[TypeResolution] Enable local variable packs.

lib/Sema/MiscDiagnostics.cpp

@@ -3549,6 +3549,16 @@ void VarDeclUsageChecker::markStoredOrInOutExpr(Expr *E, unsigned Flags) {
     OpaqueValueMap[OEE->getOpaqueValue()] = OEE->getExistentialValue();
     return markStoredOrInOutExpr(OEE->getSubExpr(), Flags);
   }
+
+  // Bind pack references in pack expansions.
+  if (auto *expansion = dyn_cast<PackExpansionExpr>(E)) {
+    for (unsigned i = 0; i < expansion->getNumBindings(); ++i) {
+      auto *opaqueValue = expansion->getOpaqueValues()[i];
+      auto *packReference = expansion->getBindings()[i];
+      OpaqueValueMap[opaqueValue] = packReference;
+    }
+    return markStoredOrInOutExpr(expansion->getPatternExpr(), Flags);
+  }
 
   // If this is an OpaqueValueExpr that we've seen a mapping for, jump to the
   // mapped value.
@@ -3626,6 +3636,15 @@ ASTWalker::PreWalkResult<Expr *> VarDeclUsageChecker::walkToExprPre(Expr *E) {
     return Action::SkipChildren(E);
   }
 
+  // If we see a PackExpansionExpr, record its pack reference bindings.
+  if (auto *expansion = dyn_cast<PackExpansionExpr>(E)) {
+    for (unsigned i = 0; i < expansion->getNumBindings(); ++i) {
+      auto *opaqueValue = expansion->getOpaqueValues()[i];
+      auto *packReference = expansion->getBindings()[i];
+      OpaqueValueMap[opaqueValue] = packReference;
+    }
+  }
+
   // Visit bindings.
   if (auto ove = dyn_cast<OpaqueValueExpr>(E)) {
     if (auto mapping = OpaqueValueMap.lookup(ove))

lib/Sema/TypeCheckType.cpp

@@ -4223,7 +4223,7 @@ NeverNullType TypeResolver::resolvePackExpansionType(PackExpansionTypeRepr *repr
     return ErrorType::get(ctx);
 
   // We might not allow variadic expansions here at all.
-  if (!options.isPackExpansionSupported()) {
+  if (!options.isPackExpansionSupported(getDeclContext())) {
     diagnose(repr->getLoc(), diag::expansion_not_allowed, pair.first);
     return ErrorType::get(ctx);
   }

lib/Sema/TypeCheckType.h

@@ -311,14 +311,19 @@ class TypeResolutionOptions {
   }
 
   /// Whether pack expansion types are supported in this context.
-  bool isPackExpansionSupported() const {
+  bool isPackExpansionSupported(DeclContext *dc) const {
     switch (context) {
     case Context::FunctionInput:
     case Context::VariadicFunctionInput:
     case Context::TupleElement:
     case Context::GenericArgument:
       return true;
 
+    // Local variable packs are supported, but property packs
+    // are not.
+    case Context::PatternBindingDecl:
+      return !dc->isTypeContext();
+
     case Context::None:
     case Context::ProtocolGenericArgument:
     case Context::Inherited:
@@ -333,7 +338,6 @@ class TypeResolutionOptions {
     case Context::InExpression:
     case Context::ExplicitCastExpr:
     case Context::ForEachStmt:
-    case Context::PatternBindingDecl:
     case Context::EditorPlaceholderExpr:
     case Context::ClosureExpr:
     case Context::InoutFunctionInput:

test/Constraints/pack-expansion-expressions.swift

@@ -33,3 +33,10 @@ func coerceExpansion<T...>(_ value: T...) {
 
   promoteToOptional(value...)
 }
+
+func localValuePack<T...>(_ t: T...) -> (T..., T...) {
+  let local = t...
+  let localAnnotated: T... = t...
+
+  return (local..., localAnnotated...)
+}