Why do some statements generate warnings in "Release" but not when compiling "Debug" mode with GCC?

I am using gcc to compile some C ++ code, and while the code compiles fine when using the Debug configuration, it gives warnings in the Release configuration. The only difference is in the compilation options:

"Debug": g++ -O0 -g3 ...

"Release": g++ -O3 ...

The message I see in the "Release" build:

../src/xml.cpp: In static member function ‘static Z<char>* XML::ReadToZ(const char*, XMLTransform*, XMLTransformData*)’:
../src/xml.cpp:5034: warning: ignoring return value of ‘size_t fread(void*, size_t, size_t, FILE*)’, declared with attribute warn_unused_result
../src/xml.cpp:5041: warning: ignoring return value of ‘size_t fread(void*, size_t, size_t, FILE*)’, declared with attribute warn_unused_result

      

        
        
        
      

    

Relevant two statements:

/* 5034 */ fread((*y).operator char *(),1,S,fp);
/* 5041 */ fread(yy.operator char *(),1,S,fp);

      

        
        
        
      

    

Why is there a difference in warnings?