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Also adjust relevant documentation

docs/markdown/Reference-manual.md

@@ -380,11 +380,12 @@ otherwise. This function supports the following keyword arguments:
 - `static` tells the dependency provider to try to get static
   libraries instead of dynamic ones (note that this is not supported
   by all dependency backends)
-- `version`, specifies the required version, a string containing a
+- `version` specifies the required version, a string containing a
   comparison operator followed by the version string, examples include
   `>1.0.0`, `<=2.3.5` or `3.1.4` for exact matching. (*Added 0.37.0*)
   You can also specify multiple restrictions by passing a list to this
   keyword argument, such as: `['>=3.14.0', '<=4.1.0']`.
+  These requirements are never met if the version is unknown.
 - other
 [library-specific](Dependencies.md#dependencies-with-custom-lookup-functionality)
 keywords may also be accepted (e.g. `modules` specifies submodules to use for
@@ -1986,7 +1987,9 @@ an external dependency with the following methods:
    with `declare_dependency()` and `pkgconfig` for system dependencies
    obtained with Pkg-config.
 
- - `version()` is the version number as a string, for example `1.2.8`
+ - `version()` is the version number as a string, for example `1.2.8`.
+   `unknown` if the dependency provider doesn't support determining the
+   version.
 
  - `partial_dependency(compile_args : false, link_args : false, links
    : false, includes : false, source : false)` (*added 0.46.0*) returns

docs/markdown/snippets/dependency_version.md

@@ -0,0 +1,14 @@
+## `dependency(version:)` now applies to all dependency types
+
+Previously, version constraints were only enforced for dependencies found using
+the pkg-config dependency provider.  These constraints now apply to dependencies
+found using any dependency provider.
+
+Some combinations of dependency, host and method do not currently support
+discovery of the version.  In these cases, the dependency will not be found if a
+version constraint is applied, otherwise the `version()` method for the
+dependency object will return `'unknown'`.
+
+(If discovering the version in one of these combinations is important to you,
+and a method exists to determine the version in that case, please file an issue
+with as much information as possible.)