[CodeComplete] Restore original type in getOperatorCompletions

In getOperatorCompletions(), for each every known operators, temporary
expressions are created and typechecked. In this process, typechecker
may set the type of the parsed expression. That may cause non-accurate
completion results or crash at worst.

rdar://problem/28188259

Author: rintaro

lib/IDE/CodeCompletion.cpp

@@ -3268,8 +3268,17 @@ class CompletionLookup final : public swift::VisibleDeclConsumer {
   }
 
   void tryPostfixOperator(Expr *expr, PostfixOperatorDecl *op) {
-    if (!expr->getType())
+    auto Ty = expr->getType();
+    if (!Ty)
       return;
+
+    SWIFT_DEFER {
+      // Restore type.
+      // FIXME: This is workaround for getTypeOfExpressionWithoutApplying()
+      // modifies type of 'expr'.
+      expr->setType(Ty);
+    };
+
     // We allocate these expressions on the stack because we know they can't
     // escape and there isn't a better way to allocate scratch Expr nodes.
     UnresolvedDeclRefExpr UDRE(op->getName(), DeclRefKind::PostfixOperator,
@@ -3344,6 +3353,9 @@ class CompletionLookup final : public swift::VisibleDeclConsumer {
                            LHS->getType()->is<AnyFunctionType>()))
       return;
 
+    // Preserve LHS type for restoring it.
+    Type LHSTy = LHS->getType();
+
     // We allocate these expressions on the stack because we know they can't
     // escape and there isn't a better way to allocate scratch Expr nodes.
     UnresolvedDeclRefExpr UDRE(op->getName(), DeclRefKind::BinaryOperator,
@@ -3356,6 +3368,7 @@ class CompletionLookup final : public swift::VisibleDeclConsumer {
       // Reset sequence.
       SE->setElement(SE->getNumElements() - 1, nullptr);
       SE->setElement(SE->getNumElements() - 2, nullptr);
+      LHS->setType(LHSTy);
       prepareForRetypechecking(SE);
 
       for (auto &element : sequence.drop_back(2)) {

test/IDE/complete_crashes.swift

@@ -246,7 +246,6 @@ func foo_38272904(a: A_38272904) {
 }
 // RDAR_38272904: Begin completions
 
-
 // rdar://problem/41159258
 // RUN: %target-swift-ide-test -code-completion -source-filename=%s -code-completion-token=RDAR41159258_1 | %FileCheck %s -check-prefix=RDAR_41159258
 // RUN: %target-swift-ide-test -code-completion -source-filename=%s -code-completion-token=RDAR41159258_2 | %FileCheck %s -check-prefix=RDAR_41159258
@@ -287,3 +286,13 @@ public final class IntStore {
   }
 }
 // RDAR_41232519: Begin completions
+
+// rdar://problem/28188259
+// RUN: %target-swift-ide-test -code-completion -code-completion-token=RDAR_28188259 -source-filename=%s | %FileCheck %s -check-prefix=RDAR_28188259
+func test_28188259(x: ((Int) -> Void) -> Void) {
+  x({_ in }#^RDAR_28188259^#)
+}
+// RDAR_28188259: Begin completions
+// RDAR_28188259-DAG: Pattern/CurrModule:                 ({#_#})[#Void#]; name=(_)
+// RDAR_28188259-DAG: Keyword[self]/CurrNominal:          .self[#(_) -> ()#]; name=self
+// RDAR_28188259: End completions