[PATCH] c++: Abbreviated function template return type [PR92187]

[Patrick Palka via Gcc-patches]

When updating an auto return type of an abbreviated function template in
splice_late_return_type, we should also propagate PLACEHOLDER_TYPE_CONSTRAINTS
(and cv-qualifiers) of the original auto node.

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK to commit?

gcc/cp/ChangeLog:

        PR c++/92187
        * pt.c (splice_late_return_type): Propagate cv-qualifiers and
        PLACEHOLDER_TYPE_CONSTRAINTS from the original auto node to the new one.

gcc/testsuite/ChangeLog:

        PR c++/92187
        * g++.dg/concepts/abbrev5.C: New test.
        * g++.dg/concepts/abbrev6.C: New test.
---
 gcc/cp/pt.c                             | 15 +++++++++++----
 gcc/testsuite/g++.dg/concepts/abbrev5.C | 15 +++++++++++++++
 gcc/testsuite/g++.dg/concepts/abbrev6.C | 12 ++++++++++++
 3 files changed, 38 insertions(+), 4 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/concepts/abbrev5.C
 create mode 100644 gcc/testsuite/g++.dg/concepts/abbrev6.C

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 0a8ec3198d2..9e39f46a090 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -29032,10 +29032,17 @@ splice_late_return_type (tree type, tree 
late_return_type)
     {
       tree idx = get_template_parm_index (*auto_node);
       if (TEMPLATE_PARM_LEVEL (idx) <= processing_template_decl)
-       /* In an abbreviated function template we didn't know we were dealing
-          with a function template when we saw the auto return type, so update
-          it to have the correct level.  */
-       *auto_node = make_auto_1 (TYPE_IDENTIFIER (*auto_node), true);
+       {
+         /* In an abbreviated function template we didn't know we were dealing
+            with a function template when we saw the auto return type, so 
update
+            it to have the correct level.  */
+         tree new_auto = make_auto_1 (TYPE_IDENTIFIER (*auto_node), false);
+         PLACEHOLDER_TYPE_CONSTRAINTS (new_auto)
+           = PLACEHOLDER_TYPE_CONSTRAINTS (*auto_node);
+         TYPE_CANONICAL (new_auto) = canonical_type_parameter (new_auto);
+         new_auto = cp_build_qualified_type (new_auto, TYPE_QUALS 
(*auto_node));
+         *auto_node = new_auto;
+       }
     }
   return type;
 }
diff --git a/gcc/testsuite/g++.dg/concepts/abbrev5.C 
b/gcc/testsuite/g++.dg/concepts/abbrev5.C
new file mode 100644
index 00000000000..de594b5c1df
--- /dev/null
+++ b/gcc/testsuite/g++.dg/concepts/abbrev5.C
@@ -0,0 +1,15 @@
+// PR c++/92187
+// { dg-do compile { target concepts } }
+
+template <typename>
+concept C = false;
+
+C auto f(auto)
+{
+  return 42; // { dg-error "deduced return type" }
+}
+
+void foo()
+{
+  f(0);
+}
diff --git a/gcc/testsuite/g++.dg/concepts/abbrev6.C 
b/gcc/testsuite/g++.dg/concepts/abbrev6.C
new file mode 100644
index 00000000000..862675e5193
--- /dev/null
+++ b/gcc/testsuite/g++.dg/concepts/abbrev6.C
@@ -0,0 +1,12 @@
+// { dg-do compile { target concepts } }
+
+const auto &f(auto)
+{
+  static int n;
+  return n;
+}
+
+void foo()
+{
+  f(5) = 0; // { dg-error "read-only" }
+}
-- 
2.26.1.107.gefe3874640