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
  

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 %<this%> redundant "
> -		     "with by-copy capture default");
> +	    pedwarn (loc, OPT_Wc__20_extensions,
> +		     "explicit by-copy capture of %<this%> "
> +		     "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 capture default only available with" "" { target c++17_down } .-1 }
>       auto s = [=, *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 t = [=, *this, *this] { return a; };// { dg-error "already captured 'this'" }
>   					     // { dg-error "'*this' capture only available with" "" { target c++14_down } .-1 }
>       auto u = [&, this, *this] { return a; };// { dg-error "already captured 'this'" }
> diff --git a/gcc/testsuite/g++.dg/cpp1z/lambda-this8.C b/gcc/testsuite/g++.dg/cpp1z/lambda-this8.C
> new file mode 100644
> index 00000000000..b87ae0af62a
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1z/lambda-this8.C
> @@ -0,0 +1,10 @@
> +// PR c++/100493
> +// { dg-do compile { target c++17 } }
> +// { dg-options "" }
> +
> +struct A {
> +  int m;
> +  void f() {
> +    [=, this] { };
> +  }
> +};
> diff --git a/gcc/testsuite/g++.dg/cpp2a/lambda-this3.C b/gcc/testsuite/g++.dg/cpp2a/lambda-this3.C
> index 3e00e68e906..bc54a4c895d 100644
> --- a/gcc/testsuite/g++.dg/cpp2a/lambda-this3.C
> +++ b/gcc/testsuite/g++.dg/cpp2a/lambda-this3.C
> @@ -1,6 +1,6 @@
>   // P0806R2
>   // { dg-do compile { target c++17 } }
> -// { dg-options "" }
> +// { dg-options "-pedantic" }
>   
>   struct X {
>     int x;
>