[llvm-bugs] [Bug 43492] New: [coroutines] Incorrect warning about control reaching end of non-void generic coroutine lambda

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=43492

            Bug ID: 43492
           Summary: [coroutines] Incorrect warning about control reaching
                    end of non-void generic coroutine lambda
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++2a
          Assignee: unassignedclangbugs at nondot.org
          Reporter: lewissbaker at gmail.com
                CC: blitzrakete at gmail.com, erik.pilkington at gmail.com,
                    llvm-bugs at lists.llvm.org, richard-llvm at metafoo.co.uk

Clang trunk, 9.0 (but not 8.0 or earlier)
Compile options: -std=c++2a -fcoroutines-ts -stdlib=libc++ -Wreturn-type

See https://godbolt.org/z/pspDDo

Compiling the following code warns about control reaching the end of a non-void
lambda:

  auto f = [](auto n) -> generator
  {
    co_yield n;
  };

Results in the following warning:

<source>:43:5: warning: control reaches end of non-void lambda [-Wreturn-type]
  };


Note that if the lambda is changed to be non-generic (ie. has argument 'int'
instead of 'auto') then the warning is not shown.

Execution running off the end of a coroutine should result in an implicit
'co_return;'.

It seems like the -Wreturn-type warning analyser is not aware of the fact that
this lambda is a coroutine and so follows different rules for determining
whether control runs off the end.

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