interpreter: Don't abort if dep isn't required and sub didn't override

mesonbuild/interpreter.py

@@ -3489,8 +3489,12 @@ external dependencies (including libraries) must go to "dependencies".''')
                         raise DependencyException(m.format(display_name))
                     return DependencyHolder(cached_dep, self.subproject)
                 else:
-                    m = 'Subproject {} did not override dependency {}'
-                    raise DependencyException(m.format(subproj_path, display_name))
+                    if required:
+                        m = 'Subproject {} did not override dependency {}'
+                        raise DependencyException(m.format(subproj_path, display_name))
+                    mlog.log('Dependency', mlog.bold(display_name), 'from subproject',
+                             mlog.bold(subproj_path), 'found:', mlog.red('NO'))
+                    return self.notfound_dependency()
             if subproject.found():
                 self.verify_fallback_consistency(dirname, varname, cached_dep)
                 dep = self.subprojects[dirname].get_variable_method([varname], {})

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

@@ -47,3 +47,10 @@ assert(d.found(), 'Should implicitly fallback')
 d = dependency('glib-2.0', required : false)
 assert(d.found())
 assert(d.type_name() == 'internal')
+
+# sub_implicit.wrap provides gobject-2.0 and we already configured that subproject,
+# so we must not return the system dependency here. But since the subproject did
+# not override that dependency and its not required, not-found should be returned.
+# Using gobject-2.0 here because some CI runners have it installed.
+d = dependency('gobject-2.0', required : false)
+assert(not d.found())

test cases/common/102 subproject subdir/subprojects/sub_implicit.wrap

@@ -2,5 +2,5 @@
 
 [provide]
 glib-2.0 = glib_dep
-dependency_names = sub_implicit_provide1
+dependency_names = sub_implicit_provide1, gobject-2.0
 sub_implicit_provide2 = sub_implicit_provide2_dep