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From: "tartanllama at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/101013] New: Rejects-valid for alias template in a
 different namespace with a lambda in unevaluated context
Date: Thu, 10 Jun 2021 10:43:03 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101013

            Bug ID: 101013
           Summary: Rejects-valid for alias template in a different
                    namespace with a lambda in unevaluated context
           Product: gcc
           Version: 11.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tartanllama at gmail dot com
  Target Milestone: ---

Code:
namespace ns {
  template <class T>
  using void_t =3D decltype([]{}());
}

template<class T>
struct oh_no {
    using type =3D ns::void_t<T>;
};

Compiler output:
<source>:8:22: error: '<expression error>' in namespace 'ns' does not name a
type
    8 |     using type =3D ns::void_t<T>;
      |                      ^~~~~~~~~

Expected behaviour:=20
Compile cleanly

Additional information:
The code compiles if oh_no and void_t are in the same namespace

GCC version and configuration options:
GCC 11.1.0
../gcc-11.1.0/configure --prefix=3D/opt/compiler-explorer/gcc-build/staging
--build=3Dx86_64-linux-gnu --host=3Dx86_64-linux-gnu --target=3Dx86_64-linu=
x-gnu
--disable-bootstrap --enable-multiarch --with-abi=3Dm64
--with-multilib-list=3Dm32,m64,mx32 --enable-multilib --enable-clocale=3Dgnu
--enable-languages=3Dc,c++,fortran,ada,go,d --enable-ld=3Dyes --enable-gold=
=3Dyes
--enable-libstdcxx-debug --enable-libstdcxx-time=3Dyes --enable-linker-buil=
d-id
--enable-lto --enable-plugins --enable-threads=3Dposix
--with-pkgversion=3DCompiler-Explorer-Build

Command line:=20
-std=3Dc++20

Compiler explorer link:
https://godbolt.org/z/EYsbbjodj=