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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - -Winvalid-constexpr fails with specialisation when interacting with automatic reference counting"
href="https://bugs.llvm.org/show_bug.cgi?id=45020">45020</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>-Winvalid-constexpr fails with specialisation when interacting with automatic reference counting
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>MacOS X
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>-New Bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>sdefresne@google.com
</td>
</tr>
<tr>
<th>CC</th>
<td>htmldeveloper@gmail.com, llvm-bugs@lists.llvm.org, neeilans@live.com, richard-llvm@metafoo.co.uk
</td>
</tr></table>
<p>
<div>
<pre>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]));
}
===</pre>
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