[CodeCompletion] Don't ignore CovariantReturnConversionExpr

in typechecked AST. This is needed to correctly get the type of the
parsed expression when the expression is a calling to a method in super
class returning 'Self'.

rdar://problem/51504896

Author: rintaro

lib/IDE/ExprContextAnalysis.cpp

@@ -138,15 +138,35 @@ class ExprFinder : public ASTWalker {
     return SM.rangeContains(Node->getSourceRange(), TargetRange);
   }
 
+  bool shouldIgnore(Expr *E) {
+    // E.g. instanceOfDerived.methodInBaseReturningSelf().#^HERE^#'
+    // When calling a method in a base class returning 'Self', the call
+    // expression itself has the type of the base class. That is wrapped with
+    // CovariantReturnConversionExpr which downcasts it to the derived class.
+    if (isa<CovariantReturnConversionExpr>(E))
+      return false;
+
+    // E.g. TypeName(#^HERE^#
+    // In this case, we want the type expression instead of a reference to the
+    // initializer.
+    if (isa<ConstructorRefCallExpr>(E))
+      return true;
+
+    // Ignore other implicit expression.
+    if (E->isImplicit())
+      return true;
+
+    return false;
+  }
+
 public:
   ExprFinder(SourceManager &SM, SourceRange TargetRange)
       : SM(SM), TargetRange(TargetRange) {}
 
   Expr *get() const { return FoundExpr; }
 
   std::pair<bool, Expr *> walkToExprPre(Expr *E) override {
-    if (TargetRange == E->getSourceRange() && !E->isImplicit() &&
-        !isa<ConstructorRefCallExpr>(E)) {
+    if (TargetRange == E->getSourceRange() && !shouldIgnore(E)) {
       assert(!FoundExpr && "non-nullptr for found expr");
       FoundExpr = E;
       return {false, nullptr};

test/IDE/complete_super_self.swift

@@ -0,0 +1,20 @@
+// RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=COVARIANT_RETURN_CONV | %FileCheck %s --check-prefix=COVARIANT_RETURN_CONV
+
+class BaseClass {
+  func returnSelf() -> Self {}
+}
+
+class DerivedClass: BaseClass {
+  var value: Int
+  func foo() {}
+}
+
+func test(value: DerivedClass) {
+  value.returnSelf().#^COVARIANT_RETURN_CONV^#
+// COVARIANT_RETURN_CONV: Begin completions, 4 items
+// COVARIANT_RETURN_CONV-DAG: Keyword[self]/CurrNominal:          self[#DerivedClass#];
+// COVARIANT_RETURN_CONV-DAG: Decl[InstanceVar]/CurrNominal:      value[#Int#];
+// COVARIANT_RETURN_CONV-DAG: Decl[InstanceMethod]/CurrNominal:   foo()[#Void#];
+// COVARIANT_RETURN_CONV-DAG: Decl[InstanceMethod]/Super:         returnSelf()[#Self#];
+// COVARIANT_RETURN_CONV: End completions
+}