GetWidth() still seems to be broken on UMaterialInstance. It compiles, but the link step fails with:
SlidingTilePuzzle.cpp.obj : error LNK2019: unresolved external symbol "public: int __cdecl UMaterialInterface::GetWidth(void)const " (?GetWidth@UMaterialInterface@@QEBAHXZ) referenced in function “protected: virtual void __cdecl ASlidingTilePuzzle::BeginPlay(void)” (?BeginPlay@ASlidingTilePuzzle@@MEAAXXZ)
This is running 4.18.3.