[llvm-bugs] [Bug 45020] New: -Winvalid-constexpr fails with specialisation when interacting with automatic reference counting

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https://bugs.llvm.org/show_bug.cgi?id=45020

            Bug ID: 45020
           Summary: -Winvalid-constexpr fails with specialisation when
                    interacting with automatic reference counting
           Product: clang
           Version: trunk
          Hardware: PC
                OS: MacOS X
            Status: NEW
          Severity: normal
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: sdefresne at google.com
                CC: htmldeveloper at gmail.com, llvm-bugs at lists.llvm.org,
                    neeilans at live.com, richard-llvm at metafoo.co.uk

Specialisation of a template to support "id" (the Objective-C type) uniformly
the same way as pointer to Objective-C object cause failure of "constexpr"
compilation.

Small reproduction case:

=== s.mm
#import <Foundation/Foundation.h>

template<typename T>
struct a {
  explicit constexpr a(__unsafe_unretained T o = nil) : o_(o) {}

  __unsafe_unretained T o_;
};

template<typename T>
struct b : a<T> {
  explicit constexpr b(T __attribute__((ns_consumed)) o = nil) : a<T>(o) {}
};

template<typename T>
struct c : b<T*> {
  explicit constexpr c(T* __attribute__((ns_consumed)) o = nil) : b<T*>(o) {}
};

template<>
struct c<id> : b<id> {
  explicit constexpr c(id __attribute__((ns_consumed)) o = nil) : b<id>(o) {}
};

void foo() {
  b<id> _0([[NSObject alloc] init]);
  c<id> _1([[NSObject alloc] init]);
  c<NSObject> _2(([[NSObject alloc] init]));
}
===

Compiling this with the following options "--std=c++14 -fobjc-arc" results in
the following error:

s.mm:23:22: error: constexpr constructor never produces a constant expression
[-Winvalid-constexpr]
  explicit constexpr c(id __attribute__((ns_consumed)) o = nil) : b<id>(o) {}
                     ^
s.mm:23:73: note: subexpression not valid in a constant expression
  explicit constexpr c(id __attribute__((ns_consumed)) o = nil) : b<id>(o) {}
                                                                        ^
I would expect the "constexpr" to fail compilation for all of b<id>, c<id> and
c<NSObject> or for none of them. However, it only fails for c<id> which makes
me think this is a bad interaction between specialisation, ARC (automatic
reference counting) and the annotations.

The following alternative way to specialise via trait does not produce any
error:

===t.mm
#import <Foundation/Foundation.h>

template<typename T>
struct a {
  explicit constexpr a(__unsafe_unretained T o = nil) : o_(o) {}

  __unsafe_unretained T o_;
};

template<typename T>
struct b : a<T> {
  explicit constexpr b(T __attribute__((ns_consumed)) o = nil) : a<T>(o) {}
};

template<typename T>
struct t {
  typedef T* tp;
};

template<>
struct t<id> {
  typedef id tp;
};

template<typename T>
struct c : b<typename t<T>::tp> {
  typedef typename t<T>::tp tp;
  explicit constexpr c(tp __attribute__((ns_consumed)) o = nil) : b<tp>(o) {}
};

void foo() {
  b<id> _0([[NSObject alloc] init]);
  c<id> _1([[NSObject alloc] init]);
  c<NSObject> _2(([[NSObject alloc] init]));
}
===

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