[Bug c++/83167] New: decltype((x)) inside lambda is considered odr-use if x is not a reference

[deaeod at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83167

            Bug ID: 83167
           Summary: decltype((x)) inside lambda is considered odr-use if x
                    is not a reference
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: deaeod at gmail dot com
  Target Milestone: ---

https://godbolt.org/g/NmA3ZP

#include <type_traits>

int main() {
    int x = 1;
    int&& y = static_cast<int&&>(x);

    auto A = []{ static_assert(std::is_same_v<decltype((x)), int&>); };
    auto B = [=]{ static_assert(std::is_same_v<decltype((x)), int&>); };

    auto C = []{ static_assert(std::is_same_v<decltype((y)), int&>); };
    auto D = [=]{ static_assert(std::is_same_v<decltype((y)), int&>); };
}

I expected this to compile cleanly, but static asserts A and B fail under g++.
A fails because x wasnt captured and B fails because the deduced type is const
int&.

Here is how i would argue for my expectation:
http://eel.is/c++draft/expr.prim.lambda#capture-11.sentence-3
    -- "An id-expression that is not an odr-use refers to the original entity,
never to a member of the closure type."
http://eel.is/c++draft/basic.def.odr#def:potentially_evaluated
    -- "An expression is potentially evaluated unless it is an unevaluated
operand or a subexpression thereof."
http://eel.is/c++draft/basic.def.odr#def:odr-used
    -- "A variable x whose name appears as a potentially-evaluated expression
ex is odr-used by ex unless [...]"
http://eel.is/c++draft/dcl.type.simple#4.sentence-3
    -- "The operand of the decltype specifier is an unevaluated operand."