Add a clearer error message for forward declared lowercase variables (#699)

* Add better error messaging for forward referenced top level variables, see #467
* add test case

src/vm/wren_compiler.c

@@ -1253,9 +1253,11 @@ static int declareVariable(Compiler* compiler, Token* token)
   // Top-level module scope.
   if (compiler->scopeDepth == -1)
   {
+    int line = -1;
     int symbol = wrenDefineVariable(compiler->parser->vm,
                                     compiler->parser->module,
-                                    token->start, token->length, NULL_VAL);
+                                    token->start, token->length,
+                                    NULL_VAL, &line);
 
     if (symbol == -1)
     {
@@ -1265,6 +1267,12 @@ static int declareVariable(Compiler* compiler, Token* token)
     {
       error(compiler, "Too many module variables defined.");
     }
+    else if (symbol == -3)
+    {
+      error(compiler,
+        "Variable '%.*s' referenced before this definition (first use at line %d).",
+        token->length, token->start, line);
+    }
 
     return symbol;
   }
@@ -2236,13 +2244,7 @@ static void name(Compiler* compiler, bool canAssign)
                                        token->start, token->length);
   if (variable.index == -1)
   {
-    if (isLocalName(token->start))
-    {
-      error(compiler, "Undefined variable.");
-      return;
-    }
-
-    // If it's a nonlocal name, implicitly define a module-level variable in
+    // Implicitly define a module-level variable in
     // the hopes that we get a real definition later.
     variable.index = wrenDeclareVariable(compiler->parser->vm,
                                          compiler->parser->module,

src/vm/wren_core.c

@@ -1150,7 +1150,7 @@ static ObjClass* defineClass(WrenVM* vm, ObjModule* module, const char* name)
 
   ObjClass* classObj = wrenNewSingleClass(vm, 0, nameString);
 
-  wrenDefineVariable(vm, module, name, nameString->length, OBJ_VAL(classObj));
+  wrenDefineVariable(vm, module, name, nameString->length, OBJ_VAL(classObj), NULL);
 
   wrenPopRoot(vm);
   return classObj;

src/vm/wren_vm.c

@@ -456,7 +456,7 @@ static ObjClosure* compileInModule(WrenVM* vm, Value name, const char* source,
       wrenDefineVariable(vm, module,
                          coreModule->variableNames.data[i]->value,
                          coreModule->variableNames.data[i]->length,
-                         coreModule->variables.data[i]);
+                         coreModule->variables.data[i], NULL);
     }
   }
 
@@ -1498,8 +1498,15 @@ int wrenDeclareVariable(WrenVM* vm, ObjModule* module, const char* name,
   return wrenSymbolTableAdd(vm, &module->variableNames, name, length);
 }
 
+// Returns `true` if [name] is a local variable name (starts with a lowercase
+// letter).
+static bool isLocalName(const char* name)
+{
+	return name[0] >= 'a' && name[0] <= 'z';
+}
+
 int wrenDefineVariable(WrenVM* vm, ObjModule* module, const char* name,
-                       size_t length, Value value)
+                       size_t length, Value value, int* line)
 {
   if (module->variables.count == MAX_MODULE_VARS) return -2;
 
@@ -1516,9 +1523,14 @@ int wrenDefineVariable(WrenVM* vm, ObjModule* module, const char* name,
   }
   else if (IS_NUM(module->variables.data[symbol]))
   {
-    // An implicitly declared variable's value will always be a number. Now we
-    // have a real definition.
+    // An implicitly declared variable's value will always be a number.
+    // Now we have a real definition.
+    if(line) *line = AS_NUM(module->variables.data[symbol]);
     module->variables.data[symbol] = value;
+
+	// If this was a localname we want to error if it was 
+	// referenced before this definition.
+	if (isLocalName(name)) symbol = -3;
   }
   else
   {

src/vm/wren_vm.h

@@ -163,12 +163,15 @@ Value wrenFindVariable(WrenVM* vm, ObjModule* module, const char* name);
 int wrenDeclareVariable(WrenVM* vm, ObjModule* module, const char* name,
                         size_t length, int line);
 
-// Adds a new top-level variable named [name] to [module].
+// Adds a new top-level variable named [name] to [module], and optionally
+// populates line with the line of the implicit first use (line can be NULL).
 //
 // Returns the symbol for the new variable, -1 if a variable with the given name
 // is already defined, or -2 if there are too many variables defined.
+// Returns -3 if this is a top-level lowercase variable (localname) that was
+// used before being defined.
 int wrenDefineVariable(WrenVM* vm, ObjModule* module, const char* name,
-                       size_t length, Value value);
+                       size_t length, Value value, int* line);
 
 // Pushes [closure] onto [fiber]'s callstack to invoke it. Expects [numArgs]
 // arguments (including the receiver) to be on the top of the stack already.

test/language/nonlocal/localname_forward_declare.wren

@@ -0,0 +1,5 @@
+if (false) {
+  System.print(a)
+}
+
+var a = 123 // expect error: Error at '123': Variable 'a' referenced before this definition (first use at line 2).