[PATCH] c++: redundant explicit 'this' capture in C++17 [PR100493]
As described in detail in the PR, in C++20 implicitly capturing 'this'
via the '=' capture default is deprecated, but in C++17 explicitly
capturing 'this' alongside a '=' capture default is ill-formed. This
means it's impossible to write a C++17 lambda that captures 'this' and
that also has a '=' capture default in a forward-compatible way with
C++20:
[=] { this; } // #1 deprecated in C++20, OK in C++17
// GCC issues a -Wdeprecated warning in C++20 mode
[=, this] { } // #2 ill-formed in C++17, OK in C++20
// GCC issues an unconditional warning in C++17 mode
This patch resolves this dilemma by downgrading the warning for #2 into
a -pedantic one. In passing, move it into the -Wc++20-extensions class
of warnings and mention that the construct in question is a C++20 one.
Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk/11?
PR c++/100493
gcc/cp/ChangeLog:
* parser.c (cp_parser_lambda_introducer): In C++17, don't
diagnose a redundant 'this' capture alongside a by-copy
capture default unless -pedantic. Move the diagnostic into
-Wc++20-extensions and improve the wording.
gcc/testsuite/ChangeLog:
* g++.dg/cpp1z/lambda-this1.C: Adjust expected diagnostics.
* g++.dg/cpp1z/lambda-this8.C: New test.
* g++.dg/cpp2a/lambda-this3.C: Compile with -pedantic in C++17
to continue to diagnose redundant 'this' captures.
---
gcc/cp/parser.c | 8 +---
gcc/testsuite/g++.dg/cpp1z/lambda-this1.C | 8
gcc/testsuite/g++.dg/cpp1z/lambda-this8.C | 10 ++
gcc/testsuite/g++.dg/cpp2a/lambda-this3.C | 2 +-
4 files changed, 20 insertions(+), 8 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/cpp1z/lambda-this8.C
diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index 65f0f112011..30790006ac9 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -11120,10 +11120,12 @@ cp_parser_lambda_introducer (cp_parser* parser, tree
lambda_expr)
if (cp_lexer_next_token_is_keyword (parser->lexer, RID_THIS))
{
location_t loc = cp_lexer_peek_token (parser->lexer)->location;
- if (cxx_dialect < cxx20
+ if (cxx_dialect < cxx20 && pedantic
&& LAMBDA_EXPR_DEFAULT_CAPTURE_MODE (lambda_expr) == CPLD_COPY)
- pedwarn (loc, 0, "explicit by-copy capture of % redundant "
-"with by-copy capture default");
+ pedwarn (loc, OPT_Wc__20_extensions,
+"explicit by-copy capture of % "
+"with by-copy capture default only available with "
+"%<-std=c++20%> or %<-std=gnu++20%>");
cp_lexer_consume_token (parser->lexer);
if (LAMBDA_EXPR_THIS_CAPTURE (lambda_expr))
pedwarn (input_location, 0,
diff --git a/gcc/testsuite/g++.dg/cpp1z/lambda-this1.C
b/gcc/testsuite/g++.dg/cpp1z/lambda-this1.C
index b13ff8b9fc6..e12330a8291 100644
--- a/gcc/testsuite/g++.dg/cpp1z/lambda-this1.C
+++ b/gcc/testsuite/g++.dg/cpp1z/lambda-this1.C
@@ -18,7 +18,7 @@ struct A {
auto i = [=] { return a; };// { dg-warning "implicit
capture" "" { target c++2a } }
auto j = [&] { return a; };
// P0409R2 - C++2A lambda capture [=, this]
-auto k = [=, this] { return a; };// { dg-error "explicit by-copy capture
of 'this' redundant with by-copy capture default" "" { target c++17_down } }
+auto k = [=, this] { return a; };// { dg-error "explicit by-copy capture
of 'this' with by-copy capture default only available with" "" { target
c++17_down } }
auto l = [&, this] { return a; };
auto m = [=, *this] { return a; };// { dg-error "'*this' capture only
available with" "" { target c++14_down } }
auto n = [&, *this] { return a; };// { dg-error "'*this' capture only
available with" "" { target c++14_down } }
@@ -27,12 +27,12 @@ struct A {
// { dg-error "'*this' capture only
available with" "" { target c++14_down } .-1 }
auto q = [=, this, *this] { return a; };// { dg-error "already captured
'this'" }
// { dg-error "'*this' capture only
available with" "" { target c++14_down } .-1 }
- // { dg-error "explicit by-copy
capture of 'this' redundant with by-copy capture default" "" { target
c++17_down } .-2 }
+ // { dg-error "explicit by-copy
capture of 'this' with by-copy capture default only available with" "" { target
c++17_down } .-2 }
auto r = [=, this, this] { return a; };// { dg-error "already captured
'this'" }
- // { dg-error "explicit by-copy
capture of 'this' redundant with by-copy capture default" "" { target
c++17_down } .-1 }
+ // { dg-error "explicit by-copy
capture of 'this' with by-copy cap
Re: [PATCH] c++: redundant explicit 'this' capture in C++17 [PR100493]
On 11/19/21 14:25, Patrick Palka wrote:
As described in detail in the PR, in C++20 implicitly capturing 'this'
via the '=' capture default is deprecated, but in C++17 explicitly
capturing 'this' alongside a '=' capture default is ill-formed. This
means it's impossible to write a C++17 lambda that captures 'this' and
that also has a '=' capture default in a forward-compatible way with
C++20:
[=] { this; } // #1 deprecated in C++20, OK in C++17
// GCC issues a -Wdeprecated warning in C++20 mode
[=, this] { } // #2 ill-formed in C++17, OK in C++20
// GCC issues an unconditional warning in C++17 mode
This patch resolves this dilemma by downgrading the warning for #2 into
a -pedantic one. In passing, move it into the -Wc++20-extensions class
of warnings and mention that the construct in question is a C++20 one.
Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk/11?
OK.
PR c++/100493
gcc/cp/ChangeLog:
* parser.c (cp_parser_lambda_introducer): In C++17, don't
diagnose a redundant 'this' capture alongside a by-copy
capture default unless -pedantic. Move the diagnostic into
-Wc++20-extensions and improve the wording.
gcc/testsuite/ChangeLog:
* g++.dg/cpp1z/lambda-this1.C: Adjust expected diagnostics.
* g++.dg/cpp1z/lambda-this8.C: New test.
* g++.dg/cpp2a/lambda-this3.C: Compile with -pedantic in C++17
to continue to diagnose redundant 'this' captures.
---
gcc/cp/parser.c | 8 +---
gcc/testsuite/g++.dg/cpp1z/lambda-this1.C | 8
gcc/testsuite/g++.dg/cpp1z/lambda-this8.C | 10 ++
gcc/testsuite/g++.dg/cpp2a/lambda-this3.C | 2 +-
4 files changed, 20 insertions(+), 8 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/cpp1z/lambda-this8.C
diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index 65f0f112011..30790006ac9 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -11120,10 +11120,12 @@ cp_parser_lambda_introducer (cp_parser* parser, tree
lambda_expr)
if (cp_lexer_next_token_is_keyword (parser->lexer, RID_THIS))
{
location_t loc = cp_lexer_peek_token (parser->lexer)->location;
- if (cxx_dialect < cxx20
+ if (cxx_dialect < cxx20 && pedantic
&& LAMBDA_EXPR_DEFAULT_CAPTURE_MODE (lambda_expr) == CPLD_COPY)
- pedwarn (loc, 0, "explicit by-copy capture of % redundant "
-"with by-copy capture default");
+ pedwarn (loc, OPT_Wc__20_extensions,
+"explicit by-copy capture of % "
+"with by-copy capture default only available with "
+"%<-std=c++20%> or %<-std=gnu++20%>");
cp_lexer_consume_token (parser->lexer);
if (LAMBDA_EXPR_THIS_CAPTURE (lambda_expr))
pedwarn (input_location, 0,
diff --git a/gcc/testsuite/g++.dg/cpp1z/lambda-this1.C
b/gcc/testsuite/g++.dg/cpp1z/lambda-this1.C
index b13ff8b9fc6..e12330a8291 100644
--- a/gcc/testsuite/g++.dg/cpp1z/lambda-this1.C
+++ b/gcc/testsuite/g++.dg/cpp1z/lambda-this1.C
@@ -18,7 +18,7 @@ struct A {
auto i = [=] { return a; }; // { dg-warning "implicit capture"
"" { target c++2a } }
auto j = [&] { return a; };
// P0409R2 - C++2A lambda capture [=, this]
-auto k = [=, this] { return a; };// { dg-error "explicit by-copy capture of 'this'
redundant with by-copy capture default" "" { target c++17_down } }
+auto k = [=, this] { return a; };// { dg-error "explicit by-copy capture of 'this' with
by-copy capture default only available with" "" { target c++17_down } }
auto l = [&, this] { return a; };
auto m = [=, *this] { return a; };// { dg-error "'*this' capture only available
with" "" { target c++14_down } }
auto n = [&, *this] { return a; };// { dg-error "'*this' capture only available
with" "" { target c++14_down } }
@@ -27,12 +27,12 @@ struct A {
// { dg-error "'*this' capture only available
with" "" { target c++14_down } .-1 }
auto q = [=, this, *this] { return a; };// { dg-error "already captured
'this'" }
// { dg-error "'*this' capture only available
with" "" { target c++14_down } .-1 }
- // { dg-error "explicit by-copy capture of 'this'
redundant with by-copy capture default" "" { target c++17_down } .-2 }
+ // { dg-error "explicit by-copy capture of 'this'
with by-copy capture default only available with" "" { target c++17_down } .-2 }
auto r = [=, this, this] { return a; };// { dg-error "already captured
'this'" }
- // { dg-error "explicit by-copy capture of 'this'
redundant with by-copy capture default" "" { target c++17_down } .-1 }
+ // { d
