Language: C

main(x){float a;for(;~scanf("%f",&a);)x=a-.1,puts(x<48?"light fly":x<51?"fly":x<54?"bantam":x<57?"feather":x<60?"light":x<64?"light welter":x<69?"welter":x<75?"light middle":x<81?"middle":x<91?"light heavy":"heavy");exit(0):}

Compiler output:
main.c:1:1: error: return type defaults to 'int' [-Wimplicit-int]
    1 | main(x){float a;for(;~scanf("%f",&a);)x=a-.1,puts(x<48?"light fly":x<51?"fly":x<54?"bantam":x<57?"feather":x<60?"light":x<64?"light welter":x<69?"welter":x<75?"light middle":x<81?"middle":x<91?"light heavy":"heavy");exit(0):}
      | ^~~~
main.c: In function 'main':
main.c:1:1: error: type of 'x' defaults to 'int' [-Wimplicit-int]
main.c:1:23: error: implicit declaration of function 'scanf' [-Wimplicit-function-declaration]
    1 | main(x){float a;for(;~scanf("%f",&a);)x=a-.1,puts(x<48?"light fly":x<51?"fly":x<54?"bantam":x<57?"feather":x<60?"light":x<64?"light welter":x<69?"welter":x<75?"light middle":x<81?"middle":x<91?"light heavy":"heavy");exit(0):}
      |                       ^~~~~
main.c:1:1: note: include '<stdio.h>' or provide a declaration of 'scanf'
  +++ |+#include <stdio.h>
    1 | main(x){float a;for(;~scanf("%f",&a);)x=a-.1,puts(x<48?"light fly":x<51?"fly":x<54?"bantam":x<57?"feather":x<60?"light":x<64?"light welter":x<69?"welter":x<75?"light middle":x<81?"middle":x<91?"light heavy":"heavy");exit(0):}
main.c:1:23: warning: incompatible implicit declaration of built-in function 'scanf' [-Wbuiltin-declaration-mismatch]
    1 | main(x){float a;for(;~scanf("%f",&a);)x=a-.1,puts(x<48?"light fly":x<51?"fly":x<54?"bantam":x<57?"feather":x<60?"light":x<64?"light welter":x<69?"welter":x<75?"light middle":x<81?"middle":x<91?"light heavy":"heavy");exit(0):}
      |                       ^~~~~
main.c:1:23: note: include '<stdio.h>' or provide a declaration of 'scanf'
main.c:1:46: error: implicit declaration of function 'puts' [-Wimplicit-function-declaration]
    1 | main(x){float a;for(;~scanf("%f",&a);)x=a-.1,puts(x<48?"light fly":x<51?"fly":x<54?"bantam":x<57?"feather":x<60?"light":x<64?"light welter":x<69?"welter":x<75?"light middle":x<81?"middle":x<91?"light heavy":"heavy");exit(0):}
      |                                              ^~~~
main.c:1:46: note: include '<stdio.h>' or provide a declaration of 'puts'
main.c:1:217: error: implicit declaration of function 'exit' [-Wimplicit-function-declaration]
    1 | main(x){float a;for(;~scanf("%f",&a);)x=a-.1,puts(x<48?"light fly":x<51?"fly":x<54?"bantam":x<57?"feather":x<60?"light":x<64?"light welter":x<69?"welter":x<75?"light middle":x<81?"middle":x<91?"light heavy":"heavy");exit(0):}
      |                                                                                                                                                                                                                         ^~~~
main.c:1:1: note: include '<stdlib.h>' or provide a declaration of 'exit'
  +++ |+#include <stdlib.h>
    1 | main(x){float a;for(;~scanf("%f",&a);)x=a-.1,puts(x<48?"light fly":x<51?"fly":x<54?"bantam":x<57?"feather":x<60?"light":x<64?"light welter":x<69?"welter":x<75?"light middle":x<81?"middle":x<91?"light heavy":"heavy");exit(0):}
main.c:1:217: warning: incompatible implicit declaration of built-in function 'exit' [-Wbuiltin-declaration-mismatch]
    1 | main(x){float a;for(;~scanf("%f",&a);)x=a-.1,puts(x<48?"light fly":x<51?"fly":x<54?"bantam":x<57?"feather":x<60?"light":x<64?"light welter":x<69?"welter":x<75?"light middle":x<81?"middle":x<91?"light heavy":"heavy");exit(0):}
      |                                                                                                                                                                                                                         ^~~~
main.c:1:217: note: include '<stdlib.h>' or provide a declaration of 'exit'
main.c:1:224: error: expected ';' before ':' token
    1 | main(x){float a;for(;~scanf("%f",&a);)x=a-.1,puts(x<48?"light fly":x<51?"fly":x<54?"bantam":x<57?"feather":x<60?"light":x<64?"light welter":x<69?"welter":x<75?"light middle":x<81?"middle":x<91?"light heavy":"heavy");exit(0):}
      |                                                                                                                                                                                                                                ^
      |                                                                                                                                                                                                                                ;