[SE-0408] Enable Pack Iteration

include/swift/AST/Decl.h

@@ -5855,6 +5855,7 @@ enum class PropertyWrapperSynthesizedPropertyKind {
 class VarDecl : public AbstractStorageDecl {
   friend class NamingPatternRequest;
   NamedPattern *NamingPattern = nullptr;
+  GenericEnvironment *OpenedElementEnvironment = nullptr;
 
 public:
   enum class Introducer : uint8_t {
@@ -5982,6 +5983,13 @@ class VarDecl : public AbstractStorageDecl {
   NamedPattern *getNamingPattern() const;
   void setNamingPattern(NamedPattern *Pat);
 
+  GenericEnvironment *getOpenedElementEnvironment() const {
+    return OpenedElementEnvironment;
+  }
+  void setOpenedElementEnvironment(GenericEnvironment *Env) {
+    OpenedElementEnvironment = Env;
+  }
+
   /// If this is a VarDecl that does not belong to a CaseLabelItem's pattern,
   /// return this. Otherwise, this VarDecl must belong to a CaseStmt's
   /// CaseLabelItem. In that case, return the first case label item of the first

include/swift/Sema/SyntacticElementTarget.h

@@ -22,15 +22,16 @@
 #include "swift/AST/Pattern.h"
 #include "swift/AST/Stmt.h"
 #include "swift/AST/TypeLoc.h"
+#include "swift/Basic/TaggedUnion.h"
 #include "swift/Sema/ConstraintLocator.h"
 #include "swift/Sema/ContextualTypeInfo.h"
 
 namespace swift {
 
 namespace constraints {
-/// Describes information about a for-each loop that needs to be tracked
-/// within the constraint system.
-struct ForEachStmtInfo {
+/// Describes information specific to a sequence
+/// in a for-each loop.
+struct SequenceIterationInfo {
   /// The type of the sequence.
   Type sequenceType;
 
@@ -47,6 +48,19 @@ struct ForEachStmtInfo {
   Expr *nextCall;
 };
 
+/// Describes information specific to a pack expansion expression
+/// in a for-each loop.
+struct PackIterationInfo {
+  /// The type of the pattern that matches the elements.
+  Type patternType;
+};
+
+/// Describes information about a for-each loop that needs to be tracked
+/// within the constraint system.
+struct ForEachStmtInfo : TaggedUnion<SequenceIterationInfo, PackIterationInfo> {
+  using TaggedUnion::TaggedUnion;
+};
+
 /// Describes the target to which a constraint system's solution can be
 /// applied.
 class SyntacticElementTarget {

lib/AST/ASTVerifier.cpp

@@ -795,6 +795,13 @@ class Verifier : public ASTWalker {
       if (!shouldVerify(cast<Stmt>(S)))
         return false;
 
+      if (auto *expansion =
+              dyn_cast<PackExpansionExpr>(S->getParsedSequence())) {
+        if (!shouldVerify(expansion)) {
+          return false;
+        }
+      }
+
       if (!S->getElementExpr())
         return true;
 
@@ -804,6 +811,11 @@ class Verifier : public ASTWalker {
     }
 
     void cleanup(ForEachStmt *S) {
+      if (auto *expansion =
+              dyn_cast<PackExpansionExpr>(S->getParsedSequence())) {
+        cleanup(expansion);
+      }
+
       if (!S->getElementExpr())
         return;
 
@@ -2605,6 +2617,16 @@ class Verifier : public ASTWalker {
         abort();
       }
 
+      // If we are performing pack iteration, variables have to carry the
+      // generic environment. Catching the missing environment here will prevent
+      // the code from being lowered.
+      if (var->getTypeInContext()->is<ErrorType>()) {
+        Out << "VarDecl is missing a Generic Environment: ";
+        var->getInterfaceType().print(Out);
+        Out << "\n";
+        abort();
+      }
+
       // The fact that this is *directly* be a reference storage type
       // cuts the code down quite a bit in getTypeOfReference.
       if (var->getAttrs().hasAttribute<ReferenceOwnershipAttr>() !=

lib/AST/Decl.cpp

@@ -7135,6 +7135,11 @@ VarDecl::VarDecl(DeclKind kind, bool isStatic, VarDecl::Introducer introducer,
 }
 
 Type VarDecl::getTypeInContext() const {
+  // If we are performing pack iteration, use the generic environment of the
+  // pack expansion expression to get the right context of a local variable.
+  if (auto *env = getOpenedElementEnvironment())
+    return GenericEnvironment::mapTypeIntoContext(env, getInterfaceType());
+
   return getDeclContext()->mapTypeIntoContext(getInterfaceType());
 }
 

lib/AST/Stmt.cpp

@@ -446,6 +446,9 @@ void ForEachStmt::setPattern(Pattern *p) {
 }
 
 Expr *ForEachStmt::getTypeCheckedSequence() const {
+  if (auto *expansion = dyn_cast<PackExpansionExpr>(getParsedSequence()))
+    return expansion;
+
   return iteratorVar ? iteratorVar->getInit(/*index=*/0) : nullptr;
 }