Improve extern member diagnostics and cover extern generic class test

lynglib/src/commonMain/kotlin/net/sergeych/lyng/Compiler.kt

@@ -7762,7 +7762,11 @@ class Compiler(
             nameStartPos = t.pos
         }
         if (parentClassCtx?.isExtern == true && !isExtern) {
-            throw ScriptError(nameStartPos, "members of extern classes/objects must be marked extern")
+            val owner = parentClassCtx.name
+            throw ScriptError(
+                nameStartPos,
+                "member '$name' in extern class/object '$owner' must be declared extern (use `extern fun $name(...)`)"
+            )
         }
         val extensionWrapperName = extTypeName?.let { extensionCallableName(it, name) }
         val classCtx = codeContexts.asReversed().firstOrNull { it is CodeContext.ClassBody } as? CodeContext.ClassBody
@@ -8788,7 +8792,12 @@ class Compiler(
         }
         val parentClassCtx = codeContexts.lastOrNull() as? CodeContext.ClassBody
         if (parentClassCtx?.isExtern == true && !isExtern) {
-            throw ScriptError(nameStartPos, "members of extern classes/objects must be marked extern")
+            val owner = parentClassCtx.name
+            val kind = if (isMutable) "var" else "val"
+            throw ScriptError(
+                nameStartPos,
+                "member '$name' in extern class/object '$owner' must be declared extern (use `extern $kind $name: ...`)"
+            )
         }
 
         val receiverNormalization = normalizeReceiverTypeDecl(receiverTypeDecl, emptySet())

lynglib/src/commonTest/kotlin/TypesTest.kt

@@ -615,6 +615,32 @@ class TypesTest {
             
         """.trimIndent())
     }
+
+    @Test
+    fun testExternGenerics() = runTest {
+        eval("""
+            extern fun f<T>(x: T): T 
+            extern class Cell<T> {
+                extern var value: T
+            }
+        """)
+    }
+
+    @Test
+    fun testExternClassMemberMustBeExternMessage() = runTest {
+        val e = assertFailsWith<ScriptError> {
+            eval(
+                """
+                extern class Cell<T> {
+                    var value: T
+                }
+                """.trimIndent()
+            )
+        }
+        assertTrue(e.message?.contains("must be declared extern") == true)
+        assertTrue(e.message?.contains("extern var value") == true)
+    }
+
 //    @Test fun nonTrivialOperatorsTest() = runTest {
 //        val s = Script.newScope()
 //        s.eval("""