[cxx-interop] Test trivial usage of std::variant from Swift

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

@@ -76,6 +76,12 @@ module StdFunction {
   export *
 }
 
+module StdVariant {
+  header "std-variant.h"
+  requires cplusplus
+  export *
+}
+
 module NoCXXStdlib {
   header "no-cxx-stdlib.h"
   requires cplusplus

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

@@ -0,0 +1,16 @@
+#include <string>
+#include <variant>
+
+using StdVariantIntOrString = std::variant<int, std::string>;
+inline StdVariantIntOrString getStdVariantInt() { return {123}; }
+inline StdVariantIntOrString getStdVariantString() { return {"abc"}; }
+
+struct HasDeletedCopyCtor {
+  int value;
+  HasDeletedCopyCtor(int value) : value(value) {}
+  HasDeletedCopyCtor(const HasDeletedCopyCtor &other) = delete;
+  HasDeletedCopyCtor(HasDeletedCopyCtor &&other) = default;
+};
+
+using StdVariantIntOrNonCopyable = std::variant<HasDeletedCopyCtor, int>;
+inline StdVariantIntOrNonCopyable getStdVariantNonCopyable() { return HasDeletedCopyCtor(123); }

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

@@ -0,0 +1,22 @@
+// RUN: %target-run-simple-swift(-I %S/Inputs -cxx-interoperability-mode=upcoming-swift -Xcc -std=c++17)
+
+// REQUIRES: executable_test
+
+import StdlibUnittest
+import StdVariant
+import CxxStdlib
+
+var StdVariantTestSuite = TestSuite("StdVariant")
+
+StdVariantTestSuite.test("index()") {
+  let variantInt = getStdVariantInt()
+  expectEqual(variantInt.index(), 0)
+
+  let variantString = getStdVariantString()
+  expectEqual(variantString.index(), 1)
+
+  let variantNonCopyable = getStdVariantNonCopyable()
+  expectEqual(variantNonCopyable.index(), 0)
+}
+
+runAllTests()