[llvm-bugs] [Bug 51524] New: '*this' captured by copy does not preserve constness of '*this' in mutable lambda

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=51524

            Bug ID: 51524
           Summary: '*this' captured by copy does not preserve constness
                    of '*this' in mutable lambda
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++2a
          Assignee: unassignedclangbugs at nondot.org
          Reporter: davveston at gmail.com
                CC: blitzrakete at gmail.com, erik.pilkington at gmail.com,
                    llvm-bugs at lists.llvm.org, richard-llvm at metafoo.co.uk

The following program:

// --------------------
#include <type_traits>

struct S;

void f(S *) = delete;  // #1
void f(const S *){};   // #2

struct S {
  void g() const {
    [*this]() mutable { f(this); }();
  }
};

int main() {
  S s{};
  s.g();
}
// ---------------

is well-formed. As per [expr.prim.lambda.closure]/12 [1]

> [...] but for purposes of name lookup, **determining the type**
> and value **of this** [...] the compound-statement is considered
> in the context of the lambda-expression. 

and [class.this]/1 [2]

> In the body of a non-static ([class.mfct]) member function, the 
> keyword this is a prvalue whose value is a pointer to the object
> for which the function is called. **The type of this** in a member 
> function whose type has a cv-qualifier-seq cv and whose class is 
> X is “pointer to cv X”.

the type of 'this' in a lambda defined in a const-qualified member function,
even when referred to from in the compound-statement of a lambda, is a
const-qualified type. This also holds even if the lambda is mutable and the
'*this' object is captured by copy. Thus, the 'f(this)' call in the program
above should resolve to overload #2, not overload #1.

However, Clang (for various versions and for {std=c++17, -std=c++2a}) rejects
the program as 'this' when referred to from within the lambda's
compound-statement is considered to be of non-const type, thus picking the
deleted overload #1. GCC and MSVC (various versions) both accept the program,
picking overload #2.

The relevant wording (afaict) is aligned e.g. with CWG 756 [3] but for a
copy-capture of the '*this' object (CWG 756 covers intentionally preserving
constness for variables captured by value, even in mutable lambdas).

[1] https://timsong-cpp.github.io/cppwp/n4861/expr.prim.lambda.closure#12
[2] https://timsong-cpp.github.io/cppwp/n4861/class.this#1
[3] http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#756

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