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SyntaxInformation[f] = {"ArgumentsPattern" -> {}};
f[] := 0;

After one ClearAll[f] or ClearAll["`*"], f remains black. It takes two to turn f blue, and it can't be in the same cell: just running ClearAll[f];ClearAll[f] is not enough. Yet one ClearAll is enough to clear FullDefinition[f].

In fact, the line f[] := 0; is extraneous; all that's needed is to define SyntaxInformation[f].

Is this a bug? Is there anything I can run once to turn everything blue?

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    $\begingroup$ Seems like a bug to me. I didn't look into this too much, but it looks like the front-end is informed about the symbol removal before the syntax-information stuff is updated. This probably confuses the front-end abojt the status of the symbol. As a workaround, clearing the syntax-information before clearing the symbol works for me: SyntaxInformation[f]=.; ClearAll[f] $\endgroup$
    – Lukas Lang
    Commented Jun 30, 2021 at 11:46
  • $\begingroup$ @LukasLang Can I enter something to clear all syntax information I've defined before a ClearAll["`*"]? $\endgroup$
    – H.v.M.
    Commented Jul 1, 2021 at 7:58
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    $\begingroup$ Something like Quiet[Unset@*SyntaxInformation@*Symbol /@ Names["`*"]] should do the job $\endgroup$
    – Lukas Lang
    Commented Jul 1, 2021 at 8:00

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It's a bug. From Wolfram Technical Support: "I have reproduced the issue and have reported it to our developers."

Workaround, from the comments by Lukas Lang: SyntaxInformation[f]=.; ClearAll[f] or Quiet[Unset@*SyntaxInformation@*Symbol /@ Names["`*"]]; ClearAll["`*"].

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