Can check if running as a subproject.

interpreter.py

@@ -614,6 +614,7 @@ class Interpreter():
                       'option' : self.func_option,
                       'get_option' : self.func_get_option,
                       'subproject' : self.func_subproject,
+                      'is_subproject' : self.func_is_subproject,
                       }
 
     def get_build_def_files(self):
@@ -730,6 +731,9 @@ class Interpreter():
     def func_option(self, nodes, args, kwargs):
         raise InterpreterException('Tried to call option() in build description file. All options must be in the option file.')
 
+    def func_is_subproject(self, nodes, args, kwargs):
+        return self.subproject != ''
+
     def func_subproject(self, nodes, args, kwargs):
         if len(args) != 1:
             raise InterpreterException('Subproject takes exactly one argument')

test cases/common/49 subproject/meson.build

@@ -2,6 +2,10 @@ project('subproj user', 'c')
 
 sub = subproject('sublib')
 
+if is_subproject()
+  error('Claimed to be a subproject even though we are the master project.')
+endif
+
 inc = sub.get_variable('i')
 lib = sub.get_variable('l')
 

test cases/common/49 subproject/sublib/meson.build

@@ -1,5 +1,9 @@
 project('subproject', 'c')
 
+if not is_subproject()
+  error('Claimed to be master project even though we are a subproject.')
+endif
+
 i = include_directories('include')
 l = shared_library('sublib', 'sublib.c', include_dirs : i, install : true)
 t = executable('simpletest', 'simpletest.c', include_dirs : i, link_with : l)