Better error message when trying to use subprojects as dependencies.

mesonbuild/build.py

@@ -533,7 +533,14 @@ class BuildTarget():
                 self.external_deps.append(dep)
                 self.process_sourcelist(dep.get_sources())
             else:
-                raise InvalidArguments('Argument is not an external dependency')
+                # This is a bit of a hack. We do not want Build to know anything
+                # about the interpreter so we can't import it and use isinstance.
+                # This should be reliable enough.
+                if hasattr(dep, 'subproject'):
+                    raise InvalidArguments('''Tried to use subproject object as a dependency.
+You probably wanted to use a dependency declared in it instead. Access it
+by calling get_variable() on the subproject object.''')
+                raise InvalidArguments('Argument is not an external dependency.')
 
     def get_external_deps(self):
         return self.external_deps

mesonbuild/interpreter.py

@@ -567,7 +567,7 @@ class SubprojectHolder(InterpreterObject):
 
     def __init__(self, subinterpreter):
         super().__init__()
-        self.subinterpreter = subinterpreter
+        self.held_object = subinterpreter
         self.methods.update({'get_variable' : self.get_variable_method,
                             })
 
@@ -577,7 +577,7 @@ class SubprojectHolder(InterpreterObject):
         varname = args[0]
         if not isinstance(varname, str):
             raise InterpreterException('Get_variable takes a string argument.')
-        return self.subinterpreter.variables[varname]
+        return self.held_object.variables[varname]
 
 class CompilerHolder(InterpreterObject):
     def __init__(self, compiler, env):