[llvm-bugs] [Bug 26398] New: Regression: -Wnull-dereference (part of -Wall) even applies to expressions that are not evaluated.

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=26398

            Bug ID: 26398
           Summary: Regression: -Wnull-dereference (part of -Wall) even
                    applies to expressions that are not evaluated.
           Product: clang
           Version: 3.8
          Hardware: PC
                OS: other
            Status: NEW
          Severity: release blocker
          Priority: P
         Component: Frontend
          Assignee: unassignedclangbugs at nondot.org
          Reporter: ed at 80386.nl
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

I just upgraded from LLVM+Clang 3.7.1 to LLVM+Clang 3.8.0rc1 and I noticed that
the following code now generates a compiler warning/error:

$ cat bla.c
#define some_generic_function(x) \
    _Generic(*(x),               \
             int: some_generic_function_int)(x)

static int *some_generic_function_int(int *x) {
  return x;
}

void some_function(void);
void some_function(void) {
  some_generic_function((int *)0);
}
$ x86_64-unknown-cloudabi-cc -O3 -Wall -Werror bla.c
bla.c:11:3: error: indirection of non-volatile null pointer will be deleted,
not trap [-Werror,-Wnull-dereference]
  some_generic_function((int *)0);
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
bla.c:2:14: note: expanded from macro 'some_generic_function'
    _Generic(*(x),               \
             ^~~~
bla.c:11:3: note: consider using __builtin_trap() or qualifying pointer with
'volatile'
bla.c:2:14: note: expanded from macro 'some_generic_function'
    _Generic(*(x),               \
             ^
1 error generated.

The warning should obviously not trigger in this case; the expression passed to
_Generic() is not evaluated. This is also not an uncommon construct. With
_Generic() you often want to match a type without any cv-qualifiers on the
pointer type. Dereferencing the expression is an easy way to get rid of those.

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