On Thu, Sep 30, 2021 at 03:34:24PM -0400, Jason Merrill wrote:
> On 9/30/21 10:50, Marek Polacek wrote:
> > This patch addresses one of my leftovers from GCC 11. C++20 introduced
> > [depr.array.comp]:
> > "Equality and relational comparisons between two operands of array type are
> > deprecated."
> > so this patch adds -Wdeprecated-array-compare (enabled by default in C++20).
>
> Why not enable it by default in all modes? It was always pretty dubious
> code.
Sure, it could be done, but it kind of complicates things: we'd probably
need a different option and a different message because it seems incorrect
to say "deprecated" in e.g. C++17 when this was only deprecated in C++20.
I'd rather not add another option; if it stays -Wdeprecated-array-compare
but -Wno-deprecated doesn't turn it off that also seems weird.
I could rename it to -Warray-compare, enable by -Wall, and only
append "is deprecated" to the warning message in C++20. Does that seem
better?
> > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> >
> > PR c++/97573
> >
> > gcc/c-family/ChangeLog:
> >
> > * c-opts.c (c_common_post_options): In C++20, turn on
> > -Wdeprecated-array-compare.
> > * c.opt (Wdeprecated-array-compare): New option.
> >
> > gcc/cp/ChangeLog:
> >
> > * typeck.c (do_warn_deprecated_array_compare): New.
> > (cp_build_binary_op): Call it for equality and relational comparisons.
> >
> > gcc/ChangeLog:
> >
> > * doc/invoke.texi: Document -Wdeprecated-array-compare.
> >
> > gcc/testsuite/ChangeLog:
> >
> > * g++.dg/tree-ssa/pr15791-1.C: Add dg-warning.
> > * g++.dg/cpp2a/array-comp1.C: New test.
> > * g++.dg/cpp2a/array-comp2.C: New test.
> > * g++.dg/cpp2a/array-comp3.C: New test.
> > ---
> > gcc/c-family/c-opts.c | 5 ++++
> > gcc/c-family/c.opt | 4 +++
> > gcc/cp/typeck.c | 28 +++++++++++++++++++
> > gcc/doc/invoke.texi | 19 ++++++++++++-
> > gcc/testsuite/g++.dg/cpp2a/array-comp1.C | 34 +++++++++++++++++++++++
> > gcc/testsuite/g++.dg/cpp2a/array-comp2.C | 31 +++++++++++++++++++++
> > gcc/testsuite/g++.dg/cpp2a/array-comp3.C | 29 +++++++++++++++++++
> > gcc/testsuite/g++.dg/tree-ssa/pr15791-1.C | 2 +-
> > 8 files changed, 150 insertions(+), 2 deletions(-)
> > create mode 100644 gcc/testsuite/g++.dg/cpp2a/array-comp1.C
> > create mode 100644 gcc/testsuite/g++.dg/cpp2a/array-comp2.C
> > create mode 100644 gcc/testsuite/g++.dg/cpp2a/array-comp3.C
> >
> > diff --git a/gcc/c-family/c-opts.c b/gcc/c-family/c-opts.c
> > index 3eaab5e1530..00b52cc5e12 100644
> > --- a/gcc/c-family/c-opts.c
> > +++ b/gcc/c-family/c-opts.c
> > @@ -962,6 +962,11 @@ c_common_post_options (const char **pfilename)
> > warn_deprecated_enum_float_conv,
> > cxx_dialect >= cxx20 && warn_deprecated);
> > + /* -Wdeprecated-array-compare is enabled by default in C++20. */
> > + SET_OPTION_IF_UNSET (&global_options, &global_options_set,
> > + warn_deprecated_array_compare,
> > + cxx_dialect >= cxx20 && warn_deprecated);
> > +
> > /* Declone C++ 'structors if -Os. */
> > if (flag_declone_ctor_dtor == -1)
> > flag_declone_ctor_dtor = optimize_size;
> > diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt
> > index 9c151d19870..a4f0ea68594 100644
> > --- a/gcc/c-family/c.opt
> > +++ b/gcc/c-family/c.opt
> > @@ -540,6 +540,10 @@ Wdeprecated
> > C C++ ObjC ObjC++ CPP(cpp_warn_deprecated) CppReason(CPP_W_DEPRECATED)
> > ; Documented in common.opt
> > +Wdeprecated-array-compare
> > +C++ ObjC++ Var(warn_deprecated_array_compare) Warning
> > +Warn about deprecated comparisons between two operands of array type.
> > +
> > Wdeprecated-copy
> > C++ ObjC++ Var(warn_deprecated_copy) Warning LangEnabledBy(C++ ObjC++,
> > Wextra)
> > Mark implicitly-declared copy operations as deprecated if the class has a
> > diff --git a/gcc/cp/typeck.c b/gcc/cp/typeck.c
> > index a2398dbe660..1e3a41104d6 100644
> > --- a/gcc/cp/typeck.c
> > +++ b/gcc/cp/typeck.c
> > @@ -40,6 +40,7 @@ along with GCC; see the file COPYING3. If not see
> > #include "attribs.h"
> > #include "asan.h"
> > #include "gimplify.h"
> > +#include "tree-pretty-print.h"
> > static tree cp_build_addr_expr_strict (tree, tsubst_flags_t);
> > static tree cp_build_function_call (tree, tree, tsubst_flags_t);
> > @@ -4725,6 +4726,21 @@ do_warn_enum_conversions (location_t loc, enum
> > tree_code code, tree type0,
> > }
> > }
> > +/* Warn about C++20 [depr.array.comp] array comparisons: "Equality
> > + and relational comparisons between two operands of array type are
> > + deprecated." */
> > +
> > +static inline void
> > +do_warn_deprecated_array_compare (location_t location, tree_code code,
> > + tree op0, tree op1)
> > +{
> > + if (warning_at (location, OPT_Wdeprecated_array_compare,
> > + "comparison between two arrays is deprecated"))
> > + inform (location, "use unary %<+%> which decays operands to pointers "
> > + "or %<&%D[0] %s &%D[0]%> to compare the addresses",
> > + op0, op_symbol_code (code), op1);
> > +}
> > +
> > /* Build a binary-operation expression without default conversions.
> > CODE is the kind of expression to build.
> > LOCATION is the location_t of the operator in the source code.
> > @@ -5289,6 +5305,11 @@ cp_build_binary_op (const op_location_t &location,
> > warning_at (location, OPT_Waddress,
> > "comparison with string literal results in "
> > "unspecified behavior");
> > + else if (TREE_CODE (TREE_TYPE (orig_op0)) == ARRAY_TYPE
> > + && TREE_CODE (TREE_TYPE (orig_op1)) == ARRAY_TYPE)
> > + do_warn_deprecated_array_compare (location, code,
> > + stripped_orig_op0,
> > + stripped_orig_op1);
> > }
> > build_type = boolean_type_node;
> > @@ -5559,6 +5580,13 @@ cp_build_binary_op (const op_location_t &location,
> > "comparison with string literal results "
> > "in unspecified behavior");
> > }
> > + else if (TREE_CODE (TREE_TYPE (orig_op0)) == ARRAY_TYPE
> > + && TREE_CODE (TREE_TYPE (orig_op1)) == ARRAY_TYPE
> > + && code != SPACESHIP_EXPR
>
> What happens if you try to compare them with <=>?
That's always an error so printing a warning is redundant.
Marek
