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

[Richard Smith via cfe-commits]

rsmith added a comment.

Thank you for working on this! Please also add a test to test/CXX/drs/dr15xx.cpp with a "// dr1579: 3.9" comment (we have a script that turns those comments into www/cxx_dr_status.html).


================
Comment at: include/clang/Sema/Sema.h:3473
@@ -3472,3 +3472,3 @@
   VarDecl *getCopyElisionCandidate(QualType ReturnType, Expr *E,
-                                   bool AllowFunctionParameters);
+                                   bool AllowParamOrMoveConstructable);
   bool isCopyElisionCandidate(QualType ReturnType, const VarDecl *VD,
----------------
Constructable -> Constructible.

================
Comment at: lib/Sema/SemaStmt.cpp:2756-2759
@@ -2755,4 +2755,6 @@
     // ... the same cv-unqualified type as the function return type ...
     if (!VDType->isDependentType() &&
-        !Context.hasSameUnqualifiedType(ReturnType, VDType))
-      return false;
+        !Context.hasSameUnqualifiedType(ReturnType, VDType)) {
+      // When considering moving this expression out, allow dissimilar types.
+      if (!AllowParamOrMoveConstructable)
+        return false;
----------------
Combine these conditions into a single `if`, and put the (cheapest) `bool` check first.

================
Comment at: lib/Sema/SemaStmt.cpp:2773-2783
@@ -2769,13 +2772,13 @@
 
   // ...non-volatile...
   if (VD->getType().isVolatileQualified()) return false;
 
   // __block variables can't be allocated in a way that permits NRVO.
   if (VD->hasAttr<BlocksAttr>()) return false;
 
   // Variables with higher required alignment than their type's ABI
   // alignment cannot use NRVO.
   if (!VD->getType()->isDependentType() && VD->hasAttr<AlignedAttr>() &&
       Context.getDeclAlign(VD) > Context.getTypeAlignInChars(VD->getType()))
     return false;
 
----------------
None of this should be checked for the `AllowParamOrMoveConstructible` case.

================
Comment at: lib/Sema/SemaStmt.cpp:2820
@@ -2814,5 +2819,3 @@
 
-    //   [...] If overload resolution fails, or if the type of the first
-    //   parameter of the selected constructor is not an rvalue reference
-    //   to the object's type (possibly cv-qualified), overload resolution
-    //   is performed again, considering the object as an lvalue.
+    InitializationKind Kind = InitializationKind::CreateDirect(
+        Value->getLocStart(), Value->getLocStart(), Value->getLocEnd());
----------------
Why are you using direct-initialization here? The return value should always be copy-initialized.


http://reviews.llvm.org/D21619