[Bug c++/62181] [C/C++] Expected new warning: "adding 'char' to a string does not append to the string" [-Wstring-plus-int]

["ryxi at stu dot xidian.edu.cn" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62181

--- Comment #5 from Xi Ruoyao <ryxi at stu dot xidian.edu.cn> ---
Some clang-5.0 warning case:

~~~
Wstring-plus-int.c:20:24: warning: adding 'int' to a string does not append to
the string [-Wstring-plus-int]
  const char *a = "aa" + 'a'; /* { dg-warning "does not append" } */
                  ~~~~~^~~~~
Wstring-plus-int.c:20:24: note: use array indexing to silence this warning
  const char *a = "aa" + 'a'; /* { dg-warning "does not append" } */
                       ^
                  &    [    ]
Wstring-plus-int.c:20:24: warning: adding 'char' to a string pointer does not
append to the string [-Wstring-plus-char]
  const char *a = "aa" + 'a'; /* { dg-warning "does not append" } */
                  ~~~~~^~~~~
Wstring-plus-int.c:20:24: note: use array indexing to silence this warning
  const char *a = "aa" + 'a'; /* { dg-warning "does not append" } */
                       ^
                  &    [    ]
Wstring-plus-int.c:21:28: warning: adding 'int' to a string does not append to
the string [-Wstring-plus-int]
  const wchar_t *b = L"aa" + L'a'; /* { dg-warning "does not append" } */
                     ~~~~~~^~~~~~
Wstring-plus-int.c:21:28: note: use array indexing to silence this warning
  const wchar_t *b = L"aa" + L'a'; /* { dg-warning "does not append" } */
                           ^
                     &     [     ]
Wstring-plus-int.c:22:24: warning: adding 'int' to a string does not append to
the string [-Wstring-plus-int]
  const char *e = "aa" + getchar(); /* { dg-warning "does not append" } */
                  ~~~~~^~~~~~~~~~~
Wstring-plus-int.c:22:24: note: use array indexing to silence this warning
  const char *e = "aa" + getchar(); /* { dg-warning "does not append" } */
                       ^
                  &    [          ]
Wstring-plus-int.c:25:21: warning: adding 'char' to a string pointer does not
append to the string [-Wstring-plus-char]
  const char *f = a + 'a'; /* { dg-warning "does not append" } */
                  ~~^~~~~
Wstring-plus-int.c:25:21: note: use array indexing to silence this warning
  const char *f = a + 'a'; /* { dg-warning "does not append" } */
                    ^
                  & [    ]
Wstring-plus-int.c:26:23: warning: adding 'char' to a string pointer does not
append to the string [-Wstring-plus-char]
  const char *g = 'a' + a; /* { dg-warning "does not append" } */
                  ~~~~^~~
Wstring-plus-int.c:26:23: note: use array indexing to silence this warning
6 warnings generated.
~~~

Note the both warnings are reported at line 22.

By the way clang -Wstring-plus-char doesn't warn for `a += 'a'`.  I think
we should handle it.

I've submitted a series of patches about this PR.  I'll adjust them to be
like clang-5.0 and resubmit them in stage 1.