Merge pull request #65445 from xedin/rdar-108064941

[Diagnostics] Suppress unused expression warning if result is discard…

Author: xedin

include/swift/AST/Types.h

@@ -2459,6 +2459,10 @@ class TupleType final : public TypeBase, public llvm::FoldingSetNode,
 
   bool containsPackExpansionType() const;
 
+  /// Check whether this tuple consists of a single unlabeled element
+  /// of \c PackExpansionType.
+  bool isSingleUnlabeledPackExpansion() const;
+
 private:
   TupleType(ArrayRef<TupleTypeElt> elements, const ASTContext *CanCtx,
             RecursiveTypeProperties properties)

lib/AST/ParameterPack.cpp

@@ -134,6 +134,14 @@ bool CanTupleType::containsPackExpansionTypeImpl(CanTupleType tuple) {
   return false;
 }
 
+bool TupleType::isSingleUnlabeledPackExpansion() const {
+  if (getNumElements() != 1)
+    return false;
+
+  const auto &elt = getElement(0);
+  return !elt.hasName() && elt.getType()->is<PackExpansionType>();
+}
+
 bool AnyFunctionType::containsPackExpansionType(ArrayRef<Param> params) {
   for (auto param : params) {
     if (param.getPlainType()->is<PackExpansionType>())

lib/Sema/TypeCheckStmt.cpp

@@ -1749,6 +1749,16 @@ Stmt *PreCheckReturnStmtRequest::evaluate(Evaluator &evaluator, ReturnStmt *RS,
 }
 
 static bool isDiscardableType(Type type) {
+  // If type is `(_: repeat ...)`, it can be discardable.
+  if (auto *tuple = type->getAs<TupleType>()) {
+    if (tuple->isSingleUnlabeledPackExpansion()) {
+      type = tuple->getElementType(0);
+    }
+  }
+
+  if (auto *expansion = type->getAs<PackExpansionType>())
+    return isDiscardableType(expansion->getPatternType());
+
   return (type->hasError() ||
           type->isUninhabited() ||
           type->lookThroughAllOptionalTypes()->isVoid());

test/Constraints/pack-expansion-expressions.swift

@@ -397,3 +397,14 @@ do {
   _ = Defaulted(t: "b", 0) // Ok
   _ = Defaulted(t: "c", 1.0, u: "d", 0) // Ok
 }
+
+// rdar://108064941 - unused result diagnostic is unaware of Void packs
+func test_no_unused_result_warning(arr: inout [Any]) {
+  func test1<each T>(_ value: (repeat each T)) {
+    repeat arr.append(each value.element) // no warning
+  }
+
+  func test2<each T>(_ value: repeat each T) {
+    ((repeat arr.append(each value))) // no warning
+  }
+}