[cinterop][tests] Add test for -Xcompile-source flag

backend.native/tests/build.gradle

@@ -3394,6 +3394,14 @@ createInterop("cmangling2") {
     it.defFile 'interop/basics/mangling2.def'
 }
 
+createInterop("auxiliaryCppSources") {
+    // We need to provide empty def file to create correct `KonanInteropTask`.
+    it.defFile 'interop/auxiliary_sources/auxiliaryCppSources.def'
+    it.headers "$projectDir/interop/auxiliary_sources/name.h"
+    it.extraOpts "-Xcompile-source", "$projectDir/interop/auxiliary_sources/name.cpp"
+    it.extraOpts "-Xsource-compiler-option", "-std=c++17"
+}
+
 if (PlatformInfo.isAppleTarget(project)) {
     createInterop("objcSmoke") {
         it.defFile 'interop/objc/objcSmoke.def'
@@ -3563,6 +3571,12 @@ interopTest("interop_mangling2") {
     interop = 'cmangling2'
 }
 
+interopTest("interop_auxiliarySources") {
+    disabled = (project.testTarget == 'wasm32') // No interop for wasm yet.
+    source = "interop/auxiliary_sources/main.kt"
+    interop = 'auxiliaryCppSources'
+}
+
 interopTest("interop_values") {
     disabled = (project.testTarget == 'wasm32') // No interop for wasm yet.
     source = "interop/basics/values.kt"

backend.native/tests/interop/auxiliary_sources/main.kt

@@ -0,0 +1,7 @@
+import auxiliaryCppSources.*
+import kotlin.test.*
+import kotlinx.cinterop.*
+
+fun main() {
+    assertEquals(name()!!.toKString(), "Hello from C++")
+}

backend.native/tests/interop/auxiliary_sources/name.cpp

@@ -0,0 +1,8 @@
+#include <string>
+#include "name.h"
+
+static std::string _name = "Hello from C++";
+
+const char* name() {
+    return _name.c_str();
+}

backend.native/tests/interop/auxiliary_sources/name.h

@@ -0,0 +1,9 @@
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+const char* name();
+
+#ifdef __cplusplus
+}
+#endif