Interpreter: Fallback when required is false but forcefallback is true

docs/markdown/Wrap-dependency-system-manual.md

@@ -189,6 +189,9 @@ endif
   `dependency('foo-1.0', required: get_option('foo_opt'))` will only
   fallback when the user sets `foo_opt` to `enabled` instead of
   `auto`.
+*Since 0.58.0* optional dependency like above will fallback to the subproject
+defined in the wrap file in the case `wrap_mode` is set to `forcefallback`
+or `force_fallback_for` contains the subproject.
 
 If it is desired to fallback for an optional dependency, the
 `fallback` or `allow_fallback` keyword arguments must be passed

docs/markdown/snippets/forcefallback.md

@@ -0,0 +1,21 @@
+## Use fallback from wrap file when force fallback
+
+Optional dependency like below will now fallback to the subproject
+defined in the wrap file in the case `wrap_mode` is set to `forcefallback`
+or `force_fallback_for` contains the subproject.
+
+```meson
+# required is false because we could fallback to cc.find_library(), but in the
+# forcefallback case this now configure the subproject.
+dep = dependency('foo-1.0', required: false)
+if not dep.found()
+  dep = cc.find_library('foo', has_headers: 'foo.h')
+endif
+```
+
+```ini
+[wrap-file]
+...
+[provide]
+dependency_names = foo-1.0
+```

mesonbuild/interpreter/interpreter.py

@@ -1598,6 +1598,11 @@ external dependencies (including libraries) must go to "dependencies".''')
             if not isinstance(allow_fallback, bool):
                 raise InvalidArguments('"allow_fallback" argument must be boolean')
 
+        wrap_mode = self.coredata.get_option(OptionKey('wrap_mode'))
+        force_fallback_for = self.coredata.get_option(OptionKey('force_fallback_for'))
+        force_fallback |= (wrap_mode == WrapMode.forcefallback or
+                           name in force_fallback_for)
+
         # If "fallback" is absent, look for an implicit fallback.
         if name and fallback is None and allow_fallback is not False:
             # Add an implicit fallback if we have a wrap file or a directory with the same name,
@@ -1609,7 +1614,8 @@ external dependencies (including libraries) must go to "dependencies".''')
             if not provider and allow_fallback is True:
                 raise InvalidArguments('Fallback wrap or subproject not found for dependency \'%s\'' % name)
             subp_name = mesonlib.listify(provider)[0]
-            if provider and (allow_fallback is True or required or self.get_subproject(subp_name)):
+            force_fallback |= subp_name in force_fallback_for
+            if provider and (allow_fallback is True or required or self.get_subproject(subp_name) or force_fallback):
                 fallback = provider
 
         if 'default_options' in kwargs and not fallback:
@@ -1640,13 +1646,7 @@ external dependencies (including libraries) must go to "dependencies".''')
             subp_name, varname = self.get_subproject_infos(fallback)
             if self.get_subproject(subp_name):
                 return self.get_subproject_dep(name, display_name, subp_name, varname, kwargs)
-
-            wrap_mode = self.coredata.get_option(OptionKey('wrap_mode'))
-            force_fallback_for = self.coredata.get_option(OptionKey('force_fallback_for'))
-            force_fallback = (force_fallback or
-                              wrap_mode == WrapMode.forcefallback or
-                              name in force_fallback_for or
-                              subp_name in force_fallback_for)
+            force_fallback |= subp_name in force_fallback_for
 
         if name != '' and (not fallback or not force_fallback):
             self._handle_featurenew_dependencies(name)

run_unittests.py

@@ -2513,6 +2513,14 @@ class AllPlatformTests(BasePlatformTests):
         self.build()
         self.run_tests()
 
+    def test_implicit_forcefallback(self):
+        testdir = os.path.join(self.unit_test_dir, '95 implicit force fallback')
+        with self.assertRaises(subprocess.CalledProcessError) as cm:
+            self.init(testdir)
+        self.init(testdir, extra_args=['--wrap-mode=forcefallback'])
+        self.new_builddir()
+        self.init(testdir, extra_args=['--force-fallback-for=something'])
+
     def test_nopromote(self):
         testdir = os.path.join(self.common_test_dir, '99 subproject subdir')
         with self.assertRaises(subprocess.CalledProcessError) as cm:

test cases/unit/95 implicit force fallback/meson.build

@@ -0,0 +1,8 @@
+project('implicit force fallback')
+
+# The dependency 'something' is provided by a subproject. Normally this won't
+# use the fallback subproject because required is false. However this unit test
+# is configured with wrap_mode=forcefallback and force_fallback_for=something
+# in which case we are expecting the fallback to be done.
+d = dependency('something', required: false)
+assert(d.found())

test cases/unit/95 implicit force fallback/subprojects/something/meson.build

@@ -0,0 +1,3 @@
+project('something')
+
+meson.override_dependency('something', declare_dependency())