[CodeCompletion] Don't discard parsed expression in incomplete ternary

If we are code-completion, we need to keep the parsed expression in the
AST. Don't discard the parsed expression if the middle expression in a
ternary expression has code-completion token. This improves the
completions for:

    let _: MyEnum = condition ? .<HERE>
    let _: MyEnum = condition ? .<HERE> :

rdar://problem/54132682

Author: rintaro

lib/Parse/ParseExpr.cpp

@@ -189,8 +189,13 @@ ParserResult<Expr> Parser::parseExprSequence(Diag<> Message,
       if (CodeCompletion)
         CodeCompletion->setLeadingSequenceExprs(SequencedExprs);
     }
-    if (Primary.isNull())
+    if (Primary.isNull()) {
+      if (HasCodeCompletion) {
+        SequencedExprs.push_back(new (Context) CodeCompletionExpr(PreviousLoc));
+        break;
+      }
       return Primary;
+    }
 
     SequencedExprs.push_back(Primary.get());
 
@@ -242,16 +247,23 @@ ParserResult<Expr> Parser::parseExprSequence(Diag<> Message,
         HasCodeCompletion = true;
       if (middle.isNull())
         return nullptr;
-      
+
       // Make sure there's a matching ':' after the middle expr.
       if (!Tok.is(tok::colon)) {
+        if (middle.hasCodeCompletion()) {
+          SequencedExprs.push_back(new (Context) IfExpr(questionLoc,
+                                                        middle.get(),
+                                                        PreviousLoc));
+          SequencedExprs.push_back(new (Context) CodeCompletionExpr(PreviousLoc));
+          goto done;
+        }
+        
         diagnose(questionLoc, diag::expected_colon_after_if_question);
-
-      Status.setIsParseError();
-      return makeParserResult(Status, new (Context) ErrorExpr(
-          {startLoc, middle.get()->getSourceRange().End}));
+        Status.setIsParseError();
+        return makeParserResult(Status, new (Context) ErrorExpr(
+            {startLoc, middle.get()->getSourceRange().End}));
       }
-      
+
       SourceLoc colonLoc = consumeToken();
 
       auto *unresolvedIf

test/IDE/complete_unresolved_members.swift

@@ -115,6 +115,11 @@
 
 // RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=TYPEPARAM_IN_CONTEXTTYPE_1 | %FileCheck %s -check-prefix=TYPEPARAM_IN_CONTEXTTYPE_1
 
+// RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=TERNARY_1 | %FileCheck %s -check-prefix=UNRESOLVED_3
+// RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=TERNARY_2 | %FileCheck %s -check-prefix=UNRESOLVED_3
+// RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=TERNARY_3 | %FileCheck %s -check-prefix=UNRESOLVED_3
+// RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=TERNARY_4 | %FileCheck %s -check-prefix=UNRESOLVED_3
+
 enum SomeEnum1 {
   case South
   case North
@@ -732,3 +737,13 @@ func testTypeParamInContextType() {
 // TYPEPARAM_IN_CONTEXTTYPE_1-DAG: Decl[StaticVar]/CurrNominal/TypeRelation[Convertible]: myProtocolOption[#MyStruct<ConcreteMyProtocol>#];
 // TYPEPARAM_IN_CONTEXTTYPE_1: End completions
 }
+
+func testTernaryOperator() {
+  let _: SomeEnum1 = true ? .#^TERNARY_1^#
+  func sync(){}
+  let _: SomeEnum1 = true ? .#^TERNARY_2^# :
+  func sync(){}
+  let _: SomeEnum1 = true ? .#^TERNARY_3^# : .South
+  func sync(){}
+  let _: SomeEnum1 = true ? .South : .#^TERNARY_4^#
+}