[Clang] errors with "conflicting types for" function declarations which only differ by return type constness

[nabijaczleweli]

$ cat a.c
int qwe(void);
const int qwe(void);
$ cc -c a.c
a.c:2:11: error: conflicting types for 'qwe'
    2 | const int qwe(void);
      |           ^
a.c:1:5: note: previous declaration is here
    1 | int qwe(void);
      |     ^
1 error generated.
$ gcc -Wall -pedantic -c a.c
$ cc --version
Debian clang version 18.0.0 (++20231231112334+c7c912cff945-1~exp1~20231231112352.433)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /bin
$ gcc --version
gcc (Debian 12.2.0-14) 12.2.0
Copyright (C) 2022 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

(and the same with gcc 13.2.0-7)

In a Draft, 6.7.6.3 Function declarators, Semantics, 4, I see

If, in the declaration "T D1", D1 has the form
        D ( parameter-type-listopt ) attribute-specifier-sequenceopt
and the type specified for ident in the declaration "T D" is "derived-declarator-type-list T", then the type specified for ident is "derived-declarator-type-list function returning the unqualified, non-atomic version of T". The optional attribute specifier sequence appertains to the function type.

This appears to imply that the types for both declarations are processed thusly (pardon the C++-like syntax):

remove_atomic<remove_cv<int>> qwe(void);
remove_atomic<remove_cv<const int>> qwe(void);

yielding

int qwe(void);
int qwe(void);

which is identical.

(Original report)

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: наб (nabijaczleweli)

``` $ cat a.c int qwe(void); const int qwe(void); $ cc -c a.c a.c:2:11: error: conflicting types for 'qwe' 2 | const int qwe(void); | ^ a.c:1:5: note: previous declaration is here 1 | int qwe(void); | ^ 1 error generated. $ gcc -Wall -pedantic -c a.c $ cc --version Debian clang version 18.0.0 (++20231231112334+c7c912cff945-1~exp1~20231231112352.433) Target: x86_64-pc-linux-gnu Thread model: posix InstalledDir: /bin $ gcc --version gcc (Debian 12.2.0-14) 12.2.0 Copyright (C) 2022 Free Software Foundation, Inc. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. ``` (and the same with gcc 13.2.0-7)

In a Draft, 6.7.6.3 Function declarators, Semantics, 4, I see
> If, in the declaration "T D1", D1 has the form
>         D ( parameter-type-listopt ) attribute-specifier-sequenceopt
> and the type specified for ident in the declaration "T D" is "derived-declarator-type-list T", then the type specified for ident is "derived-declarator-type-list function returning the unqualified, non-atomic version of T". The optional attribute specifier sequence appertains to the function type.

This appears to imply that the types for both declarations are processed thusly (pardon the C++-like syntax):

remove_atomic<remove_cv<int>> qwe(void);
remove_atomic<remove_cv<const int>> qwe(void);

yielding

int qwe(void);
int qwe(void);

which is identical.

([Original report](https://101010.pl/@sebastian@jittr.click/111770387762063846))

[shafik]

CC @AaronBallman this feel vaguely familiar but I can't find a similar issue.

[llvmbot]

@llvm/issue-subscribers-c

Author: наб (nabijaczleweli)

``` $ cat a.c int qwe(void); const int qwe(void); $ cc -c a.c a.c:2:11: error: conflicting types for 'qwe' 2 | const int qwe(void); | ^ a.c:1:5: note: previous declaration is here 1 | int qwe(void); | ^ 1 error generated. $ gcc -Wall -pedantic -c a.c $ cc --version Debian clang version 18.0.0 (++20231231112334+c7c912cff945-1~exp1~20231231112352.433) Target: x86_64-pc-linux-gnu Thread model: posix InstalledDir: /bin $ gcc --version gcc (Debian 12.2.0-14) 12.2.0 Copyright (C) 2022 Free Software Foundation, Inc. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. ``` (and the same with gcc 13.2.0-7)

In a Draft, 6.7.6.3 Function declarators, Semantics, 4, I see
> If, in the declaration "T D1", D1 has the form
>         D ( parameter-type-listopt ) attribute-specifier-sequenceopt
> and the type specified for ident in the declaration "T D" is "derived-declarator-type-list T", then the type specified for ident is "derived-declarator-type-list function returning the unqualified, non-atomic version of T". The optional attribute specifier sequence appertains to the function type.

This appears to imply that the types for both declarations are processed thusly (pardon the C++-like syntax):

remove_atomic<remove_cv<int>> qwe(void);
remove_atomic<remove_cv<const int>> qwe(void);

yielding

int qwe(void);
int qwe(void);

which is identical.

([Original report](https://101010.pl/@sebastian@jittr.click/111770387762063846))

[AaronBallman]

Duplicate of #39494