Error if a class tries to inherit from null.

Fix #246.

src/vm/wren_compiler.c

@@ -1020,9 +1020,9 @@ static int emitJump(Compiler* compiler, Code instruction)
 
 // Creates a new constant for the current value and emits the bytecode to load
 // it from the constant table.
-static void emitConstant(Compiler* compiler, Value value)
+static void emitConstant(Compiler* compiler, Value value)
 {
-  int constant = addConstant(compiler, value);
+  int constant = addConstant(compiler, value);
   
   // Compile the code to load the constant.
   emitShortArg(compiler, CODE_CONSTANT, constant);
@@ -1790,6 +1790,15 @@ static void loadThis(Compiler* compiler)
   }
 }
 
+// Pushes the value for a module-level variable implicitly imported from core.
+static void loadCoreVariable(Compiler* compiler, const char* name)
+{
+  int symbol = wrenSymbolTableFind(&compiler->parser->module->variableNames,
+                                   name, strlen(name));
+  ASSERT(symbol != -1, "Should have already defined core name.");
+  emitShortArg(compiler, CODE_LOAD_MODULE_VAR, symbol);
+}
+
 // A parenthesized expression.
 static void grouping(Compiler* compiler, bool allowAssignment)
 {
@@ -1800,13 +1809,8 @@ static void grouping(Compiler* compiler, bool allowAssignment)
 // A list literal.
 static void list(Compiler* compiler, bool allowAssignment)
 {
-  // Load the List class.
-  int listClassSymbol = wrenSymbolTableFind(
-      &compiler->parser->module->variableNames, "List", 4);
-  ASSERT(listClassSymbol != -1, "Should have already defined 'List' variable.");
-  emitShortArg(compiler, CODE_LOAD_MODULE_VAR, listClassSymbol);
-
   // Instantiate a new list.
+  loadCoreVariable(compiler, "List");
   callMethod(compiler, 0, "new()", 5);
   
   // Compile the list elements. Each one compiles to a ".add()" call.
@@ -1836,13 +1840,8 @@ static void list(Compiler* compiler, bool allowAssignment)
 // A map literal.
 static void map(Compiler* compiler, bool allowAssignment)
 {
-  // Load the Map class.
-  int mapClassSymbol = wrenSymbolTableFind(
-      &compiler->parser->module->variableNames, "Map", 3);
-  ASSERT(mapClassSymbol != -1, "Should have already defined 'Map' variable.");
-  emitShortArg(compiler, CODE_LOAD_MODULE_VAR, mapClassSymbol);
-
   // Instantiate a new map.
+  loadCoreVariable(compiler, "Map");
   callMethod(compiler, 0, "new()", 5);
 
   // Compile the map elements. Each one is compiled to just invoke the
@@ -2125,7 +2124,7 @@ static void null(Compiler* compiler, bool allowAssignment)
 // A number or string literal.
 static void literal(Compiler* compiler, bool allowAssignment)
 {
-  emitConstant(compiler, compiler->parser->value);
+  emitConstant(compiler, compiler->parser->value);
 }
 
 static void super_(Compiler* compiler, bool allowAssignment)
@@ -3002,8 +3001,8 @@ static bool method(Compiler* compiler, ClassCompiler* classCompiler,
   if (isForeign)
   {
     // Define a constant for the signature.
-    emitConstant(compiler, wrenNewString(compiler->parser->vm,
-                                         fullSignature, length));
+    emitConstant(compiler, wrenNewString(compiler->parser->vm,
+                                         fullSignature, length));
 
     // We don't need the function we started compiling in the parameter list
     // any more.
@@ -3042,8 +3041,8 @@ static void classDefinition(Compiler* compiler, bool isForeign)
   int slot = declareNamedVariable(compiler);
 
   // Make a string constant for the name.
-  emitConstant(compiler, wrenNewString(compiler->parser->vm,
-      compiler->parser->previous.start, compiler->parser->previous.length));
+  emitConstant(compiler, wrenNewString(compiler->parser->vm,
+      compiler->parser->previous.start, compiler->parser->previous.length));
 
   // Load the superclass (if there is one).
   if (match(compiler, TOKEN_IS))
@@ -3052,8 +3051,8 @@ static void classDefinition(Compiler* compiler, bool isForeign)
   }
   else
   {
-    // Create the empty class.
-    emit(compiler, CODE_NULL);
+    // Implicitly inherit from Object.
+    loadCoreVariable(compiler, "Object");
   }
 
   // Store a placeholder for the number of fields argument. We don't know

src/vm/wren_opcodes.h

@@ -164,13 +164,12 @@ OPCODE(CONSTRUCT)
 // compiler-generated constructor metaclass methods.
 OPCODE(FOREIGN_CONSTRUCT)
 
-// Creates a class. Top of stack is the superclass, or `null` if the class
-// inherits Object. Below that is a string for the name of the class. Byte
-// [arg] is the number of fields in the class.
+// Creates a class. Top of stack is the superclass. Below that is a string for
+// the name of the class. Byte [arg] is the number of fields in the class.
 OPCODE(CLASS)
 
-// Creates a foreign class. Top of stack is the superclass, or `null` if the
-// class inherits Object. Below that is a string for the name of the class.
+// Creates a foreign class. Top of stack is the superclass. Below that is a
+// string for the name of the class.
 OPCODE(FOREIGN_CLASS)
 
 // Define a method for symbol [arg]. The class receiving the method is popped

src/vm/wren_vm.c

@@ -639,27 +639,21 @@ static bool defineClass(WrenVM* vm, ObjFiber* fiber, int numFields,
 {
   // Pull the name and superclass off the stack.
   Value name = fiber->stackTop[-2];
-  Value superclassValue = fiber->stackTop[-1];
+  Value superclass = fiber->stackTop[-1];
   
   // We have two values on the stack and we are going to leave one, so discard
   // the other slot.
   fiber->stackTop--;
 
-  // Use implicit Object superclass if none given.
-  ObjClass* superclass = vm->objectClass;
-  
-  if (!IS_NULL(superclassValue))
+  Value error = validateSuperclass(vm, name, superclass, numFields);
+  if (!IS_NULL(error))
   {
-    Value error = validateSuperclass(vm, name, superclassValue, numFields);
-    if (!IS_NULL(error))
-    {
-      fiber->stackTop[-1] = error;
-      return false;
-    }
-    superclass = AS_CLASS(superclassValue);
+    fiber->stackTop[-1] = error;
+    return false;
   }
   
-  ObjClass* classObj = wrenNewClass(vm, superclass, numFields, AS_STRING(name));
+  ObjClass* classObj = wrenNewClass(vm, AS_CLASS(superclass), numFields,
+                                    AS_STRING(name));
   fiber->stackTop[-1] = OBJ_VAL(classObj);
   
   if (numFields == -1) bindForeignClass(vm, classObj, module);

test/language/inheritance/inherit_from_null.wren

@@ -0,0 +1 @@
+class Foo is null {} // expect runtime error: Class 'Foo' cannot inherit from a non-class object.