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Before this commit, Kernel#lambda can't tell the difference between a directly passed literal block and one passed with an ampersand. A block passed with an ampersand is semantically speaking already a non-lambda proc. When Kernel#lambda receives a non-lambda proc, it should simply return it. Implementation wise, when the VM calls a method with a literal block, it places the code for the block on the calling control frame and passes a pointer (block handler) to the callee. Before this commit, the VM forwards block arguments by simply forwarding the block handler, which leaves the slot for block code unused when a control frame forwards its block argument. I use the vacant space to indicate that a frame has forwarded its block argument and inspect that in Kernel#lambda to detect forwarded blocks. This is a very ad-hoc solution and relies *heavily* on the way block passing works in the VM. However, it's the most self-contained solution I have. [Bug #15620]