C++里一個帶有返回值的函數在沒有return語句的情況下在GCC里編譯通過是否可看作GCC的bug？

不能看作是 Bug。

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因為「在有返回值的函數里不 return」屬於未定義行為：

§6.6.3/2 C++03 C++11 C++14

Flowing off the end of a function is equivalent to a return with no value; this results in undefined behavior in a value-returning function.

而「未定義行為」沒有必要導致編譯失敗：

§1.3.13 C++03 / §1.3.24 C++11 C++14

Permissible undefined behavior ranges from ignoring the situation completely with unpredictable results, to behaving during translation or program execution in a documented manner characteristic of the environment (with or without the issuance of a diagnostic message), to terminating a translation or execution (with the issuance of a diagnostic message).

當然有一個特例就是 main，沒有 return 等於結尾有個 return 0：

§3.6.1/5 C++03 C++11 C++14

If control reaches the end of main without encountering a return statement, the effect is that of executing
return 0;

自然也就不會導致編譯失敗。