Why does this invalid-looking code compile success

Question

This question already has an answer here:

• Strange code that compiles with g++ 3 answers

Consider this strange program:

int main()
{
    int(*){} Is it C++14 or any other language?
}

(See a live demo here & here.)

Even though the comment // is missing, the code compiles fine without any errors & warnings even when I use -pedantic-errorsoptions in g++ 6.0. This seems like a compiler bug to me. Is it really a bug in the compiler?

Answer 1

I've run the command on my Fedora VM with g++ version 5.1.1 and found the following:

[user:~] 1 $ g++ -fdump-tree-original-raw tmp.cpp
tmp.cpp: In function ‘int main()’:
tmp.cpp:3:11: warning: extended initializer lists only available with -std=c++11 or -std=gnu++11
     int(*){} Is it C++14 or any other language?
       ^

However that still managed to compile... So I've dumped the AST and got this:

$ cat tmp.cpp.003t.original 

;; Function int main() (null)
;; enabled by -tree-original

@1      return_expr      type: @2       expr: @3      
@2      void_type        name: @4       algn: 8       
@3      init_expr        type: @5       op 0: @6       op 1: @7      
@4      type_decl        name: @8       type: @2       srcp: <built-in>:0      
                         note: artificial 
@5      integer_type     name: @9       size: @10      algn: 32      
                         prec: 32       sign: signed   min : @11     
                         max : @12     
@6      result_decl      type: @5       scpe: @13      srcp: tmp.cpp:1      
                         note: artificial              size: @10     
                         algn: 32      
@7      integer_cst      type: @5      int: 0
@8      identifier_node  strg: void     lngt: 4       
@9      type_decl        name: @14      type: @5       srcp: <built-in>:0      
                         note: artificial 
@10     integer_cst      type: @15     int: 32
@11     integer_cst      type: @5      int: -2147483648
@12     integer_cst      type: @5      int: 2147483647
@13     function_decl    name: @16      type: @17      scpe: @18     
                         srcp: tmp.cpp:1               lang: C       
                         link: extern  
@14     identifier_node  strg: int      lngt: 3       
@15     integer_type     name: @19      size: @20      algn: 128     
                         prec: 128      sign: unsigned min : @21     
                         max : @22     
@16     identifier_node  strg: main     lngt: 4       
@17     function_type    size: @23      algn: 8        retn: @5      
                         prms: @24     
@18     translation_unit_decl 
@19     identifier_node  strg: bitsizetype             lngt: 11      
@20     integer_cst      type: @15     int: 128
@21     integer_cst      type: @15     int: 0
@22     integer_cst      type: @15     int: -1
@23     integer_cst      type: @15     int: 8
@24     tree_list        valu: @2      

Which is too big to fit inside a comment but should be useful in determining what is going on. I'm still going through this but I'm just posting this information for others to build from.

Which is visualized like this .

Answer 2

This looks to be an bug/feature/issue with g++ in all of the versions I can test it on. Running

int main()
{
    int(*){} Is it C++14 or any other language?
}

On godbolt.org for all versions of g++ with no compilation flags give the following assembly ouput.

main:
    pushq   %rbp
    movq    %rsp, %rbp
    movl    $0, %eax
    leave
    ret

The only diagnosis I get is on godbolt.org and that is

!!warning: extended initializer lists only available with -std=c++0x or -std=gnu++0x

Clang, ICC and MSVS all fail to compile this.

EDIT:

From the comments zwol filed a bug with gcc on this. The bug report can be found here.