[C++ Patch/RFC] PR 53025
Paolo Carlini
paolo.carlini@oracle.com
Thu Oct 2 12:12:00 GMT 2014
Hi,
On 10/02/2014 05:22 AM, Jason Merrill wrote:
> On 10/01/2014 12:31 PM, Paolo Carlini wrote:
>> in this issue Daniel argued that the value of a noexcept expression
>> should not depend on constructor elision.
>
> I'm open to that, but I don't think it's at all clear in the standard.
Ok. Personally, I find the analysis in the audit trail rather
convincing. In practice, clang++ definitely agrees, likewise
SolarisStudio. Current EDG seems inconsistent, the templated and
non-templated cases are handled differently.
>
>> Then, in the audit trail Marc
>> tentatively suggested something like the parser.c hunk below, which just
>> disables our -felide-constructors optimization when parsing the noexcept
>> expression. Over the last couple of days, I had a look, noticed that in
>> any case we still have to handle templates, thus the pt.c hunk, and also
>> that maybe we can avoid completely disabling -felide-constructors in
>> noexcept expressions when we know for sure that the constructor at issue
>> doen't throw: for that in call.s I'm further abusing the
>> flag_elide_constructors global, in terms of using a special value of 2
>> when flag_elide_constructor is found == 1 when handling the expression.
>> The below passes testing, anyway.
>
> Why do we want to avoid completely disabling -felide-constructors,
> since it's an unevaluated context anyway?
Agreed.
> If you're going to mess with this flag you need to save/restore it in
> push/pop_to_top_level as well.
I see. But then if we have to do take care of that I find much more
clean to just add a global?!? In any case, if I understand correctly the
logic at the end of push_to_top_level, it would be tricky to reset the
flag to a sensible value (by default would be 1, but the user may have
passed -fno-elide-constructors on the command line).
The below passes testing.
Thanks!
Paolo.
////////////////////////
-------------- next part --------------
Index: cp/call.c
===================================================================
--- cp/call.c (revision 215797)
+++ cp/call.c (working copy)
@@ -7251,7 +7251,11 @@ build_over_call (struct z_candidate *cand, int fla
/* Do things the hard way. */;
else if (cand->num_convs == 1
&& (DECL_COPY_CONSTRUCTOR_P (fn)
- || DECL_MOVE_CONSTRUCTOR_P (fn)))
+ || DECL_MOVE_CONSTRUCTOR_P (fn))
+ /* It's unsafe to elide the constructor when handling
+ a noexcept-expression, it may evaluate to the wrong
+ value (c++/53025). */
+ && cp_noexcept_operand == 0)
{
tree targ;
tree arg = argarray[num_artificial_parms_for (fn)];
Index: cp/cp-tree.h
===================================================================
--- cp/cp-tree.h (revision 215797)
+++ cp/cp-tree.h (working copy)
@@ -1058,6 +1058,7 @@ struct GTY(()) saved_scope {
int unevaluated_operand;
int inhibit_evaluation_warnings;
+ int noexcept_operand;
/* If non-zero, implicit "omp declare target" attribute is added into the
attribute lists. */
int omp_declare_target_attribute;
@@ -4399,6 +4400,10 @@ extern int comparing_specializations;
extern int cp_unevaluated_operand;
+/* Nonzero if we are parsing the operand of a noexcept operator. */
+
+extern int cp_noexcept_operand;
+
/* in pt.c */
/* These values are used for the `STRICT' parameter to type_unification and
Index: cp/name-lookup.c
===================================================================
--- cp/name-lookup.c (revision 215797)
+++ cp/name-lookup.c (working copy)
@@ -6139,6 +6139,7 @@ push_to_top_level (void)
s->function_decl = current_function_decl;
s->unevaluated_operand = cp_unevaluated_operand;
s->inhibit_evaluation_warnings = c_inhibit_evaluation_warnings;
+ s->noexcept_operand = cp_noexcept_operand;
s->x_stmt_tree.stmts_are_full_exprs_p = true;
scope_chain = s;
@@ -6149,6 +6150,7 @@ push_to_top_level (void)
push_class_stack ();
cp_unevaluated_operand = 0;
c_inhibit_evaluation_warnings = 0;
+ cp_noexcept_operand = 0;
timevar_cond_stop (TV_NAME_LOOKUP, subtime);
}
@@ -6182,6 +6184,7 @@ pop_from_top_level_1 (void)
current_function_decl = s->function_decl;
cp_unevaluated_operand = s->unevaluated_operand;
c_inhibit_evaluation_warnings = s->inhibit_evaluation_warnings;
+ cp_noexcept_operand = s->noexcept_operand;
}
/* Wrapper for pop_from_top_level_1. */
Index: cp/parser.c
===================================================================
--- cp/parser.c (revision 215797)
+++ cp/parser.c (working copy)
@@ -259,6 +259,9 @@ static FILE *cp_lexer_debug_stream;
sizeof, typeof, or alignof. */
int cp_unevaluated_operand;
+/* Nonzero if we are parsing the operand of a noexcept operator. */
+int cp_noexcept_operand;
+
/* Dump up to NUM tokens in BUFFER to FILE starting with token
START_TOKEN. If START_TOKEN is NULL, the dump starts with the
first token in BUFFER. If NUM is 0, dump all the tokens. If
@@ -7156,7 +7159,9 @@ cp_parser_unary_expression (cp_parser *parser, boo
++cp_unevaluated_operand;
++c_inhibit_evaluation_warnings;
+ ++cp_noexcept_operand;
expr = cp_parser_expression (parser);
+ --cp_noexcept_operand;
--c_inhibit_evaluation_warnings;
--cp_unevaluated_operand;
Index: cp/pt.c
===================================================================
--- cp/pt.c (revision 215797)
+++ cp/pt.c (working copy)
@@ -14769,11 +14769,13 @@ tsubst_copy_and_build (tree t,
op1 = TREE_OPERAND (t, 0);
++cp_unevaluated_operand;
++c_inhibit_evaluation_warnings;
+ ++cp_noexcept_operand;
op1 = tsubst_copy_and_build (op1, args, complain, in_decl,
/*function_p=*/false,
/*integral_constant_expression_p=*/false);
--cp_unevaluated_operand;
--c_inhibit_evaluation_warnings;
+ --cp_noexcept_operand;
RETURN (finish_noexcept_expr (op1, complain));
case MODOP_EXPR:
Index: testsuite/g++.dg/cpp0x/noexcept23.C
===================================================================
--- testsuite/g++.dg/cpp0x/noexcept23.C (revision 0)
+++ testsuite/g++.dg/cpp0x/noexcept23.C (working copy)
@@ -0,0 +1,14 @@
+// PR c++/53025
+// { dg-do compile { target c++11 } }
+
+struct A {
+ A() noexcept {}
+ A(const A&) noexcept(false) {}
+};
+
+void a(A) noexcept {}
+
+void f()
+{
+ static_assert(!noexcept(a(A{})), "");
+}
Index: testsuite/g++.dg/cpp0x/noexcept24.C
===================================================================
--- testsuite/g++.dg/cpp0x/noexcept24.C (revision 0)
+++ testsuite/g++.dg/cpp0x/noexcept24.C (working copy)
@@ -0,0 +1,22 @@
+// PR c++/53025
+// { dg-do compile { target c++11 } }
+
+template<typename T>
+struct A {
+ A() noexcept {}
+ A(const A&) noexcept(false) {}
+};
+
+template<typename T>
+void a(A<T>) noexcept {}
+
+template<typename T>
+void f()
+{
+ static_assert(!noexcept(a(A<T>{})), "");
+}
+
+void g()
+{
+ f<int>();
+}
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