resolve #17, fix wrong description and example for constexpr

code/2/2.02.constexpr.cpp

@@ -38,7 +38,9 @@ int main() {
     // char arr_3[len];                  // illegal
 
     const int len_2 = len + 1;
-    char arr_4[len_2];                   // legal
+    constexpr int len_2_constexpr = 1 + 2 + 3;
+    // char arr_4[len_2];                // illegal, but ok for most of the compilers
+    char arr_4[len_2_constexpr];         // legal
 
     // char arr_5[len_foo()+5];          // illegal
     char arr_6[len_foo_constexpr() + 1]; // legal

code/2/Makefile

@@ -1,7 +1,7 @@
 all: $(patsubst %.cpp, %.out, $(wildcard *.cpp))
 
 %.out: %.cpp Makefile
-	clang++ $< -o $@ -std=c++17
+	clang++ $< -o $@ -std=c++17 -pedantic
 
 clean:
 	rm *.out