[PATCH v2] c++: Fix noexcept with unevaluated operand [PR101087]

[Marek Polacek]

On Thu, Jul 08, 2021 at 09:35:02AM -0400, Marek Polacek wrote:
> On Thu, Jul 08, 2021 at 09:30:27AM -0400, Jason Merrill wrote:
> > On 7/7/21 9:40 PM, Marek Polacek wrote:
> > > It sounds plausible that this assert
> > > 
> > >    int f();
> > >    static_assert(noexcept(sizeof(f())));
> > > 
> > > should pass: sizeof produces a std::size_t and its operand is not
> > > evaluated, so it can't throw.  noexcept should only evaluate to
> > > false for potentially evaluated operands.  Therefore I think that
> > > check_noexcept_r shouldn't walk into operands of sizeof/decltype/
> > > alignof/typeof.  Only checking cp_unevaluated_operand therein does
> > > not work, because expr_noexcept_p can be called in an unevaluated
> > > context, so I resorted to the following cp_evaluated hack.  Does
> > > that seem acceptable?
> > 
> > I suppose, but why not check for SIZEOF_EXPR/ALIGNOF_EXPR/NOEXCEPT_EXPR
> > directly?
> 
> I thought I would, but then it occurred to me that it might be better to
> rely on cp_walk_subtrees which ++/--s cp_unevaluated_operand for those
> codes.  I'd be happy to change the patch to check those codes directly;
> maybe I'm overthinking things here.

So here's v2 which checks the codes directly, via a new inline:

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

-- >8 --
It sounds plausible that this assert

  int f();
  static_assert(noexcept(sizeof(f())));

should pass: sizeof produces a std::size_t and its operand is not
evaluated, so it can't throw.  noexcept should only evaluate to
false for potentially evaluated operands.  Therefore I think that
check_noexcept_r shouldn't walk into operands of sizeof/decltype/
alignof/typeof.

	PR c++/101087

gcc/cp/ChangeLog:

	* cp-tree.h (unevaluated_p): New.
	* except.c (check_noexcept_r): Use it.  Don't walk into
	unevaluated operands.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/noexcept70.C: New test.
---
 gcc/cp/cp-tree.h                        | 13 +++++++++++++
 gcc/cp/except.c                         |  9 ++++++---
 gcc/testsuite/g++.dg/cpp0x/noexcept70.C |  5 +++++
 3 files changed, 24 insertions(+), 3 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/noexcept70.C

diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index b4501576b26..d4810c0c986 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -8465,6 +8465,19 @@ is_constrained_auto (const_tree t)
   return is_auto (t) && PLACEHOLDER_TYPE_CONSTRAINTS_INFO (t);
 }
 
+/* True if CODE, a tree code, denotes a tree whose operand is not evaluated
+   as per [expr.context], i.e., an operand to sizeof, typeof, decltype, or
+   alignof.  */
+
+inline bool
+unevaluated_p (tree_code code)
+{
+  return (code == DECLTYPE_TYPE
+	  || code == ALIGNOF_EXPR
+	  || code == SIZEOF_EXPR
+	  || code == NOEXCEPT_EXPR);
+}
+
 /* RAII class to push/pop the access scope for T.  */
 
 struct push_access_scope_guard
diff --git a/gcc/cp/except.c b/gcc/cp/except.c
index a8cea53cf91..a8acbc4b7b2 100644
--- a/gcc/cp/except.c
+++ b/gcc/cp/except.c
@@ -1033,12 +1033,15 @@ check_handlers (tree handlers)
      expression whose type is a polymorphic class type (10.3).  */
 
 static tree
-check_noexcept_r (tree *tp, int * /*walk_subtrees*/, void * /*data*/)
+check_noexcept_r (tree *tp, int *walk_subtrees, void *)
 {
   tree t = *tp;
   enum tree_code code = TREE_CODE (t);
-  if ((code == CALL_EXPR && CALL_EXPR_FN (t))
-      || code == AGGR_INIT_EXPR)
+
+  if (unevaluated_p (code))
+    *walk_subtrees = false;
+  else if ((code == CALL_EXPR && CALL_EXPR_FN (t))
+	   || code == AGGR_INIT_EXPR)
     {
       /* We can only use the exception specification of the called function
 	 for determining the value of a noexcept expression; we can't use
diff --git a/gcc/testsuite/g++.dg/cpp0x/noexcept70.C b/gcc/testsuite/g++.dg/cpp0x/noexcept70.C
new file mode 100644
index 00000000000..45a6137dd6f
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/noexcept70.C
@@ -0,0 +1,5 @@
+// PR c++/101087
+// { dg-do compile { target c++11 } }
+
+int f();
+static_assert(noexcept(sizeof(f())), "");

base-commit: 763121ccd908f52bc666f277ea2cf42110b3aad9
-- 
2.31.1