[CodeCompletion] Fix use of dangling pointers to stack memory

These pointers were already dangling, but after the recent change to
cleanup ErrorType values, we would actually dereference them while
walking the expressions.  In debug builds, this manifest as crashes.

Author: benlangmuir

lib/IDE/CodeCompletion.cpp

@@ -26,6 +26,7 @@
 #include "swift/ClangImporter/ClangModule.h"
 #include "swift/Parse/CodeCompletionCallbacks.h"
 #include "swift/Sema/IDETypeChecking.h"
+#include "swift/Basic/Defer.h" // must be included after Tokens.def.
 #include "swift/Subsystems.h"
 #include "llvm/ADT/SmallSet.h"
 #include "llvm/ADT/SmallString.h"
@@ -42,6 +43,8 @@
 #include <algorithm>
 #include <string>
 
+#undef defer // for Tokens.def; use defer_impl instead.
+
 using namespace swift;
 using namespace ide;
 
@@ -3246,6 +3249,13 @@ class CompletionLookup final : public swift::VisibleDeclConsumer {
     CodeCompletionExpr CCE((SourceRange()));
     sequence.back() = &CCE;
 
+    defer_impl {
+      // Reset sequence.
+      SE->setElement(SE->getNumElements() - 1, nullptr);
+      SE->setElement(SE->getNumElements() - 2, nullptr);
+      eraseErrorTypes(SE);
+    };
+
     Expr *expr = SE;
     if (!typeCheckCompletionSequence(const_cast<DeclContext *>(CurrDeclContext),
                                      expr)) {
@@ -3336,13 +3346,8 @@ class CompletionLookup final : public swift::VisibleDeclConsumer {
           tryPostfixOperator(LHS, cast<PostfixOperatorDecl>(op));
         break;
       case DeclKind::InfixOperator:
-        if (seenInfixOperators.insert(op->getName()).second) {
+        if (seenInfixOperators.insert(op->getName()).second)
           tryInfixOperatorCompletion(cast<InfixOperatorDecl>(op), SE);
-          // Reset sequence.
-          eraseErrorTypes(SE);
-          SE->setElement(SE->getNumElements() - 1, nullptr);
-          SE->setElement(SE->getNumElements() - 2, nullptr);
-        }
         break;
       default:
         llvm_unreachable("unexpected operator kind");