1 files changed, 47 insertions, 0 deletions
diff --git a/clang/lib/Sema/TreeTransform.h b/clang/lib/Sema/TreeTransform.h
index a74171ed5f8..6aa1dbbc442 100644
--- a/clang/lib/Sema/TreeTransform.h
+++ b/clang/lib/Sema/TreeTransform.h
@@ -10763,6 +10763,51 @@ TreeTransform<Derived>::TransformSizeOfPackExpr(SizeOfPackExpr *E) {
                                               E->getRParenLoc(), None, None);
   }
 
+  // Try to compute the result without performing a partial substitution.
+  Optional<unsigned> Result = 0;
+  for (const TemplateArgument &Arg : PackArgs) {
+    if (!Arg.isPackExpansion()) {
+      Result = *Result + 1;
+      continue;
+    }
+
+    TemplateArgumentLoc ArgLoc;
+    InventTemplateArgumentLoc(Arg, ArgLoc);
+
+    // Find the pattern of the pack expansion.
+    SourceLocation Ellipsis;
+    Optional<unsigned> OrigNumExpansions;
+    TemplateArgumentLoc Pattern =
+        getSema().getTemplateArgumentPackExpansionPattern(ArgLoc, Ellipsis,
+                                                          OrigNumExpansions);
+
+    // Substitute under the pack expansion. Do not expand the pack (yet).
+    TemplateArgumentLoc OutPattern;
+    Sema::ArgumentPackSubstitutionIndexRAII SubstIndex(getSema(), -1);
+    if (getDerived().TransformTemplateArgument(Pattern, OutPattern,
+                                               /*Uneval*/ true))
+      return true;
+
+    // See if we can determine the number of arguments from the result.
+    Optional<unsigned> NumExpansions =
+        getSema().getFullyPackExpandedSize(OutPattern.getArgument());
+    if (!NumExpansions) {
+      // No: we must be in an alias template expansion, and we're going to need
+      // to actually expand the packs.
+      Result = None;
+      break;
+    }
+
+    Result = *Result + *NumExpansions;
+  }
+
+  // Common case: we could determine the number of expansions without
+  // substituting.
+  if (Result)
+    return getDerived().RebuildSizeOfPackExpr(E->getOperatorLoc(), E->getPack(),
+                                              E->getPackLoc(),
+                                              E->getRParenLoc(), *Result, None);
+
   TemplateArgumentListInfo TransformedPackArgs(E->getPackLoc(),
                                                E->getPackLoc());
   {
@@ -10775,6 +10820,8 @@ TreeTransform<Derived>::TransformSizeOfPackExpr(SizeOfPackExpr *E) {
       return ExprError();
   }
 
+  // Check whether we managed to fully-expand the pack.
+  // FIXME: Is it possible for us to do so and not hit the early exit path?
   SmallVector<TemplateArgument, 8> Args;
   bool PartialSubstitution = false;
   for (auto &Loc : TransformedPackArgs.arguments()) {