Issue 5656: Warn about accessing the Global context explicitly

https://sourceforge.net/p/testlilyissues/issues/5656/

This is another nudge in the direction of consistency in handling contexts.

1: Add regression test for \new Global
This passes without changes: LilyPond issues a warning about \new Global.

2: Warn about trying to access the Global context explicitly
Warn for \context Global and \set Global.property as for \new Global.

[thomasmorley651, 2020-01-09 00:33:35]

I know several custom codings (defining a new grob, thus the need to reintroduce
all-grob-descriptions at Global level) using

\layout {
  \context {
    \Global
    \grobdescriptions #all-grob-descriptions
  }
}

will this be disallowed with this patch?
Then I'd object.

If it stays allowed, why disallow \context Global etc then?

[Dan Eble, 2020-01-09 03:24:34]

On 2020/01/09 00:33:35, thomasmorley651 wrote:
> I know several custom codings (defining a new grob, thus the need to
reintroduce
> all-grob-descriptions at Global level) using
> 
> \layout {
>   \context {
>     \Global
>     \grobdescriptions #all-grob-descriptions
>   }
> }
> 
> will this be disallowed with this patch?
> Then I'd object.

This patch doesn't touch context definitions, so I don't expect anything about
them to change.
 
> If it stays allowed, why disallow \context Global etc then?

*Shrug*  I'm trying to understand what was intended, cover it in tests, and fix
it if necessary.  In the future, I'd like to improve the maintainability of the
code.  If I've misinterpreted how this should work, I'm open to correction.

This is the description of the Global context:
    \description "Hard coded entry point for LilyPond.  Cannot be tuned."
Does that mean that \set Global.whatever = ... should be disallowed?  It sounds
like it, and it seems not to work in master, and the warning behavior is
inconsistent.  Try this example, if you wish.

\version "2.21.0"

\layout {
  \context {
    \Global
    \unset instrumentName
  }
  \context {
    \Score
    \unset instrumentName
  }
  \context {
    \Staff
    \unset instrumentName
  }
}

\context Global {
  %% This has no apparent effect and there is no warning.
  \set Global.instrumentName = "Global 1"
  a'1
}

\context Score {
  b'1
}

\context Score {
  %% This issues a warning but engraves the name anyway.  I think it
  %% falls back to setting the property in the current context
  %% (Score), but I haven't verified that.
  \set Global.instrumentName = "Global 2"
  c''1
}

\context Score {
  d''1
}

\context Score {
  \set Score.instrumentName = "Score"
  e''1
}

Regards,
-- 
Dan

[Dan Eble, 2020-01-14 13:12:54]

Harm,
Do you want more time to consider this change, or are you now comfortable with
my pushing it?
-- 
Dan

[Dan Eble, 2020-01-14 17:43:19]

On 2020/01/14 13:12:54, Dan Eble wrote:
> Harm,
> Do you want more time to consider this change, or are you now comfortable with
> my pushing it?

Since your primary concern was that modifying Global in a \layout block should
still work, and my testing shows that it does, I'm going to push this.

[thomasmorley651, 2020-01-14 18:53:43]

On 2020/01/14 17:43:19, Dan Eble wrote:
> On 2020/01/14 13:12:54, Dan Eble wrote:
> > Harm,
> > Do you want more time to consider this change, or are you now comfortable
with
> > my pushing it?
> 
> Since your primary concern was that modifying Global in a \layout block should
> still work, and my testing shows that it does, I'm going to push this.

Hi Dan,

sorry, wasn't online some time ...

Additionally, I was thinking about:

{
	\context Global
	\applyContext #(lambda (ctx) ...)
	...
}

Though, for which use-cases ...?
I didn't find any.

So I'm fine with the patch being pushed.

Cheers,
  Harm