Populate and use ObjFn->arity correctly (wren-lang#731)

`ObjFn->arity` was only populated for functions. Fix that with
wren call handles, and regular methods, so the stack can be properly
adjusted when invoking `wrenCallFunction`.

src/vm/wren_compiler.c

@@ -3282,6 +3282,8 @@ static void createConstructor(Compiler* compiler, Signature* signature,
   Compiler methodCompiler;
   initCompiler(&methodCompiler, compiler->parser, compiler, true);
 
+  methodCompiler.fn->arity = signature->arity;
+
   // Allocate the instance.
   emitOp(&methodCompiler, compiler->enclosingClass->isForeign
        ? CODE_FOREIGN_CONSTRUCT : CODE_CONSTRUCT);
@@ -3462,6 +3464,8 @@ static bool method(Compiler* compiler, Variable classVariable)
     error(compiler, "A constructor cannot be static.");
   }
 
+  methodCompiler.fn->arity = signature.arity;
+
   // Include the full signature in debug messages in stack traces.
   char fullSignature[MAX_METHOD_SIGNATURE];
   int length;

src/vm/wren_vm.c

@@ -1422,6 +1422,7 @@ WrenHandle* wrenMakeCallHandle(WrenVM* vm, const char* signature)
   // Create a little stub function that assumes the arguments are on the stack
   // and calls the method.
   ObjFn* fn = wrenNewFunction(vm, NULL, numParams + 1);
+  fn->arity = numParams;
 
   // Wrap the function in a closure and then in a handle. Do this here so it
   // doesn't get collected as we fill it in.
@@ -1460,9 +1461,10 @@ WrenInterpretResult wrenCall(WrenVM* vm, WrenHandle* method)
 
   // Discard any extra temporary slots. We take for granted that the stub
   // function has exactly one slot for each argument.
-  vm->fiber->stackTop = &vm->fiber->stack[closure->fn->maxSlots];
+  const int closureNumArgs = closure->fn->arity + 1;
+  vm->fiber->stackTop = &vm->fiber->stack[closureNumArgs];
 
-  wrenCallFunction(vm, vm->fiber, closure, 0);
+  wrenCallFunction(vm, vm->fiber, closure, closureNumArgs);
   WrenInterpretResult result = runInterpreter(vm, vm->fiber);
 
   // If the call didn't abort, then set up the API stack to point to the

src/vm/wren_vm.h

@@ -187,12 +187,18 @@ static inline void wrenCallFunction(WrenVM* vm, ObjFiber* fiber,
     fiber->frameCapacity = max;
   }
 
+  // Functions allows to be called with more arguments than required. So the
+  // the [fiber] [stackTop] has to be realined to match the [closure] [arity].
+  const int closureNumArgs = closure->fn->arity + 1;
+  ASSERT(closureNumArgs <= numArgs, "Expect more arguments");
+  fiber->stackTop -= numArgs - closureNumArgs;
+
   // Grow the stack if needed.
   int stackSize = (int)(fiber->stackTop - fiber->stack);
   int needed = stackSize + closure->fn->maxSlots;
   wrenEnsureStack(vm, fiber, needed);
 
-  wrenAppendCallFrame(vm, fiber, closure, fiber->stackTop - numArgs);
+  wrenAppendCallFrame(vm, fiber, closure, fiber->stackTop - closureNumArgs);
 }
 
 // Marks [obj] as a GC root so that it doesn't get collected.

test/regression/731.wren

@@ -0,0 +1,5 @@
+
+Fn.new {
+  var foo
+  System.print(foo) // expect: null
+}.call("Bug")