offsetof triggers LLVM assertion failure on StaticArray

[HertzDevil]

The following triggers an LLVM assertion failure in LLVMOffsetOfElement:

offsetof(UInt8[4], @buffer)

.../include/llvm/Support/Casting.h:579: decltype(auto) llvm::cast(From*) [with To = StructType; From = Type]: Assertion `isa<To>(Val) && "cast<Ty>() argument of incompatible type!"' failed.

The relevant code is:

crystal/src/compiler/crystal/semantic/main_visitor.cr

Lines 2607 to 2608 in 5dc9424

	if type && (type.struct? || type.is_a?(TupleInstanceType))
	offset = @program.offset_of(type.sizeof_type, ivar_index)

It breaks because the LLVM type for UInt8[4] is [4 x i8], which is an array type, not a struct type. (Crystal::Type#sizeof_type has no significant effect here.) LLVMOffsetOfElement only accepts struct types.

The same thing happens in the interpreter for pointerof, but via a different code path:

crystal/src/compiler/crystal/interpreter/compiler.cr

Lines 1528 to 1542 in 5dc9424

	private def compile_pointerof_ivar(node : ASTNode, name : String)
	closure_self = lookup_closured_var?("self")
	if closure_self
	get_closured_self_pointer(closure_self, name, node: node)
	return
	end
	index = scope.index_of_instance_var(name).not_nil!
	offset = if scope.struct?
	@context.offset_of(scope, index)
	else
	@context.instance_offset_of(scope, index)
	end
	pointerof_ivar(offset, node: node)
	end

The following is an empty prelude reproduction:

lib LibC
  fun exit(status : Int32) : NoReturn
end

def exit
  LibC.exit(0)
end

struct StaticArray(T, N)
  def to_unsafe
    pointerof(@buffer)
  end
end

x = uninitialized UInt8[4]
x.to_unsafe

Normal codegen doesn't have this issue with pointerof, as instance variable addresses there are always obtained at run time via getelementptr instructions instead, and no compile-time decision needs to be made.