PR16263: Implement current direction of core issue 1376. Binding a reference to

the result of a cast-to-reference-type lifetime-extends the object to which the
reference inside the cast binds.

This requires us to look for subobject adjustments on both the inside and the
outside of the MaterializeTemporaryExpr when looking for a temporary to
lifetime-extend (which we also need for core issue 616, and possibly 1213).

llvm-svn: 184024

1 files changed, 43 insertions, 4 deletions

diff --git a/clang/lib/Sema/SemaInit.cpp b/clang/lib/Sema/SemaInit.cpp
index cf595b49de6..863079c6d8f 100644
--- a/clang/lib/Sema/SemaInit.cpp
+++ b/clang/lib/Sema/SemaInit.cpp
@@ -5236,7 +5236,7 @@ getDeclForTemporaryLifetimeExtension(const InitializedEntity &Entity,
   case InitializedEntity::EK_Exception:
   case InitializedEntity::EK_VectorElement:
   case InitializedEntity::EK_ComplexElement:
-    llvm_unreachable("should not materialize a temporary to initialize this");
+    return 0;
   }
   llvm_unreachable("unknown entity kind");
 }
@@ -5245,7 +5245,8 @@ static void performLifetimeExtension(Expr *Init, const ValueDecl *ExtendingD);
 
 /// Update a glvalue expression that is used as the initializer of a reference
 /// to note that its lifetime is extended.
-static void performReferenceExtension(Expr *Init, const ValueDecl *ExtendingD) {
+/// \return \c true if any temporary had its lifetime extended.
+static bool performReferenceExtension(Expr *Init, const ValueDecl *ExtendingD) {
   if (InitListExpr *ILE = dyn_cast<InitListExpr>(Init)) {
     if (ILE->getNumInits() == 1 && ILE->isGLValue()) {
       // This is just redundant braces around an initializer. Step over it.
@@ -5253,12 +5254,38 @@ static void performReferenceExtension(Expr *Init, const ValueDecl *ExtendingD) {
     }
   }
 
+  // Walk past any constructs which we can lifetime-extend across.
+  Expr *Old;
+  do {
+    Old = Init;
+
+    // Step over any subobject adjustments; we may have a materialized
+    // temporary inside them.
+    SmallVector<const Expr *, 2> CommaLHSs;
+    SmallVector<SubobjectAdjustment, 2> Adjustments;
+    Init = const_cast<Expr *>(
+        Init->skipRValueSubobjectAdjustments(CommaLHSs, Adjustments));
+
+    // Per current approach for DR1376, look through casts to reference type
+    // when performing lifetime extension.
+    if (CastExpr *CE = dyn_cast<CastExpr>(Init))
+      if (CE->getSubExpr()->isGLValue())
+        Init = CE->getSubExpr();
+
+    // FIXME: Per DR1213, subscripting on an array temporary produces an xvalue.
+    // It's unclear if binding a reference to that xvalue extends the array
+    // temporary.
+  } while (Init != Old);
+
   if (MaterializeTemporaryExpr *ME = dyn_cast<MaterializeTemporaryExpr>(Init)) {
     // Update the storage duration of the materialized temporary.
     // FIXME: Rebuild the expression instead of mutating it.
     ME->setExtendingDecl(ExtendingD);
     performLifetimeExtension(ME->GetTemporaryExpr(), ExtendingD);
+    return true;
   }
+
+  return false;
 }
 
 /// Update a prvalue expression that is going to be materialized as a
@@ -5274,8 +5301,10 @@ static void performLifetimeExtension(Expr *Init, const ValueDecl *ExtendingD) {
     Init = BTE->getSubExpr();
 
   if (CXXStdInitializerListExpr *ILE =
-          dyn_cast<CXXStdInitializerListExpr>(Init))
-    return performReferenceExtension(ILE->getSubExpr(), ExtendingD);
+          dyn_cast<CXXStdInitializerListExpr>(Init)) {
+    performReferenceExtension(ILE->getSubExpr(), ExtendingD);
+    return;
+  }
 
   if (InitListExpr *ILE = dyn_cast<InitListExpr>(Init)) {
     if (ILE->getType()->isArrayType()) {
@@ -5582,6 +5611,16 @@ InitializationSequence::Perform(Sema &S,
       if (S.CheckExceptionSpecCompatibility(CurInit.get(), DestType))
         return ExprError();
 
+      // Even though we didn't materialize a temporary, the binding may still
+      // extend the lifetime of a temporary. This happens if we bind a reference
+      // to the result of a cast to reference type.
+      if (const ValueDecl *ExtendingDecl =
+              getDeclForTemporaryLifetimeExtension(Entity)) {
+        if (performReferenceExtension(CurInit.get(), ExtendingDecl))
+          warnOnLifetimeExtension(S, Entity, CurInit.get(), false,
+                                  ExtendingDecl);
+      }
+
       break;
 
     case SK_BindReferenceToTemporary: {