Make is_linearizable_convex/is_linearizable_concave abstract on Expre… (#153)

* Make is_linearizable_convex/is_linearizable_concave abstract on Expression

Enforce implementation in all Expression subclasses by uncommenting
@abc.abstractmethod decorators. Add missing implementations to indicator
(convex, not concave) and PartialProblem (delegates to is_convex/is_concave).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* remove is_smooth from max

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: Transurgeon

cvxpy/atoms/max.py

@@ -114,10 +114,6 @@ def is_atom_log_log_concave(self) -> bool:
         """
         return False
 
-    def is_smooth(self):
-        """max is not a smooth function of its args"""
-        return False
-    
     def is_incr(self, idx) -> bool:
         """Is the composition non-decreasing in argument idx?
         """

cvxpy/expressions/expression.py

@@ -348,13 +348,13 @@ def is_concave(self) -> bool:
         """
         raise NotImplementedError()
 
-    #@abc.abstractmethod
+    @abc.abstractmethod
     def is_linearizable_convex(self) -> bool:
         """Is the expression convex after linearizing all smooth subexpressions?
         """
         raise NotImplementedError()
 
-    #@abc.abstractmethod
+    @abc.abstractmethod
     def is_linearizable_concave(self) -> bool:
         """Is the expression concave after linearizing all smooth subexpressions?
         """

cvxpy/transforms/indicator.py

@@ -57,6 +57,12 @@ def is_concave(self) -> bool:
         """
         return False
 
+    def is_linearizable_convex(self) -> bool:
+        return True
+
+    def is_linearizable_concave(self) -> bool:
+        return False
+
     def is_log_log_convex(self) -> bool:
         return False
 

cvxpy/transforms/partial_optimize.py

@@ -144,6 +144,12 @@ def is_concave(self) -> bool:
         return self.args[0].is_dcp() and \
             type(self.args[0].objective) == Maximize
 
+    def is_linearizable_convex(self) -> bool:
+        return self.is_convex()
+
+    def is_linearizable_concave(self) -> bool:
+        return self.is_concave()
+
     def is_dpp(self, context: str = 'dcp') -> bool:
         """The expression is a disciplined parameterized expression.
         """