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From: "mkretz at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/106520] New: 2+ index expressions in build_op_subscript are
 incorrectly interpreted as comma expression
Date: Wed, 03 Aug 2022 22:29:15 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106520

            Bug ID: 106520
           Summary: 2+ index expressions in build_op_subscript are
                    incorrectly interpreted as comma expression
           Product: gcc
           Version: 12.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mkretz at gcc dot gnu.org
  Target Milestone: ---

Commit b38c9cf6d570f6c4c1109e00c8b81d82d0f24df3 implemented Multidimensional
subscript operator [PR102611]. However, the backwards compatibility leads to
surprising results. E.g.:

struct A
{
  void operator[](unsigned);
  void operator[](unsigned, unsigned);
};

struct B
{
  explicit operator unsigned() const;
};

void f(A a, B b)
{
  a[1];
  a[b, 2];
}

Compiles to two calls to A::operator[](unsigned) with the following
diagnostics:

<source>: In function 'void f(A, B)':
<source>:15:4: warning: top-level comma expression in array subscript chang=
ed
meaning in C++23 [-Wcomma-subscript]
   15 |   a[b, 2];
      |    ^

[https://godbolt.org/z/f6vf3x5Gv]

The user probably intended to call the two-index subscript overload. But
there's no indication why the call failed. The warning is probably puzzling=
 to
most users. It's probably not obvious to most users that the "wrong" functi=
on
gets called.

I'm not sure the compatibility issue is worth it. I think it would be bette=
r to
call build_op_subscript with unmodified complain and let code that turns on
-std=3Dc++23 break if it relies on comma expressions in subscripts.=