[cxx-interop] Add a basic test for using stlib iterator, std::iterator_traits

test/Interop/Cxx/stdlib/Inputs/module.modulemap

@@ -1,3 +1,8 @@
+module StdIteratorTraits {
+  header "std-iterator_traits.h"
+  requires cplusplus
+}
+
 module StdVector {
   header "std-vector.h"
   requires cplusplus

test/Interop/Cxx/stdlib/Inputs/std-iterator_traits.h

@@ -0,0 +1,10 @@
+#ifndef TEST_INTEROP_CXX_STDLIB_INPUTS_STD_ITERATORTRAITS_H
+#define TEST_INTEROP_CXX_STDLIB_INPUTS_STD_ITERATORTRAITS_H
+
+#include <iterator>
+
+using IteratorTraits = std::iterator_traits<int>;
+
+inline IteratorTraits initIterator() { return {}; }
+
+#endif // TEST_INTEROP_CXX_STDLIB_INPUTS_STD_ITERATOR_H

test/Interop/Cxx/stdlib/use-std-iterator_traits.swift

@@ -0,0 +1,27 @@
+// RUN: %target-run-simple-swift(-I %S/Inputs -Xfrontend -enable-cxx-interop)
+//
+// REQUIRES: executable_test
+//
+// Enable this everywhere once we have a solution for modularizing libstdc++: rdar://87654514
+// REQUIRES: OS=macosx
+
+import StdlibUnittest
+import StdIteratorTraits
+import std.iterator_traits
+
+var StdIteratorTraitsTestSuite = TestSuite("StdIteratorTraits")
+
+extension IteratorTraits : RandomAccessIteractor {
+  public var inputIt: Int { 0 }
+  public var numberToAdvance: Int { 1 }
+}
+
+StdIteratorTraitsTestSuite.test("init") {
+    let a = [12, 13, 14]
+    let iterator = IteratorTraits()
+    iterator.inputIt = a[0]
+    iterator.numberToAdvance = 1
+    expectEqual(iterator, 13)
+}
+
+runAllTests()