An unused @ discardableResult should not throw an error (#6785)

A function marked with @discardableResult should not throw an error if
its returning closure is unused.

Fixes SR-2948.

Author: erikjv

lib/Sema/TypeCheckStmt.cpp

@@ -1082,9 +1082,22 @@ void TypeChecker::checkIgnoredExpr(Expr *E) {
   // TODO: What about tuples which contain functions by-value that are
   // dead?
   if (E->getType()->is<AnyFunctionType>()) {
-    diagnose(E->getLoc(), diag::expression_unused_function)
-      .highlight(E->getSourceRange());
-    return;
+    bool isDiscardable = false;
+    if (auto *Fn = dyn_cast<ApplyExpr>(E)) {
+      if (auto *declRef = dyn_cast<DeclRefExpr>(Fn->getFn())) {
+        if (auto *funcDecl = dyn_cast<AbstractFunctionDecl>(declRef->getDecl())) {
+          if (funcDecl->getAttrs().hasAttribute<DiscardableResultAttr>()) {
+            isDiscardable = true;
+          }
+        }
+      }
+    }
+
+    if (!isDiscardable) {
+      diagnose(E->getLoc(), diag::expression_unused_function)
+          .highlight(E->getSourceRange());
+      return;
+    }
   }
 
   // If the result of this expression is of type "Never" or "()"

test/attr/attr_discardableResult.swift

@@ -176,3 +176,9 @@ func testOptionalChaining(c1: C1?, s1: S1?) {
   s1?.f2Optional() // expected-warning {{result of call to 'f2Optional()' is unused}}
   s1!.f2Optional() // expected-warning {{result of call to 'f2Optional()' is unused}}
 }
+
+@discardableResult func SR2948 (_ closure: @escaping ()->()) -> (()->()) {
+  closure()
+  return closure
+}
+SR2948({}) // okay