slices: add Concat

[6543]

Fixes #56353

[gopherbot]

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[6543]

hmm I should not have done a forcepush gerit bot does not import it - it looks like :/

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This PR (HEAD: 8645c5f) has been imported to Gerrit for code review.

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[gopherbot]

Message from Keith Randall:

Patch Set 2:

(1 comment)

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This PR (HEAD: 4af9478) has been imported to Gerrit for code review.

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[gopherbot]

Message from 6543:

Patch Set 3:

(1 comment)

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Message from Keith Randall:

Patch Set 3:

(2 comments)

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Message from 6543:

Patch Set 3:

(1 comment)

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Message from 6543:

Patch Set 4:

(2 comments)

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Message from 6543:

Patch Set 4: Code-Review+1

(1 comment)

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Message from Ian Lance Taylor:

Patch Set 4:

(1 comment)

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[6543]

-> #60929