Fix CanBeBuiltOn not being checked on flushing building foundation on placement

Author: Starkku

docs/Whats-New.md

@@ -318,6 +318,7 @@ HideLightFlashEffects=false      ; boolean
 - Fixed the bug that weapon cannot used to intercept on gound bullet if it's projectile has `AG=no` (by NetsuNegi)
 - Fixed the bug that if there's a tank in tank bunker, tank bunker use auto death by vanish will cause tank get stuck and game will crash quickly (by NetsuNegi)
 - Fixed `ROF.RandomDelay` incorrectly defaulting to 0 (no delay) instead of random value in range 0-2 (by Starkku)
+- Fixed an oversight that prevented units deploying into buildings from deploying on top of `CanBeBuiltOn` TerrainTypes (by Starkku)
 Vanilla fixes:
 - Fixed preplaced aircraft outside visible map being incorrectly flagged as crashing under certain conditions (by Starkku)
 

src/Ext/TerrainType/Hooks.Passable.cpp

@@ -147,3 +147,20 @@ DEFINE_HOOK(0x5FD2B6, OverlayClass_Unlimbo_SkipTerrainCheck, 0x9)
 
 	return Unlimbo;
 }
+
+// Buildable-upon TerrainTypes Hook #5 -> Ignore when flushing building foundations for placement.
+DEFINE_HOOK(0x45EF3A, BuildingTypeClass_FlushForPlacement_BuildableTerrain, 0x7)
+{
+	enum { Disallow = 0x45F00B, Continue = 0x45EF4A };
+
+	GET(ObjectClass* const, pObject, ESI);
+
+	if (auto const pTerrain = abstract_cast<TerrainClass*>(pObject))
+	{
+		if (!TerrainTypeExt::ExtMap.Find(pTerrain->Type)->CanBeBuiltOn)
+			return Disallow;
+	}
+
+	return Continue;
+}
+