[PATCH] c++: Move -Wdangling-reference to -Wextra [PR109642]

[PATCH] c++: Move -Wdangling-reference to -Wextra [PR109642]

[Marek Polacek]

Sadly, -Wdangling-reference generates false positives for std::span-like
user classes, and it seems imprudent to attempt to improve the heuristic
in GCC 13.  Let's move the warning to -Wextra, that will hopefully
reduce the number of false positives the users have been seeing with 13.

I'm leaving the warning in -Wall in 14 where I think I can write code
to detect std::span-like classes.

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for 13.2?

	PR c++/109642
	PR c++/109640
	PR c++/109671

gcc/c-family/ChangeLog:

	* c.opt (Wdangling-reference): Move from -Wall to -Wextra.

gcc/ChangeLog:

	* doc/invoke.texi: Document that -Wdangling-reference is
	enabled by -Wextra.
---
 gcc/c-family/c.opt  | 2 +-
 gcc/doc/invoke.texi | 3 ++-
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt
index 3333cddeece..a75038930ae 100644
--- a/gcc/c-family/c.opt
+++ b/gcc/c-family/c.opt
@@ -560,7 +560,7 @@ C ObjC C++ ObjC++ Joined RejectNegative UInteger Var(warn_dangling_pointer) Warn
 Warn for uses of pointers to auto variables whose lifetime has ended.
 
 Wdangling-reference
-C++ ObjC++ Var(warn_dangling_reference) Warning LangEnabledBy(C++ ObjC++, Wall)
+C++ ObjC++ Var(warn_dangling_reference) Warning LangEnabledBy(C++ ObjC++, Wextra)
 Warn when a reference is bound to a temporary whose lifetime has ended.
 
 Wdate-time
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index a38547f53e5..36ed1591440 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -3781,7 +3781,7 @@ where @code{std::minmax} returns @code{std::pair<const int&, const int&>}, and
 both references dangle after the end of the full expression that contains
 the call to @code{std::minmax}.
 
-This warning is enabled by @option{-Wall}.
+This warning is enabled by @option{-Wextra}.
 
 @opindex Wdelete-non-virtual-dtor
 @opindex Wno-delete-non-virtual-dtor
@@ -6126,6 +6126,7 @@ name is still supported, but the newer name is more descriptive.)
 
 @gccoptlist{-Wclobbered
 -Wcast-function-type
+-Wdangling-reference @r{(C++ only)}
 -Wdeprecated-copy @r{(C++ only)}
 -Wempty-body
 -Wenum-conversion @r{(C only)}
-- 
2.40.1

Re: [PATCH] c++: Move -Wdangling-reference to -Wextra [PR109642]

[Jason Merrill]

On 5/1/23 19:54, Marek Polacek wrote:
> Sadly, -Wdangling-reference generates false positives for std::span-like
> user classes, and it seems imprudent to attempt to improve the heuristic
> in GCC 13.  Let's move the warning to -Wextra, that will hopefully
> reduce the number of false positives the users have been seeing with 13.
> 
> I'm leaving the warning in -Wall in 14 where I think I can write code
> to detect std::span-like classes.
> 
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for 13.2?

OK.

> 	PR c++/109642
> 	PR c++/109640
> 	PR c++/109671
> 
> gcc/c-family/ChangeLog:
> 
> 	* c.opt (Wdangling-reference): Move from -Wall to -Wextra.
> 
> gcc/ChangeLog:
> 
> 	* doc/invoke.texi: Document that -Wdangling-reference is
> 	enabled by -Wextra.
> ---
>   gcc/c-family/c.opt  | 2 +-
>   gcc/doc/invoke.texi | 3 ++-
>   2 files changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt
> index 3333cddeece..a75038930ae 100644
> --- a/gcc/c-family/c.opt
> +++ b/gcc/c-family/c.opt
> @@ -560,7 +560,7 @@ C ObjC C++ ObjC++ Joined RejectNegative UInteger Var(warn_dangling_pointer) Warn
>   Warn for uses of pointers to auto variables whose lifetime has ended.
>   
>   Wdangling-reference
> -C++ ObjC++ Var(warn_dangling_reference) Warning LangEnabledBy(C++ ObjC++, Wall)
> +C++ ObjC++ Var(warn_dangling_reference) Warning LangEnabledBy(C++ ObjC++, Wextra)
>   Warn when a reference is bound to a temporary whose lifetime has ended.
>   
>   Wdate-time
> diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
> index a38547f53e5..36ed1591440 100644
> --- a/gcc/doc/invoke.texi
> +++ b/gcc/doc/invoke.texi
> @@ -3781,7 +3781,7 @@ where @code{std::minmax} returns @code{std::pair<const int&, const int&>}, and
>   both references dangle after the end of the full expression that contains
>   the call to @code{std::minmax}.
>   
> -This warning is enabled by @option{-Wall}.
> +This warning is enabled by @option{-Wextra}.
>   
>   @opindex Wdelete-non-virtual-dtor
>   @opindex Wno-delete-non-virtual-dtor
> @@ -6126,6 +6126,7 @@ name is still supported, but the newer name is more descriptive.)
>   
>   @gccoptlist{-Wclobbered
>   -Wcast-function-type
> +-Wdangling-reference @r{(C++ only)}
>   -Wdeprecated-copy @r{(C++ only)}
>   -Wempty-body
>   -Wenum-conversion @r{(C only)}