[Bug c++/55357] New: -Wshadow warns about lambda function parameters matching variables in outer scope

[Bug c++/55357] New: -Wshadow warns about lambda function parameters matching variables in outer scope

[david at doublewise dot net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55357

             Bug #: 55357
           Summary: -Wshadow warns about lambda function parameters
                    matching variables in outer scope
    Classification: Unclassified
           Product: gcc
           Version: 4.7.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: david@doublewise.net


Similar to http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30566

input:

int main() {
    int x = 1;
    auto const lambda = [](int x) {
        return x;
    };
}



output:

src/main.cpp: In lambda function:
src/main.cpp:3:30: error: declaration of 'x' shadows a previous local
[-Werror=shadow]
src/main.cpp:2:6: error: shadowed declaration is here [-Werror=shadow]
src/main.cpp: In static member function 'static int
main()::<lambda(int)>::_FUN(int)':
src/main.cpp:5:2: error: declaration of 'x' shadows a previous local
[-Werror=shadow]
src/main.cpp:2:6: error: shadowed declaration is here [-Werror=shadow]
cc1plus: all warnings being treated as errors



My lambda has an empty capture specification, so the outer x is not captured.
Note that if I change the lambda parameter's name but do not change the name of
the returned value, I get an error that "x" was not captured.

I can't decide if this is correct behavior for the warning. It would catch
errors caused by people thinking they were using the outer variables by simply
disallowing overlap, which is good. However, it's not possible to use the outer
scope variable no matter what I name my variables in the inner scope, so there
is no chance of a silent behavior change.

However, the following code should always give a warning with -Wshadow:

int main() {
    int x = 1;
    // Capture everything
    auto const lambda = [&](int x) {
        return x;
    };
}




I also note that gcc warns me about the first line the lambda appears on (line
3) and the last line (line 5).

[Bug c++/55357] -Wshadow warns about lambda function parameters matching variables in outer scope

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55357

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |diagnostic
             Status|UNCONFIRMED                 |ASSIGNED
   Last reconfirmed|                            |2013-03-05
                 CC|                            |jason at gcc dot gnu.org
         AssignedTo|unassigned at gcc dot       |jason at gcc dot gnu.org
                   |gnu.org                     |
     Ever Confirmed|0                           |1

--- Comment #1 from Jason Merrill <jason at gcc dot gnu.org> 2013-03-05 19:25:34 UTC ---
(In reply to comment #0)
> I can't decide if this is correct behavior for the warning. It would catch
> errors caused by people thinking they were using the outer variables by simply
> disallowing overlap, which is good.

I think it's appropriate to warn here; whether or not there is a default
capture seems like a subtle distinction that is easy to miss when reading the
code.

> I also note that gcc warns me about the first line the lambda appears on (line
> 3) and the last line (line 5).

That's a bug.

[Bug c++/55357] -Wshadow warns about lambda function parameters matching variables in outer scope

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55357

--- Comment #2 from Jason Merrill <jason at gcc dot gnu.org> 2013-03-17 02:38:16 UTC ---
Author: jason
Date: Sun Mar 17 02:38:01 2013
New Revision: 196739

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=196739
Log:
    PR c++/55357
    * semantics.c (maybe_add_lambda_conv_op): Clear DECL_NAME of copied
    parms to avoid duplicate -Wshadow warnings.

Added:
    trunk/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-shadow1.C
Modified:
    trunk/gcc/cp/ChangeLog
    trunk/gcc/cp/semantics.c

[Bug c++/55357] -Wshadow warns about lambda function parameters matching variables in outer scope

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55357

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|                            |FIXED
   Target Milestone|---                         |4.9.0

--- Comment #3 from Jason Merrill <jason at gcc dot gnu.org> 2013-03-23 19:22:43 UTC ---
Duplicate warning fixed for 4.9.

[Bug c++/55357] -Wshadow warns about lambda function parameters matching variables in outer scope

[jcl at nvidia dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55357

JC Liang <jcl at nvidia dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jcl at nvidia dot com

--- Comment #4 from JC Liang <jcl at nvidia dot com> ---
Could we reevaluate this case? I think it's not necessary to warning about
lambda parameters shadowing local vars in outer scope, because it's simply
won't compile.
MSVC's default-on policy C4457 doesn't have this issue. Our project is cross
platform so we really want to have parity in different compiler for this
warning.