Language: C++

 #include<iostream>
 #include<stdio.h>
 #include<string>
 #include<math.h>
 #include<iomanip>
 #include<algorithm>
 #include<string.h>
 #include<cctype>
 #include<map>
 #include<set>
 #include<vector>
 #include<sstream>
 #include<stack>
 #include<queue>
 
 using namespace std;
 
 int main()
 {
   double n;
   while(cin>>n)
   {
     switch(n)
     {
       case n>=91:
            cout<<"heavy"<<endl;
            continue;  
       case n>=81&&n<91:
            cout<<"light heavy"<<endl;
            continue; 
       case n>=75&&n<81:
            cout<<"middle"<<endl;
            continue; 
       case n>=69&&n<75:
            cout<<"light middle"<<endl;
            continue; 
       case n>=64&&n<69:
            cout<<"welter"<<endl;
            continue; 
       case n>=60&&n<64:
            cout<<"light welter"<<endl;
            continue; 
       case n>=57&&n<60:
            cout<<"light"<<endl;
            continue; 
       case n>=54&&n<57:
            cout<<"feather"<<endl;
            continue; 
       case n>=51&&n<54:
            cout<<"bantam"<<endl;
            continue; 
       case n>=48&&n<51:
            cout<<"fly"<<endl;
            continue; 
       default:
            cout<<"light fly"<<endl;
            continue;       
     }             
   }
   //while(1);
   return 0;
 }

Compiler output:
a.cc: In function 'int main()':
a.cc:23:13: error: switch quantity not an integer
   23 |      switch(n)
      |             ^
a.cc:25:13: error: the value of 'n' is not usable in a constant expression
   25 |        case n>=91:
      |             ^
a.cc:20:11: note: 'n' was not declared 'constexpr'
   20 |    double n;
      |           ^
a.cc:28:13: error: the value of 'n' is not usable in a constant expression
   28 |        case n>=81&&n<91:
      |             ^
a.cc:20:11: note: 'n' was not declared 'constexpr'
   20 |    double n;
      |           ^
a.cc:31:13: error: the value of 'n' is not usable in a constant expression
   31 |        case n>=75&&n<81:
      |             ^
a.cc:20:11: note: 'n' was not declared 'constexpr'
   20 |    double n;
      |           ^
a.cc:34:13: error: the value of 'n' is not usable in a constant expression
   34 |        case n>=69&&n<75:
      |             ^
a.cc:20:11: note: 'n' was not declared 'constexpr'
   20 |    double n;
      |           ^
a.cc:37:13: error: the value of 'n' is not usable in a constant expression
   37 |        case n>=64&&n<69:
      |             ^
a.cc:20:11: note: 'n' was not declared 'constexpr'
   20 |    double n;
      |           ^
a.cc:40:13: error: the value of 'n' is not usable in a constant expression
   40 |        case n>=60&&n<64:
      |             ^
a.cc:20:11: note: 'n' was not declared 'constexpr'
   20 |    double n;
      |           ^
a.cc:43:13: error: the value of 'n' is not usable in a constant expression
   43 |        case n>=57&&n<60:
      |             ^
a.cc:20:11: note: 'n' was not declared 'constexpr'
   20 |    double n;
      |           ^
a.cc:46:13: error: the value of 'n' is not usable in a constant expression
   46 |        case n>=54&&n<57:
      |             ^
a.cc:20:11: note: 'n' was not declared 'constexpr'
   20 |    double n;
      |           ^
a.cc:49:13: error: the value of 'n' is not usable in a constant expression
   49 |        case n>=51&&n<54:
      |             ^
a.cc:20:11: note: 'n' was not declared 'constexpr'
   20 |    double n;
      |           ^
a.cc:52:13: error: the value of 'n' is not usable in a constant expression
   52 |        case n>=48&&n<51:
      |             ^
a.cc:20:11: note: 'n' was not declared 'constexpr'
   20 |    double n;
      |           ^