On 8/2/24 3:22 PM, Marek Polacek wrote:
On Thu, Aug 01, 2024 at 05:20:43PM -0400, Jason Merrill wrote:
On 8/1/24 4:19 PM, Marek Polacek wrote:
Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

-- >8 --
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/warn/Wdangling-reference22.C: New test.
---
   gcc/cp/call.cc                                | 15 +++++++++++++--
   .../g++.dg/warn/Wdangling-reference22.C       | 19 +++++++++++++++++++
   2 files changed, 32 insertions(+), 2 deletions(-)
   create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference22.C

diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index 40cb582acc7..375256ebcc4 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -14290,8 +14290,19 @@ 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)
+                   && SCALAR_TYPE_P (TREE_TYPE (rettype))

I don't think we need to check for scalar return type, only argument type.

Oh that was a late change to keep attr-no-dangling6.C working, i.e., to
keep warning for something like

   struct X { X(int); };
   const X& get (const int& i)
   {
      return i;
   }

   void test ()
   {
     [[maybe_unused]] const X& x2 = get (10);
   }

But we already emit a -Wreturn-local-addr warning there.  So, I've dropped
the SCALAR_TYPE_P check and adjusted attr-no-dangling6.C instead:

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

OK.

-- >8 --
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.
---
  gcc/cp/call.cc                                | 14 ++++++++++--
  gcc/testsuite/g++.dg/ext/attr-no-dangling6.C  | 22 +++++++++----------
  gcc/testsuite/g++.dg/ext/attr-no-dangling7.C  |  8 +++----
  .../g++.dg/warn/Wdangling-reference22.C       | 19 ++++++++++++++++
  4 files changed, 46 insertions(+), 17 deletions(-)
  create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference22.C

diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index 40cb582acc7..a75e2e5e3af 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 235a5fd86c5..5b349e8e682 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 3c392ed409f..a5fb809e6bd 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 00000000000..0381f9313fb
--- /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);
+}

base-commit: 5ebfaf2d4994c124ce81aa0abd7eaa1529644749

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