From 150a4989ecf988595f80809abf84835c9625b33c Mon Sep 17 00:00:00 2001
From: Richard Smith <richard-llvm@metafoo.co.uk>
Date: Fri, 6 Jun 2014 17:33:35 +0000
Subject: [PATCH] Retain an expression pack expansion when the parameter pack
 expansion code asks us to.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@210355 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/Sema/TreeTransform.h             | 29 ++++++++++++++++++++++++++--
 test/SemaTemplate/pack-deduction.cpp |  7 +++++++
 2 files changed, 34 insertions(+), 2 deletions(-)

diff --git a/lib/Sema/TreeTransform.h b/lib/Sema/TreeTransform.h
index 94b2e959df..eba23a5def 100644
--- a/lib/Sema/TreeTransform.h
+++ b/lib/Sema/TreeTransform.h
@@ -2912,9 +2912,11 @@ bool TreeTransform<Derived>::TransformExprs(Expr **Inputs,
         if (Out.isInvalid())
           return true;
 
+        // FIXME: Can this happen? We should not try to expand the pack
+        // in this case.
         if (Out.get()->containsUnexpandedParameterPack()) {
-          Out = RebuildPackExpansion(Out.get(), Expansion->getEllipsisLoc(),
-                                     OrigNumExpansions);
+          Out = getDerived().RebuildPackExpansion(
+              Out.get(), Expansion->getEllipsisLoc(), OrigNumExpansions);
           if (Out.isInvalid())
             return true;
         }
@@ -2922,6 +2924,23 @@ bool TreeTransform<Derived>::TransformExprs(Expr **Inputs,
         Outputs.push_back(Out.get());
       }
 
+      // If we're supposed to retain a pack expansion, do so by temporarily
+      // forgetting the partially-substituted parameter pack.
+      if (RetainExpansion) {
+        ForgetPartiallySubstitutedPackRAII Forget(getDerived());
+
+        ExprResult Out = getDerived().TransformExpr(Pattern);
+        if (Out.isInvalid())
+          return true;
+
+        Out = getDerived().RebuildPackExpansion(
+            Out.get(), Expansion->getEllipsisLoc(), OrigNumExpansions);
+        if (Out.isInvalid())
+          return true;
+
+        Outputs.push_back(Out.get());
+      }
+
       continue;
     }
 
@@ -8618,6 +8637,9 @@ TreeTransform<Derived>::TransformLambdaScope(LambdaExpr *E,
           // Capture the transformed variable.
           getSema().tryCaptureVariable(CapturedVar, C->getLocation(), Kind);
         }
+
+        // FIXME: Retain a pack expansion if RetainExpansion is true.
+
         continue;
       }
 
@@ -9122,6 +9144,7 @@ TreeTransform<Derived>::TransformObjCDictionaryLiteral(
 
         // If any unexpanded parameter packs remain, we still have a
         // pack expansion.
+        // FIXME: Can this really happen?
         if (Key.get()->containsUnexpandedParameterPack() ||
             Value.get()->containsUnexpandedParameterPack())
           Element.EllipsisLoc = OrigElement.EllipsisLoc;
@@ -9129,6 +9152,8 @@ TreeTransform<Derived>::TransformObjCDictionaryLiteral(
         Elements.push_back(Element);
       }
 
+      // FIXME: Retain a pack expansion if RetainExpansion is true.
+
       // We've finished with this pack expansion.
       continue;
     }
diff --git a/test/SemaTemplate/pack-deduction.cpp b/test/SemaTemplate/pack-deduction.cpp
index abb76de693..f3f969e9af 100644
--- a/test/SemaTemplate/pack-deduction.cpp
+++ b/test/SemaTemplate/pack-deduction.cpp
@@ -30,3 +30,10 @@ namespace PR14841 {
     f<char, short, int>(a); // expected-error {{no matching function}}
   }
 }
+
+namespace RetainExprPacks {
+  int f(int a, int b, int c);
+  template<typename ...Ts> struct X {};
+  template<typename ...Ts> int g(X<Ts...>, decltype(f(Ts()...)));
+  int n = g<int, int>(X<int, int, int>(), 0);
+}
-- 
2.50.1