[llvm-bugs] [Bug 33039] New: Class with a default constructible const member is not default constructible

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=33039

            Bug ID: 33039
           Summary: Class with a default constructible const member is not
                    default constructible
           Product: clang
           Version: unspecified
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: wellsd2 at rpi.edu
                CC: dgregor at apple.com, llvm-bugs at lists.llvm.org

clang++ does not always mark classes with const members as default
constructible. Variants of this bug appear in older versions of clang (my
coworkers reported 3.4 and 3.7). The original context for this is here:

https://github.com/dealii/dealii/pull/4386

The problem only seems to arise when a parent class has a user-defined default
constructor and the child class uses the implicitly generated constructor
(adding a user defined do-nothing default constructor, e.g., "Child() {}", to
the example below fixes compilation, but "Child() = default;" does not).

GCC compiles this program successfully.

terminal output:

[drwells at archway zero-parent-test]$ clang++ --version
clang version 4.0.0 (tags/RELEASE_400/final)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
[drwells at archway zero-parent-test]$ clang++ -std=c++11 -Wall -Wextra -Wpedantic
parent.cc
parent.cc:32:3: error: static_assert failed "TestChild"
  static_assert(std::is_default_constructible<TestChild>::value, "TestChild");
  ^             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.

code:

#include <type_traits>

struct Parent
{
  Parent() : a {0.0}, b {1.0}
  {}

  double a;
  double b;
};

struct Child : public Parent
{};

class TestParent
{
  const Parent p;
};

class TestChild
{
  const Child c;
};

int main()
{
  static_assert(std::is_default_constructible<Parent>::value, "Parent");
  static_assert(std::is_default_constructible<Child>::value, "Child");

  const Child c;
  static_assert(std::is_default_constructible<TestParent>::value,
"TestParent");
  static_assert(std::is_default_constructible<TestChild>::value, "TestChild");
}

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