Inconsistent (missing) incompatible function pointer type warnings

[brunodefraine]

Bugzilla Link	42120
Version	trunk
OS	All
CC	@dwblaikie,@DougGregor,@zygoloid

Extended Description

Clang marks pointer conversions that add noreturn or noescape information to function types as incompatible (while removing such information is fine), but this warning is not consistent:

$ cat test.c

void fun00(int *);
void fun01(int *) __attribute__((noreturn));
void fun10(__attribute__((noescape)) int *);
void fun11(__attribute__((noescape)) int *) __attribute__((noreturn));

// OK: safe, no warning
void (*fptr01)(int *) = &fun01;
void (*fptr02)(__attribute__((noescape)) int *) = &fun11;
void (*fptr03)(int *) __attribute__((noreturn)) = &fun11;

// OK: unsafe, warning
void (*fptr11)(int *) __attribute__((noreturn)) = &fun00;
void (*fptr12)(__attribute__((noescape)) int *) = &fun00;
void (*fptr13)(__attribute__((noescape)) int *) __attribute__((noreturn)) = &fun00;

// NOT OK: also unsafe, no warning?
void (*fptr14)(__attribute__((noescape)) int *) = &fun01;
void (*fptr15)(int *) __attribute__((noreturn)) = &fun10;

$ clang -c -x c test.c
(only cases fptr11, fptr12, fptr13 are warnings; fptr14 and fptr15 pass without warning/error)

Godbolt: https://godbolt.org/z/XjFsFB

That no warnings are issued for fptr14 and fptr15 cases seems wrong. The conversions are at least as dangerous as the fptr11 and fptr12 cases, which do get warnings.

Note that C++ behavior:

$ clang -c -x c++ test.c
(cases fptr11, fptr12, fptr12, fptr13, fptr14, fptr15 are flagged as errors)

Godbolt: https://godbolt.org/z/cVjcCQ

[brunodefraine]

This behavior is implemented in file clang/lib/Sema/SemaExpr.cpp near the end of function checkPointerTypesForAssignment:

  if (!S.getLangOpts().CPlusPlus &&
      S.IsFunctionConversion(ltrans, rtrans, ltrans))
     return Sema::IncompatiblePointer;
  return ConvTy;

This is very subtle. I think the intention is to check that the conversion from RHS to LHS is an invalid function conversion and then degrade the conversion type to IncompatiblePointer. But this is implementated by checking that the inverse conversion from LHS to RHS is valid (and LHS and RHS are not identical). This is an incomplete condition: my test case demonstrates that the inverse conversion is not necessarily valid for the conversion to be invalid.

I include a patch that replaces the call to IsFunctionConversion by a more direct check that the functions are incompatible. With this patch, also cases fptr14 and fptr15 of my test case are flagged with a warning.

[brunodefraine]

incompatible-func-conv.patch

[dwblaikie]

Usually best to send patches to llvm-commits (contributing patches discussed here: https://llvm.org/docs/Contributing.html ) - they can go a bit lost when attached to bugs.