[core] Fix Pvariable placement new losing subclass identity (#15881)

esphome/cpp_generator.py

@@ -606,33 +606,43 @@ def Pvariable(id_: ID, rhs: SafeExpType, type_: "MockObj" = None) -> "MockObj":
     if isinstance(rhs, MockObj) and rhs.is_new_expr:
         # For 'new' allocations, use placement new into static storage
         # to avoid heap fragmentation on embedded devices.
-        the_type = id_.type
+        #
+        # Storage must be sized and aligned for the actual instantiated class,
+        # which may be a subclass of id_.type (e.g. `cv.declare_id(BaseClass)`
+        # combined with `SubClass.new()` — used by ili9xxx, waveshare_epaper,
+        # etc. to select a model-specific constructor). Using id_.type would
+        # run the base-class default constructor instead, silently losing any
+        # subclass initialization. Template args live on the CallExpression
+        # and are re-emitted below.
+        call_expr = rhs.base
+        assert isinstance(call_expr, CallExpression), (
+            f"Expected CallExpression for placement new, got {type(call_expr)}"
+        )
+        actual_type = rhs.new_type if rhs.new_type is not None else id_.type
+        if call_expr.template_args is not None:
+            actual_type = f"{actual_type}{call_expr.template_args}"
+        pointer_type = id_.type
         # Extract component namespace from type for memory analysis attribution
-        component_ns = _extract_component_ns(str(the_type))
+        component_ns = _extract_component_ns(str(actual_type))
         storage_name = f"{component_ns}__{id_.id}__pstorage"
 
         # Declare aligned byte array for the object storage
         CORE.add_global(
             RawStatement(
-                f"alignas({the_type}) static unsigned char {storage_name}[sizeof({the_type})];"
+                f"alignas({actual_type}) static unsigned char {storage_name}[sizeof({actual_type})];"
             )
         )
+        # Pointer declaration uses id_.type to preserve the declared base-class
+        # pointer type for downstream callers (polymorphism through base ptr).
         CORE.add_global(
             AssignmentExpression(
-                f"static {the_type}",
+                f"static {pointer_type}",
                 "*const ",
                 id_,
-                MockObj(f"reinterpret_cast<{the_type} *>({storage_name})"),
+                MockObj(f"reinterpret_cast<{pointer_type} *>({storage_name})"),
             )
         )
-        # Extract args from the CallExpression and rebuild as placement new.
-        # Template args are already encoded in the_type (e.g. GlobalsComponent<int>),
-        # so we only pass the constructor args, not template_args.
-        call_expr = rhs.base
-        assert isinstance(call_expr, CallExpression), (
-            f"Expected CallExpression for placement new, got {type(call_expr)}"
-        )
-        placement_new = CallExpression(f"new({id_.id}) {the_type}", *call_expr.args)
+        placement_new = CallExpression(f"new({id_.id}) {actual_type}", *call_expr.args)
         CORE.add(ExpressionStatement(placement_new))
     else:
         decl = VariableDeclarationExpression(id_.type, "*", id_, static=True)
@@ -869,12 +879,16 @@ class MockObj(Expression):
     Mostly consists of magic methods that allow ESPHome's codegen syntax.
     """
 
-    __slots__ = ("base", "op", "is_new_expr")
+    __slots__ = ("base", "op", "is_new_expr", "new_type")
 
-    def __init__(self, base, op=".", is_new_expr=False) -> None:
+    def __init__(self, base, op=".", is_new_expr=False, new_type=None) -> None:
         self.base = base
         self.op = op
         self.is_new_expr = is_new_expr
+        # For `is_new_expr=True` objects, `new_type` holds the class name being
+        # constructed (e.g. "ili9xxx::ILI9XXXST7789V"). Needed by Pvariable so
+        # placement new uses the actual subclass rather than id_.type.
+        self.new_type = new_type
 
     def __getattr__(self, attr: str) -> "MockObj":
         # prevent python dunder methods being replaced by mock objects
@@ -889,7 +903,9 @@ class MockObj(Expression):
 
     def __call__(self, *args: SafeExpType) -> "MockObj":
         call = CallExpression(self.base, *args)
-        return MockObj(call, self.op, is_new_expr=self.is_new_expr)
+        return MockObj(
+            call, self.op, is_new_expr=self.is_new_expr, new_type=self.new_type
+        )
 
     def __str__(self):
         return str(self.base)
@@ -903,7 +919,7 @@ class MockObj(Expression):
 
     @property
     def new(self) -> "MockObj":
-        return MockObj(f"new {self.base}", "->", is_new_expr=True)
+        return MockObj(f"new {self.base}", "->", is_new_expr=True, new_type=self.base)
 
     def template(self, *args: SafeExpType) -> "MockObj":
         """Apply template parameters to this object."""

tests/component_tests/ili9xxx/config/ili9xxx_test.yaml

@@ -0,0 +1,20 @@
+esphome:
+  name: test
+
+esp32:
+  board: esp32dev
+  framework:
+    type: arduino
+
+spi:
+  clk_pin: GPIO18
+  mosi_pin: GPIO23
+
+display:
+  - platform: ili9xxx
+    id: tft_display
+    model: ST7789V
+    cs_pin: GPIO5
+    dc_pin: GPIO17
+    reset_pin: GPIO16
+    invert_colors: false

tests/component_tests/ili9xxx/test_ili9xxx.py

@@ -0,0 +1,31 @@
+"""Tests for the ili9xxx component."""
+
+from __future__ import annotations
+
+from collections.abc import Callable
+from pathlib import Path
+
+
+def test_ili9xxx_placement_new_uses_model_subclass(
+    generate_main: Callable[[str | Path], str],
+    component_config_path: Callable[[str], Path],
+) -> None:
+    """Regression test for ili9xxx picking the right constructor under placement new.
+
+    ili9xxx declares the ID as the base ``ILI9XXXDisplay`` but constructs a
+    model-specific subclass (e.g. ``ILI9XXXST7789V``) via ``MODELS[...].new()``.
+    Pvariable must emit placement new for the subclass — otherwise the base
+    default constructor runs and the panel is left with a null init sequence
+    and 0x0 dimensions, producing a silent blank screen.
+    """
+    main_cpp = generate_main(component_config_path("ili9xxx_test.yaml"))
+
+    # Storage is sized for the subclass so the full object fits.
+    assert "sizeof(ili9xxx::ILI9XXXST7789V)" in main_cpp
+    assert "alignas(ili9xxx::ILI9XXXST7789V)" in main_cpp
+    # Pointer is declared as the base type for polymorphism.
+    assert "static ili9xxx::ILI9XXXDisplay *const tft_display" in main_cpp
+    # Placement new runs the subclass constructor — this is the actual regression fix.
+    assert "new(tft_display) ili9xxx::ILI9XXXST7789V()" in main_cpp
+    # Base-class default constructor must NOT be used.
+    assert "new(tft_display) ili9xxx::ILI9XXXDisplay()" not in main_cpp