c++: fix -Wdangling-reference false positive [PR115987]

This fixes another false positive.  When a function is taking a
temporary of scalar type that couldn't be bound to the return type
of the function, don't warn, such a program would be ill-formed.

Thanks to Jonathan for reporting the problem.

	PR c++/115987

gcc/cp/ChangeLog:

	* call.cc (do_warn_dangling_reference): Don't consider a
	temporary with a scalar type that cannot bind to the return type.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/attr-no-dangling6.C: Adjust.
	* g++.dg/ext/attr-no-dangling7.C: Likewise.
	* g++.dg/warn/Wdangling-reference22.C: New test.

4 files changed, 46 insertions, 17 deletions

diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index 40cb582..a75e2e5 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -14290,8 +14290,18 @@ do_warn_dangling_reference (tree expr, bool arg_p)
 	    /* 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;
+	    if ((arg = do_warn_dangling_reference (arg, /*arg_p=*/true)))
+	      {
+		/* If we know the temporary could not bind to the return type,
+		   don't warn.  This is for scalars only because for classes
+		   we can't be sure we are not returning its sub-object.  */
+		if (SCALAR_TYPE_P (TREE_TYPE (arg))
+		    && TYPE_REF_P (rettype)
+		    && !reference_related_p (TREE_TYPE (arg),
+					     TREE_TYPE (rettype)))
+		  continue;
+		return expr;
+	      }
 	  /* Don't warn about member functions like:
 	      std::any a(...);
 	      S& s = a.emplace<S>({0}, 0);
diff --git a/gcc/testsuite/g++.dg/ext/attr-no-dangling6.C b/gcc/testsuite/g++.dg/ext/attr-no-dangling6.C
index 235a5fd..5b349e8 100644
--- a/gcc/testsuite/g++.dg/ext/attr-no-dangling6.C
+++ b/gcc/testsuite/g++.dg/ext/attr-no-dangling6.C
@@ -12,26 +12,26 @@ struct SF { static constexpr bool value = false; };
 
 template<typename T>
 [[gnu::no_dangling(T::value)]]
-const X& get (const int& i)
+const X& get (const int& i, const X&)
 {
    return i == 0 ? x1 : x2;
 }
 
 template<bool B = true>
 [[gnu::no_dangling(B)]]
-const X& foo (const int& i)
+const X& foo (const int& i, const X&)
 {
    return i == 0 ? x1 : x2;
 }
 
 [[gnu::no_dangling(val ())]]
-const X& bar (const int& i)
+const X& bar (const int& i, const X&)
 {
    return i == 0 ? x1 : x2;
 }
 
 [[gnu::no_dangling(!val ())]]
-const X& baz (const int& i)
+const X& baz (const int& i, const X&)
 {
    return i == 0 ? x1 : x2;
 }
@@ -52,13 +52,13 @@ auto gety() -> Span<SF>;
 void
 test ()
 {
-  [[maybe_unused]] const X& x1 = get<ST> (10);	  // { dg-bogus "dangling" }
-  [[maybe_unused]] const X& x2 = get<SF> (10);	  // { dg-warning "dangling" }
-  [[maybe_unused]] const X& x3 = foo<true> (10);  // { dg-bogus "dangling" }
-  [[maybe_unused]] const X& x4 = foo<false> (10); // { dg-warning "dangling" }
-  [[maybe_unused]] const X& x7 = foo<> (10);	  // { dg-bogus "dangling" }
-  [[maybe_unused]] const X& x5 = bar (10);	  // { dg-bogus "dangling" }
-  [[maybe_unused]] const X& x6 = baz (10);	  // { dg-warning "dangling" }
+  [[maybe_unused]] const X& x1 = get<ST> (10, X{});	  // { dg-bogus "dangling" }
+  [[maybe_unused]] const X& x2 = get<SF> (10, X{});	  // { dg-warning "dangling" }
+  [[maybe_unused]] const X& x3 = foo<true> (10, X{});  // { dg-bogus "dangling" }
+  [[maybe_unused]] const X& x4 = foo<false> (10, X{}); // { dg-warning "dangling" }
+  [[maybe_unused]] const X& x7 = foo<> (10, X{});	  // { dg-bogus "dangling" }
+  [[maybe_unused]] const X& x5 = bar (10, X{});	  // { dg-bogus "dangling" }
+  [[maybe_unused]] const X& x6 = baz (10, X{});	  // { dg-warning "dangling" }
 
   [[maybe_unused]] const auto &b1 = geti()[0];	  // { dg-bogus "dangling" }
   [[maybe_unused]] const auto &b2 = gety()[0];	  // { dg-warning "dangling" }
diff --git a/gcc/testsuite/g++.dg/ext/attr-no-dangling7.C b/gcc/testsuite/g++.dg/ext/attr-no-dangling7.C
index 3c392ed..a5fb809 100644
--- a/gcc/testsuite/g++.dg/ext/attr-no-dangling7.C
+++ b/gcc/testsuite/g++.dg/ext/attr-no-dangling7.C
@@ -16,16 +16,16 @@ const X& foo(const int& i);
 bool val () { return true; }
 
 [[gnu::no_dangling(val ())]]   // { dg-error "call" }
-const X& bar (const int& i);
+const X& bar (const int& i, const X&);
 
-[[gnu::no_dangling(20)]] const X& fn1 (const int &);
+[[gnu::no_dangling(20)]] const X& fn1 (const int &, const X&);
 
 void
 test ()
 {
-  [[maybe_unused]] const X& x1 = bar (10);	  // { dg-warning "dangling" }
+  [[maybe_unused]] const X& x1 = bar (10, X{});	  // { dg-warning "dangling" }
   [[maybe_unused]] const X& x2 = foo<int> (10);	  // { dg-error "no matching" }
   [[maybe_unused]] const X& x3			  // { dg-warning "dangling" }
-    = fn1 (10);					  // { dg-error "narrowing" }
+    = fn1 (10, X{});				  // { dg-error "narrowing" }
 }
 
diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference22.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference22.C
new file mode 100644
index 0000000..0381f93
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference22.C
@@ -0,0 +1,19 @@
+// PR c++/115987
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wdangling-reference" }
+
+template <typename T>
+struct Wrapper {
+    T val;
+};
+
+template <typename T, typename FUNC>
+    const T& unwrap_2(const Wrapper<T>& r, FUNC&&) {
+  return r.val;
+}
+
+int main(int, char**) {
+  const Wrapper<int> w{1234};
+  const auto& u = unwrap_2(w, 1L);  // { dg-bogus "dangling reference" }
+  __builtin_printf("Unwrapped value : %d\n", u);
+}