[clang] C mode -Wincompatible-function-pointer-types diag disappears in case of other function type differences

[brunodf-snps]

This is explored in the following test cases. All lines marked "OK??" are cases where a diagnostic is logically missing (and gcc has a diagnostic if it understand the involved attributes).

Godbolt demo: https://godbolt.org/z/f3o8KoqGo

#ifdef __clang__
// Comment-in pragma to enable strict mode
//#pragma clang diagnostic warning "-Wincompatible-function-pointer-types-strict"
#endif

enum E { A = -1, B };

// Case 1: assignment adding noreturn

int     f1    (int);
int    (*fp1a)(int) __attribute__((noreturn)) = &f1; // NOK
enum E (*fp1b)(int) __attribute__((noreturn)) = &f1; // OK??
int    (*fp1c)()    __attribute__((noreturn)) = &f1; // OK??

#ifdef __clang__

// Case 2: assignment adding noescape on arg

int   f2   (int*                          ) __attribute__((noreturn));
int (*fp2a)(int* __attribute__((noescape))) __attribute__((noreturn)) = &f2; // NOK
int (*fp2b)(int* __attribute__((noescape)))                           = &f2; // OK??

// Case 3: assignment adding cfi_unchecked_callee

int   f3   (int*                          );
int (*fp3a)(int*                          ) __attribute__((noreturn                     )) = &f3; // NOK
int (*fp3b)(int* __attribute__((noescape)))                                                = &f3; // NOK
int (*fp3c)(int*                          ) __attribute__((noreturn,cfi_unchecked_callee)) = &f3; // OK??
int (*fp3d)(int* __attribute__((noescape))) __attribute__((         cfi_unchecked_callee)) = &f3; // OK??
int (*fp3e)(int* __attribute__((noescape))) __attribute__((noreturn,cfi_unchecked_callee)) = &f3; // OK??

#endif

// Case 4: assignment to function with no prototype

int   f4   (int);
int (*fp4a)(int) = &f4; // OK
int (*fp4b)()    = &f4; // NOK for strict?

This issue was previously reported as https://bugs.llvm.org/show_bug.cgi?id=42120

A stricter diagnostic -Wincompatible-function-pointer-types-strict was added, this catches all the cases, but this also diagnoses differences in argument and result types that are allowed in C.

[shafik]

The old issues are available in github: #41465 should we close this as a duplicate and extend the older report?

[brunodf-snps]

The old issues are available in github: #41465 should we close this as a duplicate and extend the older report?

Thanks I did not realize! I added the information to the original report and I'm closing this as a duplicate.

[llvmbot]

@llvm/issue-subscribers-c

Author: Bruno De Fraine (brunodf-snps)

Details

This is explored in the following test cases. All lines marked "OK??" are cases where a diagnostic is logically missing (and gcc has a diagnostic if it understand the involved attributes).

Godbolt demo: https://godbolt.org/z/f3o8KoqGo

#ifdef __clang__
// Comment-in pragma to enable strict mode
//#pragma clang diagnostic warning "-Wincompatible-function-pointer-types-strict"
#endif

enum E { A = -1, B };

// Case 1: assignment adding noreturn

int     f1    (int);
int    (*fp1a)(int) __attribute__((noreturn)) = &f1; // NOK
enum E (*fp1b)(int) __attribute__((noreturn)) = &f1; // OK??
int    (*fp1c)()    __attribute__((noreturn)) = &f1; // OK??

#ifdef __clang__

// Case 2: assignment adding noescape on arg

int   f2   (int*                          ) __attribute__((noreturn));
int (*fp2a)(int* __attribute__((noescape))) __attribute__((noreturn)) = &f2; // NOK
int (*fp2b)(int* __attribute__((noescape)))                           = &f2; // OK??

// Case 3: assignment adding cfi_unchecked_callee

int   f3   (int*                          );
int (*fp3a)(int*                          ) __attribute__((noreturn                     )) = &f3; // NOK
int (*fp3b)(int* __attribute__((noescape)))                                                = &f3; // NOK
int (*fp3c)(int*                          ) __attribute__((noreturn,cfi_unchecked_callee)) = &f3; // OK??
int (*fp3d)(int* __attribute__((noescape))) __attribute__((         cfi_unchecked_callee)) = &f3; // OK??
int (*fp3e)(int* __attribute__((noescape))) __attribute__((noreturn,cfi_unchecked_callee)) = &f3; // OK??

#endif

// Case 4: assignment to function with no prototype

int   f4   (int);
int (*fp4a)(int) = &f4; // OK
int (*fp4b)()    = &f4; // NOK for strict?

This issue was previously reported as https://bugs.llvm.org/show_bug.cgi?id=42120

A stricter diagnostic -Wincompatible-function-pointer-types-strict was added, this catches all the cases, but this also diagnoses differences in argument and result types that are allowed in C.