Implicit dependency fallback when a subproject wrap or dir exists

docs/markdown/Reference-manual.md

@@ -456,7 +456,9 @@ arguments:
   *(since 0.54.0)* `'subproj_dep'` argument can be omitted in the case the
   subproject used `meson.override_dependency('dependency_name', subproj_dep)`.
   In that case, the `fallback` keyword argument can be a single string instead
-  of a list of 2 strings.
+  of a list of 2 strings. *Since 0.55.0* the `fallback` keyword argument can be
+  omitted when there is a wrap file or a directory with the same `dependency_name`,
+  and subproject used `meson.override_dependency('dependency_name', subproj_dep)`.
 - `language` *(since 0.42.0)*: defines what language-specific
   dependency to find if it's available for multiple languages.
 - `method`: defines the way the dependency is detected, the default is

docs/markdown/snippets/implicit_fallback.md

@@ -0,0 +1,9 @@
+## Implicit dependency fallback
+
+`dependency('foo')` now automatically fallback if the dependency is not found on
+the system but a subproject wrap file or directory exists with the same name.
+
+That means that simply adding `subprojects/foo.wrap` is enough to add fallback
+to any `dependency('foo')` call. It is however requires that the subproject call
+`meson.override_dependency('foo', foo_dep)` to specify which dependency object
+should be used for `foo`.

mesonbuild/interpreter.py

@@ -3549,6 +3549,14 @@ external dependencies (including libraries) must go to "dependencies".''')
             return self.notfound_dependency()
 
         has_fallback = 'fallback' in kwargs
+        if not has_fallback and name:
+            # Add an implicit fallback if we have a wrap file or a directory with the same name.
+            subproject_dir_abs = os.path.join(self.environment.get_source_dir(), self.subproject_dir)
+            wrap_, directory = wrap.get_directory(subproject_dir_abs, name)
+            if wrap_ or os.path.exists(os.path.join(subproject_dir_abs, directory)):
+                kwargs['fallback'] = name
+                has_fallback = True
+
         if 'default_options' in kwargs and not has_fallback:
             mlog.warning('The "default_options" keyworg argument does nothing without a "fallback" keyword argument.',
                          location=self.current_node)

run_unittests.py

@@ -4080,6 +4080,11 @@ recommended as it is not supported on some platforms''')
                     'name': 'sub',
                     'version': '1.0'
                 },
+                {
+                    'descriptive_name': 'sub_implicit',
+                    'name': 'sub_implicit',
+                    'version': '1.0',
+                },
                 {
                     'descriptive_name': 'sub-novar',
                     'name': 'sub_novar',

test cases/common/102 subproject subdir/meson.build

@@ -25,3 +25,7 @@ dependency('sub-novar', fallback : 'sub_novar')
 # Verify a subproject can force a dependency to be not-found
 d = dependency('sub-notfound', fallback : 'sub_novar', required : false)
 assert(not d.found(), 'Dependency should be not-found')
+
+# Verify that implicit fallback works because subprojects/sub_implicit directory exists
+d = dependency('sub_implicit')
+assert(d.found(), 'Should implicitly fallback')

test cases/common/102 subproject subdir/subprojects/sub_implicit/meson.build

@@ -0,0 +1,4 @@
+project('sub_implicit', 'c', version : '1.0')
+
+dep = declare_dependency()
+meson.override_dependency('sub_implicit', dep)

test cases/linuxlike/5 dependency versions/meson.build

@@ -31,10 +31,10 @@ dependency('somebrokenlib', version : '>=1.0', required : false)
 
 # Search for an external dependency that won't be found, but must later be
 # found via fallbacks
-somelibnotfound = dependency('somelib', required : false)
+somelibnotfound = dependency('somelib1', required : false)
 assert(somelibnotfound.found() == false, 'somelibnotfound was found?')
 # Find internal dependency without version
-somelibver = dependency('somelib',
+somelibver = dependency('somelib1',
   fallback : ['somelibnover', 'some_dep'])
 assert(somelibver.type_name() == 'internal', 'somelibver should be of type "internal", not ' + somelibver.type_name())
 # Find an internal dependency again with the same name and a specific version