The compilation error of lambda expressions in lesson 2

[RTX2080]

I'm working on Lesson 2 and trying to sort the vertices of a triangle using the std::sort() function. To do this, I wrote a lambda function to compare Vec2i objects, but I'm getting a compilation error when I try to run the make command. Even when I try a simple lambda function that just adds two integers, it still gives me an error. Here is my code and Makefile, can someone please help me?

void triangle(Vec2i tmp[],TGAImage &image, TGAColor color){
    std::vector<Vec2i> p(tmp, tmp + 3);
    std::function<int(int, int)> f1 = [](int a, int b){ return a + b; };
    auto cmp = [](const Vec2i &a,const Vec2i &b)->bool{return a.y<b.y;};
//    std::sort(p.begin(),p.end(),cmp);
//
//    line(p[0],p[1],image,green);
//    line(p[1],p[2],image,green);
//    line(p[2],p[0],image,green);

}
int width = 500;
int height = 500;

int main(int argc, char** argv) {
    auto model = new Model("obj/african_head.obj");
    TGAImage image(width, height, TGAImage::RGB);
    Vec2i t0[3] = {Vec2i(10, 70),   Vec2i(50, 160),  Vec2i(70, 80)};

    triangle(t0,image,blue);

	image.flip_vertically(); // i want to have the origin at the left bottom corner of the image
	image.write_tga_file("output.tga");
	return 0;
}

SYSCONF_LINK = g++
CPPFLAGS     =
LDFLAGS      =
LIBS         = -lm

DESTDIR = ./
TARGET  = main

OBJECTS := $(patsubst %.cpp,%.o,$(wildcard *.cpp))

all: $(DESTDIR)$(TARGET)

$(DESTDIR)$(TARGET): $(OBJECTS)
	$(SYSCONF_LINK) -Wall $(LDFLAGS) -o $(DESTDIR)$(TARGET) $(OBJECTS) $(LIBS)

$(OBJECTS): %.o: %.cpp
	$(SYSCONF_LINK) -Wall $(CPPFLAGS) -c $(CFLAGS) $< -o $@

clean:
	-rm -f $(OBJECTS)
	-rm -f $(TARGET)
	-rm -f *.tga

[RTX2080]

It only shows me 『error: expected expression』.