[llvm-bugs] [Bug 33318] New: compile error related to static_assert and boost::hana

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=33318

            Bug ID: 33318
           Summary: compile error related to static_assert and boost::hana
           Product: clang
           Version: 4.0
          Hardware: Macintosh
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++14
          Assignee: unassignedclangbugs at nondot.org
          Reporter: matthias.thul at gmail.com
                CC: llvm-bugs at lists.llvm.org

I originally posted this as a question on Stack Overflow and it was suggested
to me to file this as a bug here
(https://stackoverflow.com/questions/44355720).

The following program compiles successfully using Clang versions
3.8.0-2ubuntu4, 4.0.1-svn304242-1~exp1 and 5.0.0-svn304672-1~exp1 using
"-std=c++14". The Boost library version is 1.61.0.

========== BEGIN SOURCE 1 ==========
#include <boost/hana.hpp>

namespace hana = boost::hana;

int main() {
    constexpr auto indices = hana::range<unsigned, 0, 3>();
    hana::for_each(indices, [&](auto i) {
        hana::for_each(indices, [&](auto j) {
            constexpr bool test = (i == (j == i ? j : i));
            static_assert(test, "error");
        });
    });
}
========== END SOURCE 1 ==========

The test is quite non-sensical but that is not the point. Consider now an
alternative version where the test is put inside the static_assert:

========== BEGIN SOURCE 2 ==========
#include <boost/hana.hpp>

namespace hana = boost::hana;

int main() {
    constexpr auto indices = hana::range<unsigned, 0, 3>();
    hana::for_each(indices, [&](auto i) {
        hana::for_each(indices, [&](auto j) {
            static_assert((i == (j == i ? j : i)), "error");
        });
    });
}
========== END SOURCE 2 ==========

I get a bunch of compile errors saying:

test.cpp:7:38: note: 'i' declared here
    hana::for_each(indices, [&](auto i) {
                                     ^
test.cpp:9:39: error: reference to local variable 'i' declared in enclosing
lambda expression
            static_assert((i == (j == i ? j : i)), "error");
                                      ^

The curious thing is, that when accessing i before the static_assert,
everything compiles again:

========== BEGIN SOURCE 3 ==========
#include <boost/hana.hpp>

namespace hana = boost::hana;

int main() {
    constexpr auto indices = hana::range<unsigned, 0, 3>();
    hana::for_each(indices, [&](auto i) {
        hana::for_each(indices, [&](auto j) {
            constexpr auto a = i;
            static_assert((i == (j == i ? j : i)), "error");
        });
    });
}
========== END SOURCE 3 ==========

Note in particular that the added statement is unrelated to the static_assert.

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