[ASTWalker] Fix ArgumentList walking bug

We were accidentally continuing to walk the old
argument list even if the caller had returned a
new one.

Author: hamishknight

lib/AST/ASTWalker.cpp

@@ -1342,16 +1342,17 @@ class Traversal : public ASTVisitor<Traversal, Expr*, Stmt*,
   }
 
   ArgumentList *doIt(ArgumentList *ArgList) {
-    auto Pre = Walker.walkToArgumentListPre(ArgList);
-    if (!Pre.first || !Pre.second)
-      return Pre.second;
+    bool WalkChildren;
+    std::tie(WalkChildren, ArgList) = Walker.walkToArgumentListPre(ArgList);
+    if (!WalkChildren || !ArgList)
+      return ArgList;
 
-    for (auto Idx : indices(*Pre.second)) {
+    for (auto Idx : indices(*ArgList)) {
       auto *E = doIt(ArgList->getExpr(Idx));
       if (!E) return nullptr;
       ArgList->setExpr(Idx, E);
     }
-    return Walker.walkToArgumentListPost(Pre.second);
+    return Walker.walkToArgumentListPost(ArgList);
   }
 };
 

test/IDE/complete_result_builder_rewritten_arg_list.swift

@@ -0,0 +1,33 @@
+// RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=CC | %FileCheck %s
+
+@resultBuilder
+struct Builder {
+  static func buildBlock<T>(_ components: T...) -> T {
+    fatalError()
+  }
+}
+
+enum E {
+  case e
+}
+
+struct S {
+  init(_ x: String) {}
+  func foo(_ x: String? = nil, y: E) -> S {
+    fatalError()
+  }
+}
+
+struct R {
+  @Builder
+    var foo: S {
+      // The argument list for foo gets rewritten to remove the DefaultArgumentExpr
+      // when doing a fallback type-check. Make sure we can continue to correctly
+      // recover the MemberRefExpr from DeclRefExpr for .e
+      S(E.#^CC^#).foo(y: .e)
+      // CHECK: Begin completions
+      // CHECK: Decl[EnumElement]/CurrNominal: e[#E#]; name=e
+      // CHECK: End completions
+    }
+}
+