[Bug c++/85662] New: regression since 6: "error: non-constant condition for static assertion" from __builtin_offsetof in C++

[roland at gnu dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85662

            Bug ID: 85662
           Summary: regression since 6: "error: non-constant condition for
                    static assertion" from __builtin_offsetof in C++
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: roland at gnu dot org
                CC: msebor at gcc dot gnu.org
  Target Milestone: ---

This code was accepted by GCC 6 but is rejected by GCC 7, 8, and trunk.
The error message appears to complain that the result of the __builtin_offsetof
expression has pointer type rather than size_t.


$ cat foo.cc                                                      
struct foo {                                                                    
    unsigned long x[31];                                                        
};                                                                              

struct bar {                                                                    
    bool b;                                                                     
    foo f;                                                                      
};                                                                              

static_assert(__builtin_offsetof(bar, f.x[31 - 1]) == 8 + ((31 - 1) * 8), "");  
$ ./gcc/xgcc -Bgcc/ -O2 -S foo.cc                                  
foo.cc:10:52: error: non-constant condition for static assertion                
 static_assert(__builtin_offsetof(bar, f.x[31 - 1]) == 8 + ((31 - 1) * 8), ""); 
               ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~       
foo.cc:10:52: error: value ‘8’ of type ‘bar*’ is not a constant expression      
[Exit 1]                                                                        


The equivalent C code:

struct foo {                                                                    
    unsigned long x[31];                                                        
};                                                                              

struct bar {                                                                    
    _Bool b;                                                                    
    struct foo f;                                                               
};                                                                              

_Static_assert(__builtin_offsetof(struct bar, f.x[31 - 1]) == 8 + ((31 - 1) *
8), "");                                                                        


is accepted without complaint.


bisected to r238909 (git cb7688247fdcff08df18baed1317fce5b0e9db13)

gcc/cp/ChangeLog

2016-07-30  Martin Sebor  <msebor@redhat.com>                                   

        PR c++/60760                                                            
        PR c++/71091                                                            
        * constexpr.c (cxx_eval_binary_expression): Reject invalid expressions  
        involving null pointers.                                                
        (cxx_eval_component_reference): Reject null pointer dereferences.       
        (cxx_eval_indirect_ref): Reject indirecting through null pointers.      
        (cxx_eval_constant_expression): Reject invalid expressions involving    
        null pointers.