[K/N] Migrate some cinterop tests to new test infra

native/native.tests/testData/CInterop/executable/forwardDeclarations/forwardDeclarations.def

@@ -0,0 +1,11 @@
+---
+struct StructDeclared;
+struct StructDefined { int x; };
+
+int useStructDeclared(struct StructDeclared* declared) {
+    return -1;
+}
+
+int useStructDefined(struct StructDefined* defined) {
+    return -2;
+}

native/native.tests/testData/CInterop/executable/forwardDeclarations/forwardDeclarations.kt

@@ -0,0 +1,34 @@
+// This test mostly checks frontend behaviour.
+@file:OptIn(kotlinx.cinterop.ExperimentalForeignApi::class)
+
+import forwardDeclarations.*
+import cnames.structs.StructDeclared
+
+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
+// import cnames.structs.StructUndeclared // Supported in K1 though.
+
+fun <T1 : T2, T2> checkSubtype2() {}
+
+// Here we rely on frontend reporting conflicting overloads if some of these types turn out to be the same.
+fun checkDifferentTypes(s: StructDeclared?) = 1
+fun checkDifferentTypes(s: StructDefined?) = 2
+
+fun main() {
+    checkSubtype2<StructDeclared, COpaque>()
+
+    checkSubtype2<StructDefined, CStructVar>()
+
+    val declared: StructDeclared? = null
+    val defined: StructDefined? = null
+
+    assertEquals(1, checkDifferentTypes(declared))
+    assertEquals(2, checkDifferentTypes(defined))
+
+    assertEquals(-1, useStructDeclared(declared?.ptr))
+    assertEquals(-2, useStructDefined(defined?.ptr))
+}