fix <rdar://problem/20749592> Conditional Optional binding hides compiler error

Swift SVN r28220

Author: lattner

lib/Sema/TypeCheckStmt.cpp

@@ -901,6 +901,14 @@ static void diagnoseIgnoredExpr(TypeChecker &TC, Expr *E) {
     return;
   }
 
+  // If we have an OptionalEvaluationExpr at the top level, then someone is
+  // "optional chaining" and ignoring the result.  Produce a diagnostic if it
+  // doesn't make sense to ignore it.
+  if (auto *OEE =
+        dyn_cast<OptionalEvaluationExpr>(E->getSemanticsProvidingExpr()))
+    if (auto *IIO = dyn_cast<InjectIntoOptionalExpr>(OEE->getSubExpr()))
+      return diagnoseIgnoredExpr(TC, IIO->getSubExpr());
+
   // FIXME: Complain about literals
 
   // Check if we have a call to a function marked warn_unused_result.

test/expr/expressions.swift

@@ -506,6 +506,8 @@ ruleVar = Rule("a") // expected-error {{cannot invoke initializer for type 'Rule
 class C {
   var x: C?
   init(other: C?) { x = other }
+
+  func method() {}
 }
 
 var c = C(3) // expected-error {{cannot invoke initializer for type 'C' with an argument list of type '(Int)'}} expected-note{{expected an argument list of type '(other: C?)'}}
@@ -587,8 +589,21 @@ func test() {
   (&x).method()  // expected-error {{cannot invoke 'method' with no arguments}}
 }
 
-// Unused l-value
-_ // expected-error{{'_' can only appear in a pattern or on the left side of an assignment}}
+
+// Unused results.
+func unusedExpressionResults() {
+  // Unused l-value
+  _ // expected-error{{'_' can only appear in a pattern or on the left side of an assignment}}
+
+
+  // <rdar://problem/20749592> Conditional Optional binding hides compiler error
+  let optionalc:C? = nil
+  optionalc?.method()  // ok
+  optionalc?.method  // expected-error {{expression resolves to an unused function}}
+}
+
+
+
 
 //===----------------------------------------------------------------------===//
 // Collection Literals