gcc/clang "passing const X as this discards qualifiers" in CI

Calling a non-const member function on a const object would let it modify the object. The compiler refuses because the call discards the const qualifier on this.

What this error means

A method call on a const reference or const member fails with "discards qualifiers", typically when the called method is not marked const.

gcc

app.cpp:11:14: error: passing 'const Buffer' as 'this' argument discards qualifiers [-fpermissive]
   11 |   return b.size();
      |          ~~~~~~^~
note:   in call to 'int Buffer::size()'

Common causes

How to fix it

Mark read-only methods const

Add const to member functions that do not modify the object.
This makes them callable on const objects and references.

gcc

class Buffer {
public:
  int size() const { return len_; }   // const-qualified
};

How to prevent it

Const-qualify every member function that does not mutate the object so const instances can call it.