cgen: fix implicit the optional return value to have .ok = true, in anon `fn()?{}`

vlib/v/gen/fn.v

@@ -183,7 +183,14 @@ fn (mut g Gen) gen_fn_decl(it ast.FnDecl, skip bool) {
 		default_expr := g.type_default(it.return_type)
 		// TODO: perf?
 		if default_expr == '{0}' {
+			if it.return_type.idx() == 1 && it.return_type.has_flag(.optional) {
+				// The default return for anonymous functions that return `?,
+				// should have .ok = true set, otherwise calling them with 
+				// optfn() or { panic(err) } will cause a panic:
+				g.writeln('\treturn (Option_void){.ok = true};')
+			} else {
 				g.writeln('\treturn ($type_name)$default_expr;')
+			}
 		} else {
 			g.writeln('\treturn $default_expr;')
 		}

vlib/v/tests/anon_fn_returning_question_test.v

@@ -0,0 +1,25 @@
+type FnCb = fn (a string, b int) ?
+
+fn test_calling_an_anon_function_returning_question() {
+	create_and_call_anon_function() or { panic(err) }
+}
+
+fn create_and_call_anon_function() ? {
+	x := fn (a string, b int) ? {
+		println('test')
+        // NB: the anon function does NOT return explicitly,
+        // so V should generate an implicit "OK" value and
+        // return it. Previously, it created an implicit optional
+        // filled with 0s => .ok was false, and that was treated
+        // as a failure, triggering or blocks.
+	}
+	should_not_call_block(x) ?
+	assert true
+}
+
+fn should_not_call_block(func FnCb) ? {
+	func('abc', 123) or {
+		println('this should NOT be called')
+		assert false
+	}
+}