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

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