convert CStyleCastExpr <ToVoid> to _

[Konstantin8105]

Example in clang AST tree:

|-FunctionDecl 0x26b0288 <line:1778:1, line:1783:1> line:1778:13 used interrupt_handler 'void (int)' static
| |-ParmVarDecl 0x26b0200 <col:31, col:35> col:35 used NotUsed 'int'
| `-CompoundStmt 0x26b0740 <col:43, line:1783:1>
|   |-CStyleCastExpr 0x26b03c8 <line:1779:3, col:17> 'void' <ToVoid>
|   | `-ImplicitCastExpr 0x26b03b0 <col:9, col:17> 'int' <LValueToRValue>
|   |   `-ParenExpr 0x26b0390 <col:9, col:17> 'int' lvalue
|   |     `-DeclRefExpr 0x26b0328 <col:10> 'int' lvalue ParmVar 0x26b0200 'NotUsed' 'int'

|   |   |   |   |-CStyleCastExpr 0x26bc970 <col:20, col:29> 'void' <ToVoid>
|   |   |   |   | `-ParenExpr 0x26bc950 <col:27, col:29> 'int'
|   |   |   |   |   `-IntegerLiteral 0x26bc8f0 <col:28> 'int' 0

|   |-CStyleCastExpr 0x26d01f0 <line:2210:3, col:12> 'void' <ToVoid>
|   | `-ImplicitCastExpr 0x26d01d8 <col:9, col:12> 'char **' <LValueToRValue>
|   |   `-ParenExpr 0x26d01b8 <col:9, col:12> 'char **' lvalue
|   |     `-DeclRefExpr 0x26d0150 <col:10> 'char **' lvalue ParmVar 0x26cfe60 'az' 'char **'

[elliotchance]

I'm not sure what this issue is for?

[Konstantin8105]

In according to shell.c:

|-FunctionDecl 0x24e3650 <line:3009:1, line:3020:1> line:3009:12 used captureOutputCallback 'int (void *, int, char **, char **)' static
| |-ParmVarDecl 0x24e3450 <col:34, col:40> col:40 used pArg 'void *'
| |-ParmVarDecl 0x24e34c0 <col:46, col:50> col:50 used nArg 'int'
| |-ParmVarDecl 0x24e3538 <col:56, col:63> col:63 used azArg 'char **'
| |-ParmVarDecl 0x24e35b0 <col:70, col:77> col:77 used az 'char **'
| `-CompoundStmt 0x24e4360 <col:80, line:3020:1>
|   |-DeclStmt 0x24e3800 <line:3010:3, col:34>
|   | `-VarDecl 0x24e3728 <col:3, col:30> col:14 used p 'ShellText *' cinit
|   |   `-CStyleCastExpr 0x24e37d8 <col:18, col:30> 'ShellText *' <BitCast>
|   |     `-ImplicitCastExpr 0x24e37c0 <col:30> 'void *' <LValueToRValue>
|   |       `-DeclRefExpr 0x24e3788 <col:30> 'void *' lvalue ParmVar 0x24e3450 'pArg' 'void *'
|   |-DeclStmt 0x24e3888 <line:3011:3, col:8>
|   | `-VarDecl 0x24e3828 <col:3, col:7> col:7 used i 'int'
|   |-CStyleCastExpr 0x24e3940 <line:3012:3, col:12> 'void' <ToVoid>
|   | `-ImplicitCastExpr 0x24e3928 <col:9, col:12> 'char **' <LValueToRValue>
|   |   `-ParenExpr 0x24e3908 <col:9, col:12> 'char **' lvalue
|   |     `-DeclRefExpr 0x24e38a0 <col:10> 'char **' lvalue ParmVar 0x24e35b0 'az' 'char **'
|   |-IfStmt 0x24e3a40 <line:3013:3, col:25>

/*
** This is the callback routine from sqlite3_exec() that appends all
** output onto the end of a ShellText object.
*/
static int captureOutputCallback(void *pArg, int nArg, char **azArg, char **az){
  ShellText *p = (ShellText*)pArg;
  int i;
  UNUSED_PARAMETER(az);
  if( azArg==0 ) return 0;
//.....

/*
** Used to prevent warnings about unused parameters
*/
#define UNUSED_PARAMETER(x) (void)(x)

Example of C code for tests:

int main(){
        char **az;
        (void)(az);
        return 0;
}

In AST:

`-FunctionDecl 0x21a44c8 <1.c:1:1, line:5:1> line:1:5 main 'int ()'
  `-CompoundStmt 0x21a4768 <col:11, line:5:1>
    |-DeclStmt 0x21a4650 <line:2:9, col:18>
    | `-VarDecl 0x21a45f0 <col:9, col:16> col:16 used az 'char **'
    |-CStyleCastExpr 0x21a4708 <line:3:9, col:18> 'void' <ToVoid>
    | `-ImplicitCastExpr 0x21a46f0 <col:15, col:18> 'char **' <LValueToRValue>
    |   `-ParenExpr 0x21a46d0 <col:15, col:18> 'char **' lvalue
    |     `-DeclRefExpr 0x21a4668 <col:16> 'char **' lvalue Var 0x21a45f0 'az' 'char **'
    `-ReturnStmt 0x21a4750 <line:4:9, col:16>
      `-IntegerLiteral 0x21a4730 <col:16> 'int' 0

Result of Go code:

package main

func main() {
        __init()
        var az [][]byte
        (az)
        return
}
func __init() {
}

○ → go build 1.go
# command-line-arguments
./1.go:8: az evaluated but not used

[Konstantin8105]

In according to sqlite3.c:
In C code:

SQLITE_PRIVATE sqlite3_int64 sqlite3StatusValue(int op){
  wsdStatInit;
  assert( op>=0 && op<ArraySize(wsdStat.nowValue) );
  assert( op>=0 && op<ArraySize(statMutex) );
  assert( sqlite3_mutex_held(statMutex[op] ? sqlite3Pcache1Mutex()
                                           : sqlite3MallocMutex()) );
  return wsdStat.nowValue[op];
}

In AST tree:

|-FunctionDecl 0x28793d0 prev 0x2803c88 <line:19129:1, line:19136:1> line:19129:22 used sqlite3StatusValue 'sqlite3_int64 (int)' static
| |-ParmVarDecl 0x2879348 <col:41, col:45> col:45 used op 'int'
| `-CompoundStmt 0x2879820 <col:48, line:19136:1>
|   |-NullStmt 0x2879480 <line:19130:14>
|   |-ParenExpr 0x2879538 <line:19131:3, col:14> 'void'
|   | `-CStyleCastExpr 0x2879510 <col:4, col:13> 'void' <ToVoid>
|   |   `-ParenExpr 0x28794f0 <col:11, col:13> 'int'
|   |     `-IntegerLiteral 0x2879490 <col:12> 'int' 0

Go result:

func sqlite3StatusValue(op int) sqlite3_int64 {
        (0)
        (0)
        (0)
        return sqlite3Stat.nowValue[op]
}

[Konstantin8105]

In my point of view, in Go interpretation it is like : _ = variable