V: Redeclaration of variable with no linkage

[tankf33der]

V version: V 0.1.24 778a1cc

OS: Linux lambda 4.19.91-1-MANJARO #1 SMP Sat Dec 21 11:18:44 UTC 2019 x86_64 GNU/Linux

What did you do?

import os

fn main () {
    mut s := ''
    s = os.read_file("mike.dat") or {
        panic('boom')
    }
    println(s)
}

What did you expect to see?

content of file mike.dat

What did you see instead?

$ v run utf.v
/home/mpech/.cache/v/utf.tmp.c: In function ‘main__main’:
/home/mpech/.cache/v/utf.tmp.c:5229:10: error: redeclaration of ‘s’ with no linkage
 5229 |  string  s   ;
      |          ^
/home/mpech/.cac...
(Use `v -g` to print the entire error message)

V error: C error. This should never happen. 
Please create a GitHub issue: https://github.com/vlang/v/issues/new/choose

[ricardobeat]

C output:

void main__main () {
string s= tos3("") ;
Option_string tmp1 =   os__read_file ( tos3("mike.dat") ) ;
 string  s   ;
 if (!tmp1 .ok) {
 string err = tmp1 . error;
 int errcode = tmp1 . ecode;
 v_panic ( tos3("boom") ) ;
 }
  s    = *(string*)tmp1.data;
 ;
 println ( s ) ;
 }

The gen_handle_option_or_else method that generates the optional value always redeclares the target variable first: https://github.com/vlang/v/blob/ae4c8af674e70486210222a304f191eacef6e600/vlib/compiler/gen_c.v#L119-L122

Which leads to the duplicate string s declaration above.

Seeing as C does not have the ability to check if a variable has been initialized, the V compiler has to be aware of the existing variable. I'm unfamiliar with the compiler, but maybe passing p.known_var(var_name) as an argument to gen_var_decl in https://github.com/vlang/v/blob/ae4c8af674e70486210222a304f191eacef6e600/vlib/compiler/parser.v#L1690

[tankf33der]

I can still fail it another way:

$ v -v

V 0.1.24 c24a1b3

fn mike() ?int {
    return 122
}
fn main()  {
    mut a := 0
    a = mike()
    or {
        panic('boom')
    }
    a++
    println('$a')
}

$ v run ff.v 
/home/mpech/.cache/v/ff.tmp.c: In function ‘main__main’:
/home/mpech/.cache/v/ff.tmp.c:3909:7: error: redeclaration of ‘a’ with no linkage
 3909 |  int  a   ;
      |       ^
/home/mpech/.cache/v/ff.t...
(Use `v -g` to print the entire error message)

V error: C error. This should never happen. 
Please create a GitHub issue: https://github.com/vlang/v/issues/new/choose

[ricardobeat]

is_var_decl is set to false explicitly just earlier during assignment: https://github.com/vlang/v/blob/master/vlib/compiler/aparser.v#L1605-L1607

Setting it back to true before handing it to gen_handle_option_or_else call in line 1623 fixes this case too, but this just feels wrong, and I'm a bit lost on what's going on in those three lines above. I would expect an initialization assignment to fail after this patch, but it doesn't... I'll have to defer to someone with more knowledge of this.

[alexesprit]

This issue was also reported in #3389, but IMO that has a better title, so I'm closing this one. :)