[PATCH] c++: Fix return type deduction with an abbreviated function template

[Patrick Palka <ppalka@redhat.com>]

This patch fixes two issues with return type deduction in the presence of an
abbreviated function template.

The first issue (PR 69448) is that if a placeholder auto return type contains
any modifiers such as & or *, then the abbreviated-function-template
compensation in splice_late_return_type does not get performed for the
underlying auto node, leading to incorrect return type deduction.  This happens
because splice_late_return_type checks for a placeholder auto with is_auto,
which does not look through modifiers.  This patch replaces the use of is_auto
with find_type_usage, but it first refactors find_type_usage to return a pointer
to the matched tree so that we next subsequently replace it.

The second issue (PR 80471) is that the AUTO_IS_DECLTYPE flag is not being
preserved in splice_late_return_type when compensating for an abbreviated
function template, leading to us treating a decltype(auto) return type as if it
was an auto return type.  Fixed by propagating AUTO_IS_DECLTYPE.  The test for
PR 80471 is adjusted to expect the correct behavior.

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

gcc/cp/ChangeLog:

	PR c++/69448
	PR c++/80471
	* type-utils.h (find_type_usage): Refactor to take a tree * and to
	return a tree *, and update documentation accordingly.
	* pt.c (splice_late_return_type): Use find_type_usage to find and
	replace a possibly nested auto node.  Preserve the AUTO_IS_DECLTYPE
	flag when replacing the node.
	(type_uses_auto): Adjust the call to find_type_usage.

gcc/testsuite/ChangeLog:

	PR c++/69448
	PR c++/80471
	* g++.dg/concepts/abbrev3.C: New test.
	* g++.dg/cpp2a/concepts-pr80471.C: Adjust a static_assert to expect the
	correct behavior.
---
 gcc/cp/pt.c                                   | 26 ++++++++++++-------
 gcc/cp/type-utils.h                           | 26 +++++++++----------
 gcc/testsuite/g++.dg/concepts/abbrev3.C       | 11 ++++++++
 gcc/testsuite/g++.dg/cpp2a/concepts-pr80471.C |  2 +-
 4 files changed, 41 insertions(+), 24 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/concepts/abbrev3.C

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 2fb52caa5d4..c83aaa7c7e1 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -28904,17 +28904,21 @@ do_auto_deduction (tree type, tree init, tree auto_node,
 tree
 splice_late_return_type (tree type, tree late_return_type)
 {
-  if (is_auto (type))
-    {
-      if (late_return_type)
-	return late_return_type;
+  if (is_auto (type) && late_return_type)
+    return late_return_type;
 
-      tree idx = get_template_parm_index (type);
+  if (tree *auto_node = find_type_usage (&type, is_auto))
+    {
+      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.  */
-	return make_auto_1 (TYPE_IDENTIFIER (type), 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_node = make_auto_1 (TYPE_IDENTIFIER (*auto_node), true);
+	  AUTO_IS_DECLTYPE (new_node) = AUTO_IS_DECLTYPE (*auto_node);
+	  *auto_node = new_node;
+	}
     }
   return type;
 }
@@ -28960,8 +28964,10 @@ type_uses_auto (tree type)
       else
 	return NULL_TREE;
     }
+  else if (tree *tp = find_type_usage (&type, is_auto))
+    return *tp;
   else
-    return find_type_usage (type, is_auto);
+    return NULL_TREE;
 }
 
 /* Report ill-formed occurrences of auto types in ARGUMENTS.  If
diff --git a/gcc/cp/type-utils.h b/gcc/cp/type-utils.h
index 680b2497a36..4ad0d822119 100644
--- a/gcc/cp/type-utils.h
+++ b/gcc/cp/type-utils.h
@@ -20,36 +20,36 @@ along with GCC; see the file COPYING3.  If not see
 #ifndef GCC_CP_TYPE_UTILS_H
 #define GCC_CP_TYPE_UTILS_H
 
-/* Returns the first tree within T that is directly matched by PRED.  T may be a
-   type or PARM_DECL and is incrementally decomposed toward its type-specifier
-   until a match is found.  NULL_TREE is returned if PRED does not match any
-   part of T.
+/* Returns a pointer to the first tree within *TP that is directly matched by
+   PRED.  *TP may be a type or PARM_DECL and is incrementally decomposed toward
+   its type-specifier until a match is found.  NULL is returned if PRED does not
+   match any part of *TP.
 
-   This is primarily intended for detecting whether T uses `auto' or a concept
+   This is primarily intended for detecting whether *TP uses `auto' or a concept
    identifier.  Since either of these can only appear as a type-specifier for
    the declaration in question, only top-level qualifications are traversed;
    find_type_usage does not look through the whole type.  */
 
-inline tree
-find_type_usage (tree t, bool (*pred) (const_tree))
+inline tree *
+find_type_usage (tree *tp, bool (*pred) (const_tree))
 {
-  enum tree_code code;
+  tree t = *tp;
   if (pred (t))
-    return t;
+    return tp;
 
-  code = TREE_CODE (t);
+  enum tree_code code = TREE_CODE (t);
 
   if (code == POINTER_TYPE || code == REFERENCE_TYPE
       || code == PARM_DECL || code == OFFSET_TYPE
       || code == FUNCTION_TYPE || code == METHOD_TYPE
       || code == ARRAY_TYPE)
-    return find_type_usage (TREE_TYPE (t), pred);
+    return find_type_usage (&TREE_TYPE (t), pred);
 
   if (TYPE_PTRMEMFUNC_P (t))
     return find_type_usage
-      (TREE_TYPE (TYPE_PTRMEMFUNC_FN_TYPE (t)), pred);
+      (&TREE_TYPE (TYPE_PTRMEMFUNC_FN_TYPE (t)), pred);
 
-  return NULL_TREE;
+  return NULL;
 }
 
 #endif // GCC_CP_TYPE_UTILS_H
diff --git a/gcc/testsuite/g++.dg/concepts/abbrev3.C b/gcc/testsuite/g++.dg/concepts/abbrev3.C
new file mode 100644
index 00000000000..ba2a648156e
--- /dev/null
+++ b/gcc/testsuite/g++.dg/concepts/abbrev3.C
@@ -0,0 +1,11 @@
+// PR c++/69448
+// { dg-do compile { target c++14 } }
+// { dg-additional-options "-fconcepts" }
+
+long x;
+
+auto& f(auto) { return x; }
+auto* g(auto) { return &x; }
+
+long& r = f(1);
+long* p = g(1);
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-pr80471.C b/gcc/testsuite/g++.dg/cpp2a/concepts-pr80471.C
index d5fa5a536d3..6ea6164b417 100644
--- a/gcc/testsuite/g++.dg/cpp2a/concepts-pr80471.C
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-pr80471.C
@@ -18,6 +18,6 @@ int main()
 {
   int i;
   static_assert(is_same< decltype(f(i)), int& >, "");
-  static_assert(is_same< decltype(g(i)), int  >, "");
+  static_assert(is_same< decltype(g(i)), int& >, "");
   static_assert(is_same< decltype(z(i)), int& >, "");
 }
-- 
2.25.0.191.gde93cc14ab