[TypeChecker] Add subscript + trailing closure tests for keypath dynamic member lookup

Author: xedin

lib/Sema/CSApply.cpp

@@ -1573,7 +1573,7 @@ namespace {
 
         Expr *trailingClosure = nullptr;
         if (SE->hasTrailingClosure())
-          trailingClosure = arguments.pop_back_val();
+          trailingClosure = arguments.back();
 
         componentExpr = SubscriptExpr::create(
             ctx, dotExpr, SE->getStartLoc(), arguments, SE->getArgumentLabels(),

test/attr/attr_dynamic_member_lookup.swift

@@ -587,3 +587,20 @@ func keypath_with_incorrect_return_type(_ arr: Lens<Array<Int>>) {
     let _ = arr[idx]
   }
 }
+
+struct WithTrailingClosure {
+  subscript(fn: () -> Int) -> Int {
+    get { return fn() }
+    nonmutating set { _ = fn() + newValue }
+  }
+
+  subscript(offset: Int, _ fn: () -> Int) -> Int {
+    get { return offset + fn() }
+  }
+}
+
+func keypath_with_trailing_closure_subscript(_ ty: inout SubscriptLens<WithTrailingClosure>) {
+  _ = ty[0] { 42 } // expected-error {{subscript index of type '() -> Int' in a key path must be Hashable}}
+  _ = ty[] { 42 }  // expected-error {{subscript index of type '() -> Int' in a key path must be Hashable}}
+  _ = ty[] { 42 } = 0 // expected-error {{subscript index of type '() -> Int' in a key path must be Hashable}}
+}