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

[Marek Polacek]

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?

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

	PR c++/101087

gcc/cp/ChangeLog:

	* except.c (check_noexcept_r): Don't walk into unevaluated
	operands.
	(expr_noexcept_p): Use cp_evaluated.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/noexcept70.C: New test.
---
 gcc/cp/except.c                         | 14 +++++++++++---
 gcc/testsuite/g++.dg/cpp0x/noexcept70.C |  5 +++++
 2 files changed, 16 insertions(+), 3 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/noexcept70.C

diff --git a/gcc/cp/except.c b/gcc/cp/except.c
index a8cea53cf91..6f97ac40b4b 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 (cp_unevaluated_operand)
+    *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
@@ -1155,6 +1158,11 @@ expr_noexcept_p (tree expr, tsubst_flags_t complain)
   if (expr == error_mark_node)
     return false;
 
+  /* Even though the operand of noexcept is an _unevaluated_ operand,
+     temporarily clearing cp_unevaluated_operand allows us to check it
+     in check_noexcept_r, to handle noexcept(sizeof(f())).  It could be
+     set when we are called in the context of synthesized_method_walk.  */
+  cp_evaluated ev;
   fn = cp_walk_tree_without_duplicates (&expr, check_noexcept_r, 0);
   if (fn)
     {
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: a110855667782dac7b674d3e328b253b3b3c919b
-- 
2.31.1