From: Marek Polacek <polacek@redhat.com>
Date: Tue, 31 Mar 2026 20:52:10 +0000 (-0400)
Subject: c++/reflection: spurious error with constrained return types [PR124457]
X-Git-Tag: basepoints/gcc-17~269
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=6f12a4a8d4e62d82d6f8fd79ff4dffe30f387c4d;p=thirdparty%2Fgcc.git

c++/reflection: spurious error with constrained return types [PR124457]

Here we are emitting a bogus error in get_reflection because it
got something for which is_auto was true: the constrained auto
coming from make_constrained_auto.  We represent the
return-type-requirement as a constrained auto which is in fact
a placeholder, but in this case we don't want the error.

We can move the error from get_reflection to the parser to avoid
emitting the error.

	PR c++/124457

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_reflect_expression): Check is_auto here
	instead of...
	* reflect.cc (get_reflection): ...here.

gcc/testsuite/ChangeLog:

	* g++.dg/reflect/concept1.C: New test.

Reviewed-by: Patrick Palka <ppalka@redhat.com>
---

diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index 62ae24f559b..bbebc125765 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -10155,6 +10155,15 @@ cp_parser_reflect_expression (cp_parser *parser)
 	 ^^B <int> is a type alias though.  */
       if (TYPE_P (t) && !type_alias_p)
 	t = strip_typedefs (t);
+      /* [expr.reflect] If the type-id designates a placeholder type, R is
+	 ill-formed.  This check is here rather than in get_reflection so
+	 that we don't wrongly error for a return-type-requirement which is
+	 represented as a constrained auto.  */
+      if (is_auto (t))
+	{
+	  error_at (loc, "%<^^%> cannot be applied to a placeholder type");
+	  return error_mark_node;
+	}
       return get_reflection (loc, t);
     }
   /* Try an id-expression.  */
diff --git a/gcc/cp/reflect.cc b/gcc/cp/reflect.cc
index 3c0e029975d..b066576eba5 100644
--- a/gcc/cp/reflect.cc
+++ b/gcc/cp/reflect.cc
@@ -123,16 +123,9 @@ get_reflection (location_t loc, tree t, reflect_kind kind/*=REFLECT_UNDEF*/)
 {
   STRIP_ANY_LOCATION_WRAPPER (t);
 
-  /* [expr.reflect] If the type-id designates a placeholder type, R is
-     ill-formed.  */
-  if (is_auto (t))
-    {
-      error_at (loc, "%<^^%> cannot be applied to a placeholder type");
-      return error_mark_node;
-    }
   /* Constant template parameters and pack-index-expressions cannot
      appear as operands of the reflection operator.  */
-  else if (PACK_INDEX_P (t))
+  if (PACK_INDEX_P (t))
     {
       error_at (loc, "%<^^%> cannot be applied to a pack index");
       return error_mark_node;
diff --git a/gcc/testsuite/g++.dg/reflect/concept1.C b/gcc/testsuite/g++.dg/reflect/concept1.C
new file mode 100644
index 00000000000..2f3d475432a
--- /dev/null
+++ b/gcc/testsuite/g++.dg/reflect/concept1.C
@@ -0,0 +1,14 @@
+// PR c++/124457
+// { dg-do compile { target c++26 } }
+// { dg-additional-options "-freflection" }
+
+template <class T, auto t = ^^T>
+concept True = true;
+
+template <class T>
+concept AlsoTrue = requires (T t) {
+    { t } -> True;
+};
+void f1(True auto x);
+template <True T> void f2(T );
+auto f3(int) -> True auto;