cmd/vet: No warning for some statements with no effect

[gopherbot]

by tmadams@google.com:

What does 'go version' print?

go version go1.2 linux/amd64


What steps reproduce the problem?
If possible, include a link to a program on play.golang.org.

In this toy example (which represents a pattern of bugs that crops up fairly frequently
for our team), I have forgotten to make err a named return value in my deferred
function. The intent was to propagate any error that occurred during execution of the
deffered function, but since I forgot to make err a named return value, the error gets
dropped.

http://play.golang.org/p/MD8tetH4ZI


What happened?

The program compiled and ran without error.


What should have happened instead?

The compiler should have errored that the statement "err = e" had no effect.
Bonus points if it suggested that I might have meant to use a named return value.

[ianlancetaylor]

Comment 1:

I don't think it is feasible or useful to issue this error in the compiler.
I think it might be possible in the vet tool.

Labels changed: added repo-tools, release-none.

[adonovan]

Comment 2:

So what's the pattern?  A deferred anon function assigns to an enclosing local that is
never
used within the deferred function, and is not a named return parameter?  This isn't
necessarily a bug, as it's very hard to prove that the store is dead.  Consider:
http://play.golang.org/p/5p_IUn56qA
defer/panic code is so rare (and so inherently complex) that I doubt it would be worth
looking for this pattern in vet.