Sema: Fix @discardableResult for Self-returning methods on classes and protocols

When we form calls to these methods, the ApplyExpr is wrapped in an
additional conversion. Dig through these to produce the right diagnostic
(or not, if the method is @discardableResult).

Fixes <https://bugs.swift.org/browse/SR-1890>.

Author: slavapestov

lib/Sema/TypeCheckStmt.cpp

@@ -1017,13 +1017,17 @@ void TypeChecker::checkIgnoredExpr(Expr *E) {
     return;
   }
 
-  // Drill through noop expressions we don't care about, like ParanExprs.
+  // Drill through noop expressions we don't care about.
   auto valueE = E;
   while (1) {
     valueE = valueE->getValueProvidingExpr();
 
     if (auto *OEE = dyn_cast<OpenExistentialExpr>(valueE))
       valueE = OEE->getSubExpr();
+    else if (auto *CRCE = dyn_cast<CovariantReturnConversionExpr>(valueE))
+      valueE = CRCE->getSubExpr();
+    else if (auto *EE = dyn_cast<ErasureExpr>(valueE))
+      valueE = EE->getSubExpr();
     else
       break;
   }

test/ClangModules/AppKit_test.swift

@@ -16,7 +16,7 @@ class MyDocument : NSDocument {
 
 func test(_ url: URL, controller: NSDocumentController) {
   try! NSDocument(contentsOf: url, ofType: "") // expected-warning{{result of 'NSDocument' initializer is unused}}
-  try! MyDocument(contentsOf: url, ofType: "") // expected-warning {{expression of type 'MyDocument' is unused}}
+  try! MyDocument(contentsOf: url, ofType: "") // expected-warning{{result of 'NSDocument' initializer is unused}}
 
   try! controller.makeDocument(withContentsOf: url, ofType: "")
 }

test/attr/attr_discardableResult.swift

@@ -41,9 +41,16 @@ class C1 {
   func f1() -> Int { }
 
   func f2() -> Int { }
+
+  @discardableResult
+  func me() -> Self { return self }
+
+  func reallyMe() -> Self { return self }
 }
 
-func testFunctionsInClass(c1 : C1) {
+class C2 : C1 {}
+
+func testFunctionsInClass(c1 : C1, c2: C2) {
   C1.f1Static()     // okay
   C1.f2Static()     // expected-warning {{result of call to 'f2Static()' is unused}}
   _ = C1.f2Static() // okay
@@ -60,6 +67,15 @@ func testFunctionsInClass(c1 : C1) {
   c1.f1()           // okay
   c1.f2()           // expected-warning {{result of call to 'f2()' is unused}}
   _ = c1.f2()       // okay
+
+  c1.me()           // okay
+  c2.me()           // okay
+
+  c1.reallyMe()     // expected-warning {{result of call to 'reallyMe()' is unused}}
+  c2.reallyMe()     // expected-warning {{result of call to 'reallyMe()' is unused}}
+
+  _ = c1.reallyMe() // okay
+  _ = c2.reallyMe() // okay
 }
 
 struct S1 {
@@ -92,6 +108,18 @@ func testFunctionsInStruct(s1 : S1) {
   _ = s1.f2()       // okay
 }
 
+protocol P1 {
+  @discardableResult
+  func me() -> Self
+
+  func reallyMe() -> Self
+}
+
+func testFunctionsInExistential(p1 : P1) {
+  p1.me()           // okay
+  p1.reallyMe()     // expected-warning {{result of call to 'reallyMe()' is unused}}
+  _ = p1.reallyMe() // okay
+}
 
 let x = 4
 "Hello \(x+1) world"  // expected-warning {{expression of type 'String' is unused}}