https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102136
Bug ID: 102136
Summary: bogus warnings for function calls in unevaluated
contexts
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: msebor at gcc dot gnu.org
Target Milestone: ---
GCC implements a number of warnings in the front ends that are intended to
detect bugs that would occur during the evaluation of the checked expression.
But in contexts where the expression is not evaluated such as in operands to
_Alignof or sizeof the warnings are false positives.
Using the result of a function call in such a context might be way of
determining the size of the return type of the function. In most cases it's
easy to avoid these false positives by providing a valid argument. Where it's
harder is in calls to pure virtual member functions as in the test case below:
$ cat t.C && gcc -S -Wall t.C
struct A { virtual int f () = 0; };
enum {
e0 = sizeof __builtin_strlen (0),
e1 = sizeof (__typeof__ (__builtin_strlen (0))),
e2 = __alignof__ __builtin_printf ("%s", 0),
e3 = sizeof ((A*)0)->f ()
};
t.C:4:32: warning: argument 1 null where non-null expected [-Wnonnull]
4 | e0 = sizeof __builtin_strlen (0),
| ~~~~~~~~~~~~~~~~~^~~
<built-in>: note: in a call to function ‘long unsigned int
__builtin_strlen(const char*)’ declared ‘nonnull’
t.C:5:45: warning: argument 1 null where non-null expected [-Wnonnull]
5 | e1 = sizeof (__typeof__ (__builtin_strlen (0))),
| ~~~~~~~~~~~~~~~~~^~~
<built-in>: note: in a call to function ‘long unsigned int
__builtin_strlen(const char*)’ declared ‘nonnull’
t.C:6:40: warning: format ‘%s’ expects argument of type ‘char*’, but argument 2
has type ‘int’ [-Wformat=]
6 | e2 = __alignof__ __builtin_printf ("%s", 0),
| ~^ ~
| | |
| | int
| char*
| %d
t.C:7:26: warning: ‘this’ pointer is null [-Wnonnull]
7 | e3 = sizeof ((A*)0)->f ()
| ~~~~~~~~~~~^~
t.C:1:24: note: in a call to non-static member function ‘virtual int A::f()’
1 | struct A { virtual int f () = 0; };
| ^
