c++: -Wdangling-reference and lambda false warning [PR109640]

-Wdangling-reference checks if a function receives a temporary as its
argument, and only warns if any of the arguments was a temporary.  But
we should not warn when the temporary represents a lambda or we generate
false positives as in the attached testcases.

	PR c++/113256
	PR c++/111607
	PR c++/109640

gcc/cp/ChangeLog:

	* call.cc (do_warn_dangling_reference): Don't warn if the temporary
	is of lambda type.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/Wdangling-reference14.C: New test.
	* g++.dg/warn/Wdangling-reference15.C: New test.
	* g++.dg/warn/Wdangling-reference16.C: New test.

4 files changed, 72 insertions, 3 deletions

diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index 1f5ff41..77f51ba 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -14123,7 +14123,10 @@ do_warn_dangling_reference (tree expr, bool arg_p)
       tree e = expr;
       while (handled_component_p (e))
 	e = TREE_OPERAND (e, 0);
-      if (!reference_like_class_p (TREE_TYPE (e)))
+      tree type = TREE_TYPE (e);
+      /* If the temporary represents a lambda, we don't really know
+	 what's going on here.  */
+      if (!reference_like_class_p (type) && !LAMBDA_TYPE_P (type))
 	return expr;
     }
 
@@ -14180,10 +14183,10 @@ do_warn_dangling_reference (tree expr, bool arg_p)
 	       initializing this reference parameter.  */
 	    if (do_warn_dangling_reference (arg, /*arg_p=*/true))
 	      return expr;
-	  /* Don't warn about member function like:
+	  /* Don't warn about member functions like:
 	      std::any a(...);
 	      S& s = a.emplace<S>({0}, 0);
-	     which constructs a new object and returns a reference to it, but
+	     which construct a new object and return a reference to it, but
 	     we still want to detect:
 	       struct S { const S& self () { return *this; } };
 	       const S& s = S().self();
diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference14.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference14.C
new file mode 100644
index 0000000..92b38a9
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference14.C
@@ -0,0 +1,22 @@
+// PR c++/113256
+// { dg-do compile { target c++14 } }
+// { dg-options "-Wdangling-reference" }
+
+#include <utility>
+#include <cassert>
+
+template<class M, class T, class A> auto bind(M T::* pm, A)
+{
+    return [=]( auto&& x ) -> M const& { return x.*pm; };
+}
+
+template<int I> struct arg {};
+
+arg<1> _1;
+
+int main()
+{
+    std::pair<int, int> pair;
+    int const& x = bind( &std::pair<int, int>::first, _1 )( pair ); // { dg-bogus "dangling reference" }
+    assert( &x == &pair.first );
+}
diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference15.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference15.C
new file mode 100644
index 0000000..c39577d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference15.C
@@ -0,0 +1,31 @@
+// PR c++/111607
+// { dg-do compile { target c++20 } }
+// { dg-options "-Wdangling-reference" }
+
+#include <variant>
+
+struct S {
+	constexpr S(int i_) : i(i_) {}
+	S(S const &) = delete;
+	S & operator=(S const &) = delete;
+	S(S &&) = delete;
+	S & operator=(S &&) = delete;
+	int i;
+};
+
+struct A {
+	S s{0};
+};
+
+using V = std::variant<A>;
+
+consteval auto f(V const & v) {
+  auto const & s = std::visit([](auto const & v) -> S const & { return v.s; }, v); // { dg-bogus "dangling reference" }
+  return s.i;
+}
+
+int main() {
+	constexpr V a{std::in_place_type<A>};
+	constexpr auto i = f(a);
+	return i;
+}
diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference16.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference16.C
new file mode 100644
index 0000000..9199692
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference16.C
@@ -0,0 +1,13 @@
+// PR c++/109640
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wdangling-reference" }
+
+bool
+fn0 ()
+{
+    int a;
+    int&& i = [](int& r) -> int&& { return static_cast<int&&>(r); }(a); // { dg-bogus "dangling reference" }
+    auto const l = [](int& r) -> int&& { return static_cast<int&&>(r); };
+    int&& j = l(a);
+    return &i == &j;
+}