[Clang][P1061] Add stuctured binding packs (#121417)

This is an implementation of P1061 Structure Bindings Introduce a Pack
without the ability to use packs outside of templates. There is a couple
of ways the AST could have been sliced so let me know what you think.
The only part of this change that I am unsure of is the
serialization/deserialization stuff. I followed the implementation of
other Exprs, but I do not really know how it is tested. Thank you for
your time considering this.

---------

Co-authored-by: Yanzuo Liu <zwuis@outlook.com>

1 files changed, 28 insertions, 1 deletions

diff --git a/clang/lib/Sema/SemaTemplateInstantiate.cpp b/clang/lib/Sema/SemaTemplateInstantiate.cpp
index b4fa23d..12e98a3 100644
--- a/clang/lib/Sema/SemaTemplateInstantiate.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiate.cpp
@@ -1583,6 +1583,10 @@ namespace {
     /// pack.
     ExprResult TransformFunctionParmPackExpr(FunctionParmPackExpr *E);
 
+    // Transform a ResolvedUnexpandedPackExpr
+    ExprResult
+    TransformResolvedUnexpandedPackExpr(ResolvedUnexpandedPackExpr *E);
+
     QualType TransformFunctionProtoType(TypeLocBuilder &TLB,
                                         FunctionProtoTypeLoc TL) {
       // Call the base version; it will forward to our overridden version below.
@@ -1848,7 +1852,8 @@ bool TemplateInstantiator::AlreadyTransformed(QualType T) {
   if (T.isNull())
     return true;
 
-  if (T->isInstantiationDependentType() || T->isVariablyModifiedType())
+  if (T->isInstantiationDependentType() || T->isVariablyModifiedType() ||
+      T->containsUnexpandedParameterPack())
     return false;
 
   getSema().MarkDeclarationsReferencedInType(Loc, T);
@@ -2473,6 +2478,15 @@ TemplateInstantiator::TransformDeclRefExpr(DeclRefExpr *E) {
     if (PD->isParameterPack())
       return TransformFunctionParmPackRefExpr(E, PD);
 
+  if (BindingDecl *BD = dyn_cast<BindingDecl>(D); BD && BD->isParameterPack()) {
+    BD = cast_or_null<BindingDecl>(TransformDecl(BD->getLocation(), BD));
+    if (!BD)
+      return ExprError();
+    if (auto *RP =
+            dyn_cast_if_present<ResolvedUnexpandedPackExpr>(BD->getBinding()))
+      return TransformResolvedUnexpandedPackExpr(RP);
+  }
+
   return inherited::TransformDeclRefExpr(E);
 }
 
@@ -2637,6 +2651,19 @@ TemplateInstantiator::TransformTemplateTypeParmType(TypeLocBuilder &TLB,
   return Result;
 }
 
+ExprResult TemplateInstantiator::TransformResolvedUnexpandedPackExpr(
+    ResolvedUnexpandedPackExpr *E) {
+  if (getSema().ArgumentPackSubstitutionIndex != -1) {
+    assert(static_cast<unsigned>(getSema().ArgumentPackSubstitutionIndex) <
+               E->getNumExprs() &&
+           "ArgumentPackSubstitutionIndex is out of range");
+    return TransformExpr(
+        E->getExpansion(getSema().ArgumentPackSubstitutionIndex));
+  }
+
+  return inherited::TransformResolvedUnexpandedPackExpr(E);
+}
+
 QualType TemplateInstantiator::TransformSubstTemplateTypeParmPackType(
     TypeLocBuilder &TLB, SubstTemplateTypeParmPackTypeLoc TL,
     bool SuppressObjCLifetime) {