Re: [PATCH] c++: Quash bogus -Wunused-value with new [PR107797]

[Jason Merrill via Gcc-patches]

On 1/19/23 21:03, Marek Polacek wrote:
We shouldn't emit "right operand of comma operator has no effect"
when that comma operator was created by the compiler for "new int{}".
convert_to_void/COMPOUND_EXPR already checks warning_suppressed_p so
we can just suppress -Wunused-value.
Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

        PR c++/107797

gcc/cp/ChangeLog:

        * cvt.cc (ocp_convert): copy_warning when creating a new
        COMPOUND_EXPR.
        * init.cc (build_new_1): Suppress -Wunused-value on
        compiler-generated COMPOUND_EXPRs.

gcc/testsuite/ChangeLog:

        * g++.dg/warn/Wunused-value-1.C: New test.
---
  gcc/cp/cvt.cc                               |  6 ++++--
  gcc/cp/init.cc                              |  2 ++
  gcc/testsuite/g++.dg/warn/Wunused-value-1.C | 12 ++++++++++++
  3 files changed, 18 insertions(+), 2 deletions(-)
  create mode 100644 gcc/testsuite/g++.dg/warn/Wunused-value-1.C

diff --git a/gcc/cp/init.cc b/gcc/cp/init.cc
index f816c474cef..52e96fbe590 100644
--- a/gcc/cp/init.cc
+++ b/gcc/cp/init.cc
@@ -3800,6 +3800,8 @@ build_new_1 (vec<tree, va_gc> **placement, tree type, 
tree nelts,
    if (cookie_expr)
      rval = build2 (COMPOUND_EXPR, TREE_TYPE (rval), cookie_expr, rval);
  
+  suppress_warning (rval, OPT_Wunused_value);

This makes sense, but IIUC since rval is built with no location, this 
just sets nowarning_flag?

    if (rval == data_addr && TREE_CODE (alloc_expr) == TARGET_EXPR)
      /* If we don't have an initializer or a cookie, strip the TARGET_EXPR
         and return the call (which doesn't need to be adjusted).  */
diff --git a/gcc/cp/cvt.cc b/gcc/cp/cvt.cc
index 0cbfd8060cb..17827d06a4a 100644
--- a/gcc/cp/cvt.cc
+++ b/gcc/cp/cvt.cc
@@ -711,8 +711,10 @@ ocp_convert (tree type, tree expr, int convtype, int flags,
        return error_mark_node;
       if (e == TREE_OPERAND (expr, 1))
        return expr;
-      return build2_loc (EXPR_LOCATION (expr), COMPOUND_EXPR, TREE_TYPE (e),
-                        TREE_OPERAND (expr, 0), e);
+      e = build2_loc (EXPR_LOCATION (expr), COMPOUND_EXPR, TREE_TYPE (e),
+                     TREE_OPERAND (expr, 0), e);
+      copy_warning (e, expr);

And so I don't know what effect this would have; copy_warning doesn't 
seem to propagate nowarning_flag, which seems like a bug.

+      return e;
     }
 
   complete_type (type);

diff --git a/gcc/testsuite/g++.dg/warn/Wunused-value-1.C 
b/gcc/testsuite/g++.dg/warn/Wunused-value-1.C
new file mode 100644
index 00000000000..2ba5587fce0
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wunused-value-1.C
@@ -0,0 +1,12 @@
+// PR c++/107797
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wunused" }
+
+void
+g ()
+{
+  (long) new int{};
+  long(new int{});
+  (long) new int();
+  long(new int());
+}

base-commit: 86caab6c5d1e26e1c54c3dceacc873d6e27bfc09