constexpr value in a lambda

[llvmbot]

Bugzilla Link	25627
Version	unspecified
OS	Linux
Attachments	as explained
Reporter	LLVM Bugzilla Contributor
CC	@DaemonSnake

Extended Description

If I am correct the following code should be valid c++14:

template<int j> void g() {}
int main()
{
  constexpr int i = 0;
  auto f = []{ g<i>(); };
  return 0;
}

However, clang does not compile this code.
It does compile with when capturing [=], [i] or [&].
It also compiles if the i is made static.

This is related to a stackoverflow-question:
http://stackoverflow.com/questions/33873788/how-can-i-use-a-constexpr-value-in-a-lambda

clang version:
Debian clang version 3.8.0-svn247818-1~exp1 (trunk) (based on LLVM 3.8.0)
compile command:

clang++-3.8 -std=c++14 capture_constexpr.cpp

compiler error:
capture_constexpr.cpp:11:7: error: variable 'i' cannot be implicitly captured in a lambda with no capture-default specified
    g<i>();
      ^
capture_constexpr.cpp:7:17: note: 'i' declared here
  constexpr int i = 0;
                ^
capture_constexpr.cpp:10:12: note: lambda expression begins here
  auto f = []{ // compiles with gcc 4.9.1, not with clang 3.8
           ^
1 error generated.

[llvmbot]

Sorry about the delayed attention - I became aware of this bug just earlier today.

Patch submitted for review : https://reviews.llvm.org/D31588

[llvmbot]

fixed by http://llvm.org/viewvc/llvm-project?view=revision&revision=303492

[DaemonSnake]

This issue was only partially fixed.
The same code in a template context won't compile:

//works

void f0()
{
   constexpr int i = 0;
   []{ return i; };
}

//error

template<class T>
void f1()
{
   constexpr int i = 0;
   []{ return i; }; //i cannot be implicitly captured ...
}

godbolt link: https://godbolt.org/z/3Sc46U

console output:

<source>:5:4: warning: expression result unused [-Wunused-value]

   []{ return i; };

   ^~~~~~~~~~~~~~~

<source>:13:15: error: variable 'i' cannot be implicitly captured in a lambda with no capture-default specified

   []{ return i; };

              ^

<source>:12:18: note: 'i' declared here

   constexpr int i = 0;

                 ^

<source>:13:4: note: lambda expression begins here

   []{ return i; };

   ^

1 warning and 1 error generated.

Compiler returned: 1

[llvmbot]

A patch to address this bug is under review: https://reviews.llvm.org/D92733

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[cor3ntin]

No longer crashes but behavior still incorrect for the template case