Function `wildcard` doesn't see $# - makefile

Question

I have the following Makefile:
.PHONY: all clean run
INC_DIR = include
SRC_DIR = src
KERNEL_NAME = mykernel
AS = as
ASPARAMS = --32
CXX = g++
CXXPARAMS = -m32 -ffreestanding -fno-exceptions -fno-rtti -I$(INC_DIR)
#CXXPARAMS = -m32 -fno-use-cxa-atexit -nostdlib -fno-builtin -fno-rtti -fno-exceptions -fno-leading-underscore -I$(INC_DIR)
LDPARAMS = -melf_i386
BUILD_DIR = build
ISO_DIR = $(BUILD_DIR)/iso
ISO_BOOT = $(ISO_DIR)/boot
GRUB_CONFIG = $(ISO_BOOT)/grub/grub.cfg
SRC = $(wildcard $(SRC_DIR)/*.cpp)
SRC += $(wildcard $(SRC_DIR)/**/*.cpp)
SRC += $(wildcard $(SRC_DIR)/*.s)
OBJS = $(SRC:=.o)
OBJS := $(addprefix $(BUILD_DIR)/, $(OBJS))
.SECONDARY: $(OBJS)
all: $(KERNEL_NAME).iso
run: $(KERNEL_NAME).iso
qemu-system-i386 -cdrom $<
$(KERNEL_NAME).iso: $(GRUB_CONFIG) $(ISO_BOOT)/$(KERNEL_NAME).bin
grub-mkrescue -o $# $(ISO_DIR)
$(ISO_BOOT)/$(KERNEL_NAME).bin: $(SRC_DIR)/linker.ld $(OBJS)
ld $(LDPARAMS) -T $^ -o $# -nostdlib
$(GRUB_CONFIG):
mkdir -p $(dir $#)
echo 'set timeout=0' > $(GRUB_CONFIG)
echo 'set default=0' >> $(GRUB_CONFIG)
echo '' >> $(GRUB_CONFIG)
echo 'menuentry "$(KERNEL_NAME) OS" {' >> $(GRUB_CONFIG)
echo ' multiboot2 /boot/$(KERNEL_NAME).bin' >> $(GRUB_CONFIG)
echo ' boot' >> $(GRUB_CONFIG)
echo '}' >> $(GRUB_CONFIG)
$(BUILD_DIR)/%.cpp.o: %.cpp
echo $(dir $#)
echo $(wildcard (dir $#))
#ifeq ($(wildcard (dir $#)),)
# echo "mkdir -p $(dir $#)" ;
# $(CXX) $(CXXPARAMS) $< -c -o $#
#endif
$(BUILD_DIR)/%.s.o: %.s
#if [ ! -d $(dir $#) ]; then \
echo "mkdir -p $(dir $#)" ; \
mkdir -p $(dir $#) ; \
fi
$(AS) $(ASPARAMS) $< -o $#
clean:
rm -rf $(KERNEL_NAME).iso $(BUILD_DIR)
Why do I get the output with empty echo by running make? The directory build/src exists.
echo build/src/
build/src/
echo
echo build/src/
build/src/
echo
echo build/src/
build/src/
echo
echo build/src/
build/src/
echo
echo build/src/
build/src/
echo
echo build/src/
build/src/
echo
echo build/src/gui/
build/src/gui/
echo
echo build/src/gui/
build/src/gui/
echo
ld -melf_i386 -T src/linker.ld build/src/Keyboard.cpp.o build/src/VGArray.cpp.o build/src/mykernel.cpp.o build/src/TextCursor.cpp.o build/src/VideoContext.cpp.o build/src/GDTable.cpp.o build/src/gui/DrawContext.cpp.o build/src/gui/Window.cpp.o build/src/loader.s.o -o build/iso/boot/mykernel.bin -nostdlib
ld: cannot find build/src/Keyboard.cpp.o: No such file or directory
ld: cannot find build/src/VGArray.cpp.o: No such file or directory
ld: cannot find build/src/mykernel.cpp.o: No such file or directory
ld: cannot find build/src/TextCursor.cpp.o: No such file or directory
ld: cannot find build/src/VideoContext.cpp.o: No such file or directory
ld: cannot find build/src/GDTable.cpp.o: No such file or directory
ld: cannot find build/src/gui/DrawContext.cpp.o: No such file or directory
ld: cannot find build/src/gui/Window.cpp.o: No such file or directory
Makefile:33: recipe for target 'build/iso/boot/mykernel.bin' failed
make: *** [build/iso/boot/mykernel.bin] Error 1
I tried to make a minimal example, but it works right when build directory exists.
.PHONY: all clean
CXX = g++
OPTS = -Wall -Wextra
BUILD_DIR = build
APP = $(BUILD_DIR)/main
SRC_DIR = src
SRC = $(wildcard $(SRC_DIR)/*.cpp)
all: $(APP)
$(APP): $(APP).o
$(BUILD_DIR)/%.o: $(SRC_DIR)/%.cpp
echo $(dir $#)
echo $(wildcard $(dir $#))
clean:
rm -rf $(BUILD_DIR) main *.o

Answer 1

Replace echo $(wildcard (dir $#)) by echo $(wildcard $(dir $#)) and things should work as you expect.
Note: SRC += $(wildcard $(SRC_DIR)/**/*.cpp) will not recurse in subdirectories. If you believed it would, try:
SRC = $(shell find $(SRC_DIR) -type f -name '*.cpp')
SRC += $(wildcard $(SRC_DIR)/*.s)