Swift return expression

直接上例子：

Expression following 'return' is treated as an argument of the 'return'
为何上述代码会出现warning？

我们直接看Swift官方语法说明：

A return statement can consist of only the return keyword, or it can consist of the return keyword followed by an expression, as shown below.

• return
• return expression

When a return statement is followed by an expression, the value of the expression is returned to the calling function or method.

看到这里，就清楚warning的来源了，a = a + 1 被当做了return 的 expression。

进一步研究，编译器如何解析return呢？在 ParseStmt.cpp 中 Parser::parseStmtReturn 函数中，我们找到了如下注释：

  // Handle the ambiguity between consuming the expression and allowing the
  // enclosing stmt-brace to get it by eagerly eating it unless the return is
  // followed by a '}', ';', statement or decl start keyword sequence.

按照该注释，要想消除该warning，很简单，在 return 后面添加}或者;即可。当然，此处只是举个例子，会出现新的warning。