[cfe-dev] Local extern declarations

[Richard Pennington]

Hi,

I'm compiling binutils with clang and have run across a small problem.
Some parts of binutils use something like:

int i;

void f()
{
	extern int i;
}

with -Werror -Wall. This causes -Wshadow to complain about a shadowed 
variable.
My first thought was to add

   if (D->getLinkage() == ExternalLinkage)
     return;

to CheckShadow() in SemaDecl.cxx, which worked for binutils, but when I 
tried to update the warn-shadow.c clang test case, I noticed what might 
be a deeper problem. I added this to warn-shadow.c:

void f() {
     extern int i;
}

When I compile with my Sema modification turned off, I get:

warn-shadow.c:34:16: error: declaration shadows a variable in the global 
scope [-Wshadow]
     extern int i;
                ^
warn-shadow.c:3:5: note: previous declaration is here 

int i;          // expected-note 3 {{previous declaration is here}}
     ^

Which I expect, but then I get

warn-shadow.c:47:16: error: declaration shadows a local variable 
[-Wshadow]
void test4(int i) { // expected-warning {{declaration shadows a variable 
in the global scope}}
                ^
warn-shadow.c:34:16: note: previous declaration is here 

     extern int i;
                ^

The problem is that my "extern int i;" becomes the previous declaration. 
Notice the "shadows a local variable".

It seems to me that local extern declarations that shadow file scope 
extern declarations should either be discarded, or not made visible 
outside of the scope of their declaration for the purposes of detecting 
shadowing (or anything else for that matter).

Thoughts?

-Rich