Fix handling of dependency('', fallback: ['subproject', 'dep'])

Also extend a test case to cover this.

mesonbuild/interpreter.py

@@ -2811,10 +2811,9 @@ external dependencies (including libraries) must go to "dependencies".''')
             mlog.log('Dependency', mlog.bold(name), 'skipped: feature', mlog.bold(feature), 'disabled')
             return DependencyHolder(NotFoundDependency(self.environment))
 
-        if name == '':
-            if required:
-                raise InvalidArguments('Dependency is both required and not-found')
-            return DependencyHolder(NotFoundDependency(self.environment))
+        # writing just "dependency('')" is an error, because it can only fail
+        if name == '' and required and 'fallback' not in kwargs:
+            raise InvalidArguments('Dependency is both required and not-found')
 
         if '<' in name or '>' in name or '=' in name:
             raise InvalidArguments('Characters <, > and = are forbidden in dependency names. To specify'
@@ -2840,14 +2839,15 @@ external dependencies (including libraries) must go to "dependencies".''')
             exception = None
             dep = NotFoundDependency(self.environment)
 
-            # Search for it outside the project
-            if self.coredata.wrap_mode != WrapMode.forcefallback or 'fallback' not in kwargs:
+            # Unless a fallback exists and is forced ...
+            if self.coredata.wrap_mode == WrapMode.forcefallback and 'fallback' in kwargs:
+                exception = DependencyException("fallback for %s not found" % name)
+            # ... search for it outside the project
+            elif name != '':
                 try:
                     dep = dependencies.find_external_dependency(name, self.environment, kwargs)
                 except DependencyException as e:
                     exception = e
-            else:
-                exception = DependencyException("fallback for %s not found" % name)
 
             # Search inside the projects list
             if not dep.found():

test cases/common/171 not-found dependency/meson.build

@@ -9,3 +9,5 @@ assert(dep.type_name() == 'not-found', 'dependency should be of type "not-found"
 
 library('testlib', 'testlib.c', dependencies: [dep])
 subdir('sub', if_found: dep)
+
+subdep = dependency('', fallback: ['trivial', 'trivial_dep'])

test cases/common/171 not-found dependency/subprojects/trivial/meson.build

@@ -0,0 +1,3 @@
+project('trivial subproject', 'c')
+trivial_lib = static_library('trivial', 'trivial.c', install: false)
+trivial_dep = declare_dependency(link_with: trivial_lib)

test cases/common/171 not-found dependency/subprojects/trivial/trivial.c

@@ -0,0 +1,3 @@
+int subfunc() {
+    return 42;
+}