[Bug c++/98326] New: ICE: in create_tmp_var, at gimple-expr.c:482, converting stateless generic-lambda to function pointer

["leni536 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98326

            Bug ID: 98326
           Summary: ICE: in create_tmp_var, at gimple-expr.c:482,
                    converting stateless generic-lambda to function
                    pointer
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: leni536 at gmail dot com
  Target Milestone: ---

version: g++ (Compiler-Explorer-Build) 10.2.0
arguments: -O2 -std=c++17 -pedantic-errors

```
struct A {
    A() = default;
    A(const A&) {}
};

void (*fptr)(A) = [](auto){};
```

<source>: In static member function 'static constexpr decltype
(((const<lambda(auto:1)>*)0)->operator()<auto:1>(static_cast<auto:1&&>(<anonymous>)))<lambda(auto:1)>::_FUN(auto:1)
[with auto:1 = A]':
<source>:6:28: internal compiler error: in create_tmp_var, at gimple-expr.c:482
    6 | void (*fptr)(A) = [](auto){};
      |                            ^


The ICE seems to happen when the by-value parameter's type is not trivially
copyable. It can also be reproduced with a non-trivial destructor.

If the copy-constructor is deleted then it fails to compile with a non-ice
error.

Related: PR 86943

In my understanding gcc tries to copy/move the by-value parameter in the free
function to pass it to `closure{}(args)`. I don't think that copying/moving the
by-value argument is correct. The effect of calling the resulting function
pointer should be equivalent to calling the operator() on the closure object,
it's not expressed in terms of forwarding the parameters:

https://timsong-cpp.github.io/cppwp/n4659/expr.prim.lambda.closure#8

It's more precisely spelled out in C++20, as there it can be expressed it in
terms of a default constructed object of the closure type:

http://eel.is/c++draft/expr.prim.lambda.closure#10.sentence-1