[Interop][Test] Add tests for unions

backend.native/tests/build.gradle

@@ -3469,6 +3469,10 @@ createInterop("cmangling_keywords2") {
     it.defFile 'interop/basics/mangling_keywords2.def'
 }
 
+createInterop("cunion") {
+    it.defFile 'interop/basics/cunion.def'
+}
+
 createInterop("auxiliaryCppSources") {
     // We need to provide empty def file to create correct `KonanInteropTask`.
     it.defFile 'interop/auxiliary_sources/auxiliaryCppSources.def'
@@ -3718,6 +3722,12 @@ interopTest("interop_withSpaces") {
     }
 }
 
+interopTest("interop_union") {
+    disabled = (project.testTarget == 'wasm32') // No interop for wasm yet.
+    interop = 'cunion'
+    source = "interop/basics/union.kt"
+}
+
 task interop_convert(type: KonanLocalTest) {
     source = "codegen/intrinsics/interop_convert.kt"
 }

backend.native/tests/interop/basics/cunion.def

@@ -0,0 +1,17 @@
+---
+typedef union {
+    short s;
+    long long ll;
+} BasicUnion;
+
+typedef struct {
+    union {
+        int i;
+        float f;
+    } as;
+} StructWithUnion;
+
+typedef union {
+    unsigned int i : 31;
+    unsigned char b : 1;
+} Packed;

backend.native/tests/interop/basics/union.kt

@@ -0,0 +1,25 @@
+import cunion.*
+import kotlinx.cinterop.*
+import kotlin.test.*
+
+fun main() {
+    memScoped {
+        val basicUnion = alloc<BasicUnion>()
+        for (value in Short.MIN_VALUE..Short.MAX_VALUE) {
+            basicUnion.ll = value.toLong()
+            assertEquals(value.toShort(), basicUnion.s)
+        }
+    }
+    memScoped {
+        val struct = alloc<StructWithUnion>()
+        struct.`as`.i = Float.NaN.toRawBits()
+        assertEquals(Float.NaN, struct.`as`.f)
+    }
+    memScoped {
+        val union = alloc<Packed>()
+        union.b = 1u
+        assertEquals(1u, union.i)
+        union.i = 0u
+        assertEquals(0u, union.b)
+    }
+}