Native: add a test for cinterop forward declarations

kotlin-native/backend.native/tests/interop/forwardDeclarations/forwardDeclarations.kt

@@ -0,0 +1,45 @@
+// This test mostly checks frontend behaviour.
+
+import cForwardDeclarations.*
+
+// Note: there are no practical cases for this type, but it works in K1 so better to have a test for this.
+import cnames.structs.StructUndeclared
+
+import kotlin.test.assertEquals
+import kotlinx.cinterop.COpaque
+import kotlinx.cinterop.CStructVar
+import kotlinx.cinterop.ptr
+// The test should also check that these references can't be resolved, but the test infra doesn't support this yet:
+// import cForwardDeclarations.StructUndeclared
+
+fun <T1 : T2, T2> checkSubtype2() {}
+fun <T1 : T2, T2 : T3, T3> checkSubtype3() {}
+fun <T1 : T2, T2 : T3, T3 : T4, T4> checkSubtype4() {}
+
+// Here we rely on frontend reporting conflicting overloads if some of these types turn out to be the same.
+fun checkDifferentTypes(s: StructUndeclared?) = 0
+fun checkDifferentTypes(s: StructDeclared?) = 1
+fun checkDifferentTypes(s: StructDefined?) = 2
+
+fun main() {
+    // Checking it is the same type:
+    checkSubtype3<StructUndeclared, cnames.structs.StructUndeclared, StructUndeclared>()
+    checkSubtype2<StructUndeclared, COpaque>()
+
+    checkSubtype4<StructDeclared, cnames.structs.StructDeclared, cForwardDeclarations.StructDeclared, StructDeclared>()
+    checkSubtype2<StructDeclared, COpaque>()
+
+    checkSubtype4<StructDefined, cnames.structs.StructDefined, cForwardDeclarations.StructDefined, StructDefined>()
+    checkSubtype2<StructDefined, CStructVar>()
+
+    val undeclared: StructUndeclared? = null
+    val declared: StructDeclared? = null
+    val defined: StructDefined? = null
+
+    assertEquals(0, checkDifferentTypes(undeclared))
+    assertEquals(1, checkDifferentTypes(declared))
+    assertEquals(2, checkDifferentTypes(defined))
+
+    assertEquals(-1, useStructDeclared(declared?.ptr))
+    assertEquals(-2, useStructDefined(defined?.ptr))
+}