I.e. in src/range.h change:
bool operator==(const QCPRange& other) { return lower == other.lower && upper == other.upper; } bool operator!=(const QCPRange& other) { return !(*this == other); }
to
bool operator==(const QCPRange& other) const { return lower == other.lower && upper == other.upper; } bool operator!=(const QCPRange& other) const { return !(*this == other); }
Without this comparing two const references for equality will fail to compile.