When we capture the current object (*this) implicitly, it is always captured by
reference, even if the capture default is =. This is surprising and irregular
so this proposal <http://wg21.link/p0806r2> deprecates implicit capture of this
via [=] (not via [&]). Users should now use [=, this] (same as [=], added by
P0409R2), or [=, *this].
This patch adds a -Wdeprecated warning to address it.
Bootstrapped/regtested on x86_64-linux, ok for trunk?
2018-08-11 Marek Polacek <pola...@redhat.com>
P0806R2 - Deprecate implicit capture of this via [=]
* lambda.c (add_default_capture): Formatting fixes. Warn about
deprecated implicit capture of this via [=].
* g++.dg/cpp2a/lambda-this1.C: New test.
* g++.dg/cpp2a/lambda-this2.C: New test.
* g++.dg/cpp2a/lambda-this3.C: New test.
--- gcc/cp/lambda.c
+++ gcc/cp/lambda.c
@@ -695,14 +695,10 @@ tree
add_default_capture (tree lambda_stack, tree id, tree initializer)
{
bool this_capture_p = (id == this_identifier);
-
tree var = NULL_TREE;
-
tree saved_class_type = current_class_type;
- tree node;
-
- for (node = lambda_stack;
+ for (tree node = lambda_stack;
node;
node = TREE_CHAIN (node))
{
@@ -711,15 +707,26 @@ add_default_capture (tree lambda_stack, tree id, tree
initializer)
current_class_type = LAMBDA_EXPR_CLOSURE (lambda);
if (DECL_PACK_P (initializer))
initializer = make_pack_expansion (initializer);
- var = add_capture (lambda,
- id,
- initializer,
- /*by_reference_p=*/
- (this_capture_p
- || (LAMBDA_EXPR_DEFAULT_CAPTURE_MODE (lambda)
- == CPLD_REFERENCE)),
- /*explicit_init_p=*/false);
+ var = add_capture (lambda, id, initializer,
+ /*by_reference_p=*/
+ (this_capture_p
+ || (LAMBDA_EXPR_DEFAULT_CAPTURE_MODE (lambda)
+ == CPLD_REFERENCE)),
+ /*explicit_init_p=*/false);
initializer = convert_from_reference (var);
+
+ /* Warn about deprecated implicit capture of this via [=]. */
+ if (cxx_dialect >= cxx2a
+ && this_capture_p
+ && LAMBDA_EXPR_DEFAULT_CAPTURE_MODE (lambda) == CPLD_COPY
+ && !in_system_header_at (LAMBDA_EXPR_LOCATION (lambda)))
+ {
+ if (warning_at (LAMBDA_EXPR_LOCATION (lambda), OPT_Wdeprecated,
+ "implicit capture of %qE via %<[=]%> is deprecated "
+ "in C++20", this_identifier))
+ inform (LAMBDA_EXPR_LOCATION (lambda), "add explicit %<this%> or "
+ "%<*this%> capture");
+ }
}
current_class_type = saved_class_type;
--- gcc/testsuite/g++.dg/cpp2a/lambda-this1.C
+++ gcc/testsuite/g++.dg/cpp2a/lambda-this1.C
@@ -0,0 +1,51 @@
+// P0806R2
+// { dg-do compile }
+// { dg-options "-std=c++2a" }
+
+struct X {
+ int x;
+ void foo (int n) {
+ auto a1 = [=] { x = n; }; // { dg-warning "implicit capture" }
+ auto a2 = [=, this] { x = n; };
+ auto a3 = [=, *this]() mutable { x = n; };
+ auto a4 = [&] { x = n; };
+ auto a5 = [&, this] { x = n; };
+ auto a6 = [&, *this]() mutable { x = n; };
+
+ auto a7 = [=] { // { dg-warning "implicit capture" }
+ auto a = [=] { // { dg-warning "implicit capture" }
+ auto a2 = [=] { x = n; }; // { dg-warning "implicit capture" }
+ };
+ };
+
+ auto a8 = [=, this] {
+ auto a = [=, this] {
+ auto a2 = [=, this] { x = n; };
+ };
+ };
+
+ auto a9 = [=, *this]() mutable {
+ auto a = [=, *this]() mutable {
+ auto a2 = [=, *this]() mutable { x = n; };
+ };
+ };
+
+ auto a10 = [&] {
+ auto a = [&] {
+ auto a2 = [&] { x = n; };
+ };
+ };
+
+ auto a11 = [&, this] {
+ auto a = [&, this] {
+ auto a2 = [&, this] { x = n; };
+ };
+ };
+
+ auto a12 = [&, *this]() mutable {
+ auto a = [&, *this]() mutable {
+ auto a2 = [&, *this]() mutable { x = n; };
+ };
+ };
+ }
+};
--- gcc/testsuite/g++.dg/cpp2a/lambda-this2.C
+++ gcc/testsuite/g++.dg/cpp2a/lambda-this2.C
@@ -0,0 +1,51 @@
+// P0806R2
+// { dg-do compile }
+// { dg-options "-std=c++2a -Wno-deprecated" }
+
+struct X {
+ int x;
+ void foo (int n) {
+ auto a1 = [=] { x = n; }; // { dg-bogus "implicit capture" }
+ auto a2 = [=, this] { x = n; };
+ auto a3 = [=, *this]() mutable { x = n; };
+ auto a4 = [&] { x = n; };
+ auto a5 = [&, this] { x = n; };
+ auto a6 = [&, *this]() mutable { x = n; };
+
+ auto a7 = [=] { // { dg-bogus "implicit capture" }
+ auto a = [=] { // { dg-bogus "implicit capture" }
+ auto a2 = [=] { x = n; }; // { dg-bogus "implicit capture" }
+ };
+ };
+
+ auto a8 = [=, this] {
+ auto a = [=, this] {
+ auto a2 = [=, this] { x = n; };
+ };
+ };
+
+ auto a9 = [=, *this]() mutable {
+ auto a = [=, *this]() mutable {
+ auto a2 = [=, *this]() mutable { x = n; };
+ };
+ };
+
+ auto a10 = [&] {
+ auto a = [&] {
+ auto a2 = [&] { x = n; };
+ };
+ };
+
+ auto a11 = [&, this] {
+ auto a = [&, this] {
+ auto a2 = [&, this] { x = n; };
+ };
+ };
+
+ auto a12 = [&, *this]() mutable {
+ auto a = [&, *this]() mutable {
+ auto a2 = [&, *this]() mutable { x = n; };
+ };
+ };
+ }
+};
--- gcc/testsuite/g++.dg/cpp2a/lambda-this3.C
+++ gcc/testsuite/g++.dg/cpp2a/lambda-this3.C
@@ -0,0 +1,55 @@
+// P0806R2
+// { dg-do compile }
+// { dg-options "-std=c++17" }
+
+struct X {
+ int x;
+ void foo (int n) {
+ auto a1 = [=] { x = n; }; // { dg-bogus "implicit capture" }
+ auto a2 = [=, this] { x = n; };
+ // { dg-warning "explicit by-copy capture" "" { target c++17_down } .-1 }
+ auto a3 = [=, *this]() mutable { x = n; };
+ auto a4 = [&] { x = n; };
+ auto a5 = [&, this] { x = n; };
+ auto a6 = [&, *this]() mutable { x = n; };
+
+ auto a7 = [=] { // { dg-bogus "implicit capture" }
+ auto a = [=] { // { dg-bogus "implicit capture" }
+ auto a2 = [=] { x = n; }; // { dg-bogus "implicit capture" }
+ };
+ };
+
+ auto a8 = [=, this] {
+ // { dg-warning "explicit by-copy capture" "" { target c++17_down } .-1 }
+ auto a = [=, this] {
+ // { dg-warning "explicit by-copy capture" "" { target c++17_down } .-1 }
+ auto a2 = [=, this] { x = n; };
+ // { dg-warning "explicit by-copy capture" "" { target c++17_down } .-1 }
+ };
+ };
+
+ auto a9 = [=, *this]() mutable {
+ auto a = [=, *this]() mutable {
+ auto a2 = [=, *this]() mutable { x = n; };
+ };
+ };
+
+ auto a10 = [&] {
+ auto a = [&] {
+ auto a2 = [&] { x = n; };
+ };
+ };
+
+ auto a11 = [&, this] {
+ auto a = [&, this] {
+ auto a2 = [&, this] { x = n; };
+ };
+ };
+
+ auto a12 = [&, *this]() mutable {
+ auto a = [&, *this]() mutable {
+ auto a2 = [&, *this]() mutable { x = n; };
+ };
+ };
+ }
+};
