[llvm-bugs] [Bug 35255] New: [rejects valid] constexpr non-literal variable not usable in lambda without capture or local class

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=35255

            Bug ID: 35255
           Summary: [rejects valid] constexpr non-literal variable not
                    usable in lambda without capture or local class
           Product: clang
           Version: unspecified
          Hardware: PC
                OS: other
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++11
          Assignee: unassignedclangbugs at nondot.org
          Reporter: bastienPenava at gmail.com
                CC: dgregor at apple.com, llvm-bugs at lists.llvm.org

Tested through compiler explorer with the version 6.0.0 (trunk 317548).
Link of the example: https://godbolt.org/g/Q91sd1

code:
struct A { constexpr A() {} };

template<class T>
void func(T x)
{
    constexpr auto y = x();
    (void)[] { auto x = y; }; //with non literal: "error: variable 'y' cannot
be implicitly captured in a lambda with no capture-default specified"
    struct A { decltype(y) value = y; }; //with non-literal: "error: reference
to local variable 'y' declared in enclosing function ..."
}

int main()
{
    func([]{return 3;}); //no error
    func([]{return A{}; }); //error
}

log:
7 : <source>:7:25: error: variable 'y' cannot be implicitly captured in a
lambda with no capture-default specified
    (void)[] { auto x = y; };
                        ^
14 : <source>:14:5: note: in instantiation of function template specialization
'func<(lambda at
/tmp/compiler-explorer-compiler117108-60-mn1ptp.k9dm5i2j4i/example.cpp:14:10)>'
requested here
    func([]{return A{}; });
    ^
6 : <source>:6:20: note: 'y' declared here
    constexpr auto y = x();
                   ^
7 : <source>:7:11: note: lambda expression begins here
    (void)[] { auto x = y; };
          ^
8 : <source>:8:36: error: reference to local variable 'y' declared in enclosing
function 'func<(lambda at
/tmp/compiler-explorer-compiler117108-60-mn1ptp.k9dm5i2j4i/example.cpp:14:10)>'
    struct A { decltype(y) value = y; };
                                   ^
8 : <source>:8:12: note: in instantiation of default member initializer
'func((lambda at
/tmp/compiler-explorer-compiler117108-60-mn1ptp.k9dm5i2j4i/example.cpp:14:10))::A::value'
requested here
    struct A { decltype(y) value = y; };
           ^
14 : <source>:14:5: note: in instantiation of function template specialization
'func<(lambda at
/tmp/compiler-explorer-compiler117108-60-mn1ptp.k9dm5i2j4i/example.cpp:14:10)>'
requested here
    func([]{return A{}; });
    ^
6 : <source>:6:20: note: 'y' declared here
    constexpr auto y = x();
                   ^

version:
clang version 6.0.0 (trunk 317548)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /opt/compiler-explorer/clang-trunk/bin
Found candidate GCC installation:
/opt/compiler-explorer/gcc-7.1.0/lib/gcc/x86_64-linux-gnu/7.1.0
Selected GCC installation:
/opt/compiler-explorer/gcc-7.1.0/lib/gcc/x86_64-linux-gnu/7.1.0
Candidate multilib: .;@m64
Selected multilib: .;@m64
 "/opt/compiler-explorer/clang-trunk-20171107/bin/clang-6.0" -cc1 -triple
x86_64-unknown-linux-gnu -S -disable-free -disable-llvm-verifier
-discard-value-names -main-file-name example.cpp -mrelocation-model static
-mthread-model posix -mdisable-fp-elim -fmath-errno -masm-verbose
-mconstructor-aliases -munwind-tables -fuse-init-array -target-cpu x86-64
-dwarf-column-info -debug-info-kind=limited -dwarf-version=4
-debugger-tuning=gdb -v -coverage-notes-file
/tmp/compiler-explorer-compiler117108-60-ffz0kd.942tymn29/output.gcno
-resource-dir /opt/compiler-explorer/clang-trunk-20171107/lib/clang/6.0.0
-c-isystem /usr/include/x86_64-linux-gnu -cxx-isystem
/usr/include/x86_64-linux-gnu -internal-isystem
/opt/compiler-explorer/gcc-7.1.0/lib/gcc/x86_64-linux-gnu/7.1.0/../../../../include/c++/7.1.0
-internal-isystem
/opt/compiler-explorer/gcc-7.1.0/lib/gcc/x86_64-linux-gnu/7.1.0/../../../../include/c++/7.1.0/x86_64-linux-gnu
-internal-isystem
/opt/compiler-explorer/gcc-7.1.0/lib/gcc/x86_64-linux-gnu/7.1.0/../../../../include/c++/7.1.0/backward
-internal-isystem /usr/local/include -internal-isystem
/opt/compiler-explorer/clang-trunk-20171107/lib/clang/6.0.0/include
-internal-externc-isystem /usr/include/x86_64-linux-gnu
-internal-externc-isystem /include -internal-externc-isystem /usr/include
-std=c++1z -fdeprecated-macro -fdebug-compilation-dir /compiler-explorer
-ferror-limit 19 -fmessage-length 0 -fobjc-runtime=gcc -fcxx-exceptions
-fexceptions -fdiagnostics-show-option -mllvm --x86-asm-syntax=intel -o
/tmp/compiler-explorer-compiler117108-60-ffz0kd.942tymn29/output.s -x c++
<source>
clang -cc1 version 6.0.0 based upon LLVM 6.0.0svn default target
x86_64-unknown-linux-gnu
ignoring nonexistent directory "/include"
ignoring duplicate directory "/usr/include/x86_64-linux-gnu"
#include "..." search starts here:
#include <...> search starts here:
 /usr/include/x86_64-linux-gnu

/opt/compiler-explorer/gcc-7.1.0/lib/gcc/x86_64-linux-gnu/7.1.0/../../../../include/c++/7.1.0

/opt/compiler-explorer/gcc-7.1.0/lib/gcc/x86_64-linux-gnu/7.1.0/../../../../include/c++/7.1.0/x86_64-linux-gnu

/opt/compiler-explorer/gcc-7.1.0/lib/gcc/x86_64-linux-gnu/7.1.0/../../../../include/c++/7.1.0/backward
 /usr/local/include
 /opt/compiler-explorer/clang-trunk-20171107/lib/clang/6.0.0/include
 /usr/include

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