This adds two new methods, that are conceptually related in the same way
that `enable_auto_if` and `disable_auto_if` are. They are different
however, in that they will always replace an `auto` value with an
`enabled` or `disabled` value, or error if the feature is in the
opposite state (calling `feature(disabled).enable_if(true)`, for
example). This matters when the feature will be passed to
dependency(required : …)`, which has different behavior when passed an
enabled feature than an auto one.
The `disable_if` method will be controversial, I'm sure, since it
can be expressed via `feature.require()` (`feature.require(not
condition) == feature.disable_if(condition)`). I have two defences of
this:
1) `feature.require` is difficult to reason about, I would expect
require to be equivalent to `feature.enable_if(condition)`, not to
`feature.disable_if(not condition)`.
2) mixing `enable_if` and `disable_if` in the same call chain is much
clearer than mixing `require` and `enable_if`:
```meson
get_option('feat') \
.enable_if(foo) \
.disable_if(bar) \
.enable_if(opt)
```
vs
```meson
get_option('feat') \
.enable_if(foo) \
.require(not bar) \
.enable_if(opt)
```
In the first chain it's immediately obvious what is happening, in the
second, not so much, especially if you're not familiar with what
`require` means.
It's always been strange to me we don't have an opposite method of the
`disable_auto_if` method, but I've been pressed to find a case where we
_need_ one, because `disable_auto_if` can't be logically contorted to
work. I finally found the case where they're not equivalent: when you
don't want to convert to a boolean:
```meson
f = get_option('feat').disable_auto_if(not foo)
g = get_option('feat').enable_auto_if(foo)
dep1 = dependency('foo', required : f)
dep2 = dependency('foo', required : g)
```
Link to feature options consistently, and point out that it controls
"whether" the function finds what it's trying to find. This clues people
in to the fact that disabled features exist.
Claiming that "it should literally never be used ever no matter what" is
confusing and wrong -- it's definitely useful sometimes, but does result
in downsides, like not tracking inter-target dependencies correctly.
Ref: #10901
It is often useful to check the found version of a program without
checking whether you can successfully find
`find_program('foo', required: false, version: '>=XXX')`
This is currently allowed, and is used in at least a few projects. It
was not intended to work or documented, but it does and since it is in
use a full deprecation period must be used. A warning has also been
added for values < 0, which have surprising behavior.
All kwargs inherited from has_header need to be prefixed `header_` so we
cannot just do straight inheritance. And the part of the description
that highlighted the way kwargs are derived and evolved, went entirely
missing.
Fixes#9551
In the refman rewrite, these functions vanished. I noticed this when I
went looking at the docs for extract_all_objects(), or should I say I
tried to go looking.
Xavier Claessens