[mypyc] Fix cross-group constructor calls to cached non-native classes (#22069)

This was broken when using incremental builds.

Author: JukkaL

mypyc/irbuild/prepare.py

@@ -210,11 +210,11 @@ def load_type_map(mapper: Mapper, modules: list[MypyFile], deser_ctx: DeserMaps)
                     continue
                 mapper.type_to_ir[node.node] = ir
                 mapper.symbol_fullnames.add(node.node.fullname)
-                # Trait/builtin-base classes have an ir.ctor FuncDecl
-                # but no emitted CPyDef_<ctor>, so a cross-group direct
-                # call would hit an undefined symbol. Mirror the skip
-                # in prepare_init_method.
-                if not ir.is_trait and not ir.builtin_base:
+                # Non-extension, trait and builtin-base classes have no
+                # emitted CPyDef_<ctor>, so a cross-group direct call
+                # would hit an undefined symbol. Mirror the conditions
+                # under which prepare_init_method registers a ctor.
+                if ir.is_ext_class and not ir.is_trait and not ir.builtin_base:
                     mapper.func_to_decl[node.node] = ir.ctor
 
     for module in modules:

mypyc/test-data/run-multimodule.test

@@ -1421,6 +1421,27 @@ import native
 0
 None
 
+[case testIncrementalCompilationWithNonExtClass]
+import other_a
+[file other_a.py]
+from other_b import C
+[file other_a.py.2]
+from other_b import C
+c = C(5)
+print(c.x)
+[file other_b.py]
+from mypy_extensions import mypyc_attr
+
+@mypyc_attr(native_class=False)
+class C:
+    def __init__(self, x: int) -> None:
+        self.x = x
+[file driver.py]
+import native
+[out]
+[out2]
+5
+
 [case testIncrementalCompilationWithNonClassTypeDef]
 import other_a
 [file other_a.py]