[llvm-bugs] [Bug 42303] New: Default parameter of unique_ptr instantiates causing failure with incomplete type:

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=42303

            Bug ID: 42303
           Summary: Default parameter of unique_ptr instantiates causing
                    failure with incomplete type:
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: erich.keane at intel.com
                CC: blitzrakete at gmail.com, dgregor at apple.com,
                    erik.pilkington at gmail.com, llvm-bugs at lists.llvm.org,
                    richard-llvm at metafoo.co.uk

#include <memory>
struct Base;
  class Class {
       Class ( std::unique_ptr<Base>&& combat_targets = nullptr);
   };


The above reproducer results in:
bash-4.2$ ../../llvm-project/build/bin/clang++ --version
clang version 9.0.0 (/iusers/ekeane1/workspaces/llvm-project/clang
689509edab4d073ec3bca7d433808ae48a6cddd8)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir:
/export/iusers/ekeane1/workspaces/xmain/repro/../../llvm-project/build/bin

bash-4.2$ ../../llvm-project/build/bin/clang++ test.cpp
In file included from test.icx.cpp:1:
In file included from
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../include/c++/4.8.5/memory:81:
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../include/c++/4.8.5/bits/unique_ptr.h:65:16:
error: invalid application of 'sizeof' to an incomplete type 'Base'
        static_assert(sizeof(_Tp)>0,
                      ^~~~~~~~~~~
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../include/c++/4.8.5/bits/unique_ptr.h:184:4:
note: in instantiation of member function
'std::default_delete<Base>::operator()' requested here
          get_deleter()(__ptr);
          ^
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../include/c++/4.8.5/bits/unique_ptr.h:157:50:
note: in instantiation of member function 'std::unique_ptr<Base,
std::default_delete<Base> >::~unique_ptr' requested here
      constexpr unique_ptr(nullptr_t) noexcept : unique_ptr() { }
                                                 ^
test.icx.cpp:3:12: note: forward declaration of 'Base'


This appears to be a regression, as current godbolt (r363523) doesn't seem to
repro the issue. https://godbolt.org/z/Z_2tlY

We seem to be instantiating the default parameter before the function is used,
so this no longer works (it does in GCC as well).  Is this intentional?

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