --git a/gcc/cp/decl2.cc b/gcc/cp/decl2.cc
index d2b29208ed5..d7e9980ff1e 100644
--- a/gcc/cp/decl2.cc
+++ b/gcc/cp/decl2.cc
@@ -5804,10 +5804,15 @@ mark_used (tree decl, tsubst_flags_t complain)
      actually used until after overload resolution.  */
   if (BASELINK_P (decl))
     {
-      decl = BASELINK_FUNCTIONS (decl);
-      if (really_overloaded_fn (decl))
+      tree fns = BASELINK_FUNCTIONS (decl);
+      if (really_overloaded_fn (fns))
 	return true;
-      decl = OVL_FIRST (decl);
+      fns = OVL_FIRST (fns);
+      if (!mark_used (fns, complain))
+	return false;
+      /* We might have deduced its return type.  */
+      TREE_TYPE (decl) = TREE_TYPE (fns);
+      return true;
     }
 
   if (!DECL_P (decl))
diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 7758758f40d..f92d022a59a 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -22645,6 +22645,10 @@ unify_one_argument (tree tparms, tree targs, tree parm, tree arg,
 	      return unify_success (explain_p);
 	    }
 
+	  /* Force auto deduction now.  Use tf_none to avoid redundant
+	     deprecated warning on deprecated-14.C.  */
+	  mark_single_function (arg, tf_none);
+
 	  arg_expr = arg;
 	  arg = unlowered_expr_type (arg);
 	  if (arg == error_mark_node)
diff --git a/gcc/testsuite/g++.dg/cpp1y/auto-fn62.C b/gcc/testsuite/g++.dg/cpp1y/auto-fn62.C
new file mode 100644
index 00000000000..9c2bff1ccf3
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/auto-fn62.C
@@ -0,0 +1,14 @@
+// PR c++/105623
+// { dg-do compile { target c++14 } }
+
+template <class T>
+auto g(T fn) { }
+
+template<typename>
+struct base {
+  static auto value() { }
+};
+
+struct S : base<void> {
+  static void f() { g(value); }
+};