c++: -Wdangling-reference with reference wrapper [PR107532]

Here, -Wdangling-reference triggers where it probably shouldn't, causing
some grief.  The code in question uses a reference wrapper with a member
function returning a reference to a subobject of a non-temporary object:

  const Plane & meta = fm.planes().inner();

I've tried a few approaches, e.g., checking that the member function's
return type is the same as the type of the enclosing class (which is
the case for member functions returning *this), but that then breaks
Wdangling-reference4.C with std::optional<std::string>.

This patch adjusts do_warn_dangling_reference so that we look through
reference wrapper classes (meaning, has a reference member and a
constructor taking the same reference type, or is std::reference_wrapper
or std::ranges::ref_view) and don't warn for them, supposing that the
member function returns a reference to a non-temporary object.

	PR c++/107532

gcc/cp/ChangeLog:

	* call.cc (reference_like_class_p): New.
	(do_warn_dangling_reference): Add new bool parameter.  See through
	reference_like_class_p.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/Wdangling-reference8.C: New test.
	* g++.dg/warn/Wdangling-reference9.C: New test.

3 files changed, 182 insertions, 14 deletions

diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index 048b2b0..3dfa12a 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -13779,6 +13779,52 @@ std_pair_ref_ref_p (tree t)
   return true;
 }
 
+/* Return true if a class CTYPE is either std::reference_wrapper or
+   std::ref_view, or a reference wrapper class.  We consider a class
+   a reference wrapper class if it has a reference member and a
+   constructor taking the same reference type.  */
+
+static bool
+reference_like_class_p (tree ctype)
+{
+  if (!CLASS_TYPE_P (ctype))
+    return false;
+
+  /* Also accept a std::pair<const T&, const T&>.  */
+  if (std_pair_ref_ref_p (ctype))
+    return true;
+
+  tree tdecl = TYPE_NAME (TYPE_MAIN_VARIANT (ctype));
+  if (decl_in_std_namespace_p (tdecl))
+    {
+      tree name = DECL_NAME (tdecl);
+      return (name
+	      && (id_equal (name, "reference_wrapper")
+		  || id_equal (name, "ref_view")));
+    }
+  for (tree fields = TYPE_FIELDS (ctype);
+       fields;
+       fields = DECL_CHAIN (fields))
+    {
+      if (TREE_CODE (fields) != FIELD_DECL || DECL_ARTIFICIAL (fields))
+	continue;
+      tree type = TREE_TYPE (fields);
+      if (!TYPE_REF_P (type))
+	continue;
+      /* OK, the field is a reference member.  Do we have a constructor
+	 taking its type?  */
+      for (tree fn : ovl_range (CLASSTYPE_CONSTRUCTORS (ctype)))
+	{
+	  tree args = FUNCTION_FIRST_USER_PARMTYPE (fn);
+	  if (args
+	      && same_type_p (TREE_VALUE (args), type)
+	      && TREE_CHAIN (args) == void_list_node)
+	    return true;
+	}
+    }
+  return false;
+}
+
 /* Helper for maybe_warn_dangling_reference to find a problematic CALL_EXPR
    that initializes the LHS (and at least one of its arguments represents
    a temporary, as outlined in maybe_warn_dangling_reference), or NULL_TREE
@@ -13793,12 +13839,37 @@ std_pair_ref_ref_p (tree t)
      const int& y = (f(1), 42); // NULL_TREE
      const int& z = f(f(1)); // f(f(1))
 
-   EXPR is the initializer.  */
+   EXPR is the initializer.  If ARG_P is true, we're processing an argument
+   to a function; the point is to distinguish between, for example,
+
+     Ref::inner (&TARGET_EXPR <D.2839, F::foo (fm)>)
+
+   where we shouldn't warn, and
+
+     Ref::inner (&TARGET_EXPR <D.2908, F::foo (&TARGET_EXPR <...>)>)
+
+   where we should warn (Ref is a reference_like_class_p so we see through
+   it.  */
 
 static tree
-do_warn_dangling_reference (tree expr)
+do_warn_dangling_reference (tree expr, bool arg_p)
 {
   STRIP_NOPS (expr);
+
+  if (arg_p && expr_represents_temporary_p (expr))
+    {
+      /* An attempt to reduce the number of -Wdangling-reference
+	 false positives concerning reference wrappers (c++/107532).
+	 When we encounter a reference_like_class_p, we don't warn
+	 just yet; instead, we keep recursing to see if there were
+	 any temporaries behind the reference-wrapper class.  */
+      tree e = expr;
+      while (handled_component_p (e))
+	e = TREE_OPERAND (e, 0);
+      if (!reference_like_class_p (TREE_TYPE (e)))
+	return expr;
+    }
+
   switch (TREE_CODE (expr))
     {
     case CALL_EXPR:
@@ -13831,7 +13902,7 @@ do_warn_dangling_reference (tree expr)
 	     std::pair<const int&, const int&> v = std::minmax(1, 2);
 	   which also creates a dangling reference, because std::minmax
 	   returns std::pair<const T&, const T&>(b, a).  */
-	if (!(TYPE_REF_OBJ_P (rettype) || std_pair_ref_ref_p (rettype)))
+	if (!(TYPE_REF_OBJ_P (rettype) || reference_like_class_p (rettype)))
 	  return NULL_TREE;
 
 	/* Here we're looking to see if any of the arguments is a temporary
@@ -13844,14 +13915,13 @@ do_warn_dangling_reference (tree expr)
 	    if (!DECL_NONSTATIC_MEMBER_FUNCTION_P (fndecl)
 		&& !TYPE_REF_P (TREE_TYPE (arg)))
 	      continue;
-	    /* It could also be another call taking a temporary and returning
-	       it and initializing this reference parameter.  */
-	    if (do_warn_dangling_reference (arg))
-	      return expr;
 	    STRIP_NOPS (arg);
 	    if (TREE_CODE (arg) == ADDR_EXPR)
 	      arg = TREE_OPERAND (arg, 0);
-	    if (expr_represents_temporary_p (arg))
+	    /* Recurse to see if the argument is a temporary.  It could also
+	       be another call taking a temporary and returning it and
+	       initializing this reference parameter.  */
+	    if (do_warn_dangling_reference (arg, /*arg_p=*/true))
 	      return expr;
 	  /* Don't warn about member function like:
 	      std::any a(...);
@@ -13868,15 +13938,15 @@ do_warn_dangling_reference (tree expr)
 	return NULL_TREE;
       }
     case COMPOUND_EXPR:
-      return do_warn_dangling_reference (TREE_OPERAND (expr, 1));
+      return do_warn_dangling_reference (TREE_OPERAND (expr, 1), arg_p);
     case COND_EXPR:
-      if (tree t = do_warn_dangling_reference (TREE_OPERAND (expr, 1)))
+      if (tree t = do_warn_dangling_reference (TREE_OPERAND (expr, 1), arg_p))
 	return t;
-      return do_warn_dangling_reference (TREE_OPERAND (expr, 2));
+      return do_warn_dangling_reference (TREE_OPERAND (expr, 2), arg_p);
     case PAREN_EXPR:
-      return do_warn_dangling_reference (TREE_OPERAND (expr, 0));
+      return do_warn_dangling_reference (TREE_OPERAND (expr, 0), arg_p);
     case TARGET_EXPR:
-      return do_warn_dangling_reference (TARGET_EXPR_INITIAL (expr));
+      return do_warn_dangling_reference (TARGET_EXPR_INITIAL (expr), arg_p);
     default:
       return NULL_TREE;
     }
@@ -13919,7 +13989,7 @@ maybe_warn_dangling_reference (const_tree decl, tree init)
     = make_temp_override (global_dc->dc_warn_system_headers,
 			  (!in_system_header_at (DECL_SOURCE_LOCATION (decl))
 			   || global_dc->dc_warn_system_headers));
-  if (tree call = do_warn_dangling_reference (init))
+  if (tree call = do_warn_dangling_reference (init, /*arg_p=*/false))
     {
       auto_diagnostic_group d;
       if (warning_at (DECL_SOURCE_LOCATION (decl), OPT_Wdangling_reference,
diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
new file mode 100644
index 0000000..330de1f
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
@@ -0,0 +1,77 @@
+// PR c++/107532
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wdangling-reference" }
+
+struct Plane { unsigned int bytesused; };
+
+// Passes a reference through. Does not change lifetime.
+template <typename T>
+struct Ref {
+    const T& i_;
+    Ref(const T & i) : i_(i) {}
+    const T & inner();
+};
+
+struct FrameMetadata {
+    Ref<const Plane> planes() const { return p_; }
+
+    Plane p_;
+};
+
+void bar(const Plane & meta);
+void foo(const FrameMetadata & fm)
+{
+    const Plane & meta = fm.planes().inner();
+    bar(meta);
+    const Plane & meta2 = FrameMetadata().planes().inner(); // { dg-warning "dangling reference" }
+    bar(meta2);
+}
+
+struct S {
+  const S& self () { return *this; }
+} s;
+
+const S& r1 = s.self();
+const S& r2 = S().self(); // { dg-warning "dangling reference" }
+
+struct D {
+};
+
+struct C {
+  D d;
+  Ref<const D> get() const { return d; }
+};
+
+struct B {
+  C c;
+  const C& get() const { return c; }
+  B();
+};
+
+struct A {
+  B b;
+  const B& get() const { return b; }
+};
+
+void
+g (const A& a)
+{
+  const auto& d1 = a.get().get().get().inner();
+  (void) d1;
+  const auto& d2 = A().get().get().get().inner(); // { dg-warning "dangling reference" }
+  (void) d2;
+  const auto& d3 = A().b.get().get().inner(); // { dg-warning "dangling reference" }
+  (void) d3;
+  const auto& d4 = a.b.get().get().inner();
+  (void) d4;
+  const auto& d5 = a.b.c.get().inner();
+  (void) d5;
+  const auto& d6 = A().b.c.get().inner(); // { dg-warning "dangling reference" }
+  (void) d6;
+  Plane p;
+  Ref<Plane> r(p);
+  const auto& d7 = r.inner();
+  (void) d7;
+  const auto& d8 = Ref<Plane>(p).inner();
+  (void) d8;
+}
diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference9.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
new file mode 100644
index 0000000..9ad83f7
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
@@ -0,0 +1,21 @@
+// PR c++/107532
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wdangling-reference" }
+
+#include <functional>
+
+struct X { int n; };
+
+struct S {
+  std::reference_wrapper<const X> wrapit() const { return x; }
+  X x;
+};
+
+void
+g (const S& s)
+{
+  const auto& a1 = s.wrapit().get();
+  (void) a1;
+  const auto& a2 = S().wrapit().get(); // { dg-warning "dangling reference" }
+  (void) a2;
+}