interpreter: Already configured fallback should be used for optional dep

mesonbuild/interpreter.py

@@ -3585,13 +3585,15 @@ external dependencies (including libraries) must go to "dependencies".''')
             return self.notfound_dependency()
 
         has_fallback = 'fallback' in kwargs
-        if not has_fallback and name and required:
+        if not has_fallback and name:
             # Add an implicit fallback if we have a wrap file or a directory with the same name,
             # but only if this dependency is required. It is common to first check for a pkg-config,
             # then fallback to use find_library() and only afterward check again the dependency
-            # with a fallback.
+            # with a fallback. If the fallback has already been configured then we have to use it
+            # even if the dependency is not required.
             provider = self.environment.wrap_resolver.find_dep_provider(name)
-            if provider:
+            dirname = mesonlib.listify(provider)[0]
+            if provider and (required or dirname in self.subprojects):
                 kwargs['fallback'] = provider
                 has_fallback = True
 

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

@@ -40,3 +40,10 @@ assert(d.found(), 'Should implicitly fallback')
 # sub_implicit_provide2.
 d = dependency('sub_implicit_provide2')
 assert(d.found(), 'Should implicitly fallback')
+
+# sub_implicit.wrap provides glib-2.0 and we already configured that subproject,
+# so we must not return the system dependency here. Using glib-2.0 here because
+# some CI runners have it installed.
+d = dependency('glib-2.0', required : false)
+assert(d.found())
+assert(d.type_name() == 'internal')

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

@@ -1,5 +1,6 @@
 [wrap-file]
 
 [provide]
+glib-2.0 = glib_dep
 dependency_names = sub_implicit_provide1
 sub_implicit_provide2 = sub_implicit_provide2_dep

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

@@ -6,3 +6,6 @@ meson.override_dependency('sub_implicit_provide1', dep)
 
 # This one is not overriden but the wrap file tells the variable name to use.
 sub_implicit_provide2_dep = dep
+
+# This one is not overriden but the wrap file tells the variable name to use.
+glib_dep = dep