Apparently COMAL created around 1975 had 'em. Anything earlier?
(This Q&A over at the langdev stack doesn't answer this question.)
2 Answers
The BCPL Reference Manual, Memorandum-M-352, Massachusetts Institute of Technology, July 21, 1967 specified in section 2.1.2:
(b) User’s comment may be included in a program between a double slash '//' and the end of the line.
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BCPL was the first language, as the earlier CPL used vertical bars.
Wikipedia states CPL first appeared in 1963. The history articles on the C language refer back to CPL 1963.
According to Section 27 Comments on page 33 of the Combined Programming Language (CPL), Elementary Programming Manual, Edition II (Cambridge) January 1966, comments are two vertical bars and continue to the end of the line.
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7@Bergi Given the timing of this and njuffa's answer, it looks like this was intended as some kind of counter-argument to their quote from the BCPL Reference Manual, Memorandum-M-352 and another-dave's comment on it. (i.e. "BCPL used them. Maybe CPL did." "No it didn't.")– ssokolowCommented Oct 14, 2023 at 19:17
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5"No it didn't" - that's how I read it. Maybe the first line of this answer could use a little punctuation: "BCPL, as the earlier...". And while it's almost not-an-answer, it's a useful contribution to establishing the claim of BCPL.– daveCommented Oct 14, 2023 at 19:49
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2The first word is an answer and likely the correct answer (albeit a duplicate), which is why I said 'almost'.– daveCommented Oct 14, 2023 at 21:52
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5@another-dave Oh, thank you! Yes, I was misreading as "BCPL, like CPL, used vertical bars", as opposed to "BCPL, since CPL used vertical bars". I agree this is a worthwhile answer then. It unfortunately won't let me revert my downvote unless it's edited, so I've proposed a clarifying edit :) Commented Oct 14, 2023 at 21:58
// random stufforiginate with just making a joke at the expense of OS/360 JCL?