[llvm-bugs] [Bug 47713] New: Instantiation backtrace omits important element

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=47713

            Bug ID: 47713
           Summary: Instantiation backtrace omits important element
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++17
          Assignee: unassignedclangbugs at nondot.org
          Reporter: bballo at mozilla.com
                CC: blitzrakete at gmail.com, erik.pilkington at gmail.com,
                    llvm-bugs at lists.llvm.org, richard-llvm at metafoo.co.uk

Given the following (invalid) code:

  struct Noncopyable {
    Noncopyable(const Noncopyable&) = delete;
  };

  template <class T>
  struct Wrapper2 {
    T storage;
    Wrapper2(const T& val) : storage{val} {}
  };

  template <class T>
  struct Wrapper {
    Wrapper2<T> storage;
    constexpr Wrapper(const T& val) : storage{val} {}
  };

  Wrapper<Noncopyable> foo(Noncopyable& a) { return Wrapper<Noncopyable>(a); }

compiling with clang (any recent version including trunk) with -std=c++17 gives
the following errors:

  test.cpp:8:28: error: call to deleted constructor of 'Noncopyable'
    Wrapper2(const T& val) : storage{val} {}
                           ^      ~~~~~
  test.cpp:14:37: note: in instantiation of member function
'Wrapper2<Noncopyable>::Wrapper2' requested here
    constexpr Wrapper(const T& val) : storage{val} {}
                                    ^
  test.cpp:2:3: note: 'Noncopyable' has been explicitly marked deleted here
    Noncopyable(const Noncopyable&) = delete;
    ^

Observe that the backtrace omits a key element, the one that points to the
point of instantiation of Wrapper<Noncopyable> on the last line of the file.

If you image the wrapper classes being defined in utility headers, and the
point of instantiation being somewhere in a large translation unit, seeing this
error can leave you searching for a long time to find the point of
instantiation that you need to actually fix.

Making any of the following changes makes the desired backtrace element appear:

  * Making the constructor of Wrapper not constexpr
  * Making the constructor of Wrapper2 constexpr
  * Using -std=c++14 rather than -std=c++17

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