Scope and assignment to return values

 1 class Foo{
 2 
 3 };
 4 
 5 Foo func1(){
 6   Foo f;
 7   return f;
 8 }
 9 
10 int func2(){
11   int a = 2;
12   return a;
13 }
14 
15 int main(int argc, char** argv){
16   Foo var1;
17   func1() = var1; //OK
18   func2() = 1; //error: expression is not assignable
19   return 0;
20 }

What is the fundamental reason that assignment to the return value that is of built-in type is not allowed but assignment to a return value that is user-defined type is allowed? How is the memory managed that allows one but not the other?