[PATCH] D21619: [Sema] Implement C++14's DR1579: Prefer moving id-expression out of functions

[Erik Pilkington via cfe-commits]

erik.pilkington created this revision.
erik.pilkington added reviewers: rsmith, faisalv.
erik.pilkington added a subscriber: cfe-commits.

DR1579 says that when returning a id-expression from a function, we should attempt to return said expression by move first, then fallback to copy.

This patch does this by generalizing `PerformMoveOrCopyElision`, which previously did this when returning a id-expression that was a parameter.

This patch allows the following code to compile, for example:
```
struct Base {};
struct Derived : public Base {};
std::unique_ptr<Base> f() {
  std::unique_ptr<Derived> d;
  return d;
}
```
The DR in question: http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1579
Pointed out by PR27785.

Thanks!

http://reviews.llvm.org/D21619

Files:
  include/clang/Sema/Initialization.h
  include/clang/Sema/Sema.h
  lib/Sema/SemaStmt.cpp
  test/SemaCXX/rval-references.cpp

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