Constexpr non-scalar variable not usable in lambda without capture or local class

[DaemonSnake]

Bugzilla Link	35255
Version	trunk
OS	other
CC	@davidstone,@DougGregor,@JohelEGP

Extended Description

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

[davidstone]

It looks like you have two separate test cases here. The first test case can be reduced to

void a() {
	constexpr auto x = 0;
	[] { x; };
}

I believe clang is correct to reject this program.

The second test case is

struct A { };

void g() {
	constexpr auto x = A();
	struct B { A value = x; };
}

clang also rejects this program, and I agree that it's surprising that it is accepted if all of the A in g are replaced with int, but I also believe clang is correct to reject this program.

[DaemonSnake]

From: https://en.cppreference.com/w/cpp/language/lambda
A lambda expression can read the value of a variable without capturing it if the variable has const non-volatile integral or enumeration type and has been initialized with a constant expression, or is constexpr and has no mutable members.

So the example you've provided is actually a reject-valid here:

void a() {
	constexpr auto x = 0;
	[] { x; }; //error here
}

but even if, it doesn't explain why being in a dependent scope (template) would turn it into a valid:

template<class T = int>
void func()
{
    constexpr auto y = 0;
    [] { auto x = y; }; //no longer an error here
}

int main()
{
    func();
}

[shafik]

CC @cor3ntin

[llvmbot]

@llvm/issue-subscribers-clang-frontend