Allow empty AnnAssigns (var decls without values)

trans/transpiler/phases/emit_cpp/block.py

@@ -140,9 +140,12 @@ class BlockVisitor(NodeVisitor):
         yield ";"
 
     def visit_AnnAssign(self, node: ast.AnnAssign) -> Iterable[str]:
-        if node.value is None:
-            raise NotImplementedError(node, "empty value")
-        yield from self.visit_lvalue(node.target)
-        yield " = "
-        yield from self.expr().visit(node.value)
+        # if node.value is None:
+        #     raise NotImplementedError(node, "empty value")
+        yield from self.visit_lvalue(node.target, node.is_declare)
+        if node.value:
+            yield " = "
+            yield from self.expr().visit(node.value)
         yield ";"
+
+

trans/transpiler/phases/typing/block.py

@@ -56,6 +56,19 @@ class ScoperBlockVisitor(ScoperVisitor):
         ty = self.get_type(node.value)
         try:
             node.is_declare = self.visit_assign_target(target, ty)
+        except IncompatibleTypesError as e:
+            raise IncompatibleTypesError(f"`{ast.unparse(node)}: {e}") from e
+
+    def visit_AnnAssign(self, node: ast.AnnAssign):
+        if node.value is not None:
+            raise NotImplementedError(node)
+        if node.simple != 1:
+            raise NotImplementedError(node)
+        if not isinstance(node.target, ast.Name):
+            raise NotImplementedError(node)
+        ty = self.visit_annotation(node.annotation)
+        try:
+            node.is_declare = self.visit_assign_target(node.target, ty)
         except IncompatibleTypesError as e:
             raise IncompatibleTypesError(f"`{ast.unparse(node)}: {e}")