[Bug c++/94624] New: Nested lambda mutable capture of outer lambda non-mutable capture is not allowed

[Bug c++/94624] New: Nested lambda mutable capture of outer lambda non-mutable capture is not allowed

[cuzdav at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94624

            Bug ID: 94624
           Summary: Nested lambda mutable capture of outer lambda
                    non-mutable capture is not allowed
           Product: gcc
           Version: tree-ssa
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: cuzdav at gmail dot com
  Target Milestone: ---

An inner mutable lambda with a simple copy-capture of the outer lambda's
immutable variable is not allowed to be modified.  

This applies to all versions of g++ since mutable lambdas were introduced.

Live example
https://godbolt.org/z/RkMeYL


Full source code to reproduce:


////////////////////////////////////////////////
template <typename F> void foo(F const&) {}
template <typename F> void bar(F const&) {}

void bad() {
    int x = 5;
    foo([x]() { 
        bar([x]() mutable {
            x = 10;    // << modify local copy in mutable lambda should be ok 
        });
    });
}
////////////////////////////////////////////////


In lambda function:
8:15: error: assignment of read-only variable 'x'

    8 |             x = 10;

      |             ~~^~~~

Compiler returned: 1


Adding "mutable" to the outer lambda works around this, so somehow the
read-only behavior in the outer lambda is leaking into the inner lambda, even
though it is a simple copy and the underlying type is "int".

Additionally, clang accepts this all the way back to version 5.

[Bug c++/94624] Assignment to nested mutable lambda's capture (of outer lambda non-mutable capture) is not allowed

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94624

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
   Target Milestone|---                         |12.0
                 CC|                            |ppalka at gcc dot gnu.org
         Resolution|---                         |DUPLICATE

--- Comment #1 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Looks like this is fixed for GCC 12 by r12-6065-g89cf57ea35d1e0 aka the fix for
PR94376.

*** This bug has been marked as a duplicate of bug 94376 ***