How would using an existing structure definition where an undefined type is found break anything?
Compilers already know what you want to do as it will print an error such as: "unknown type name ‘Vec’; use ‘struct’ keyword to refer to the type".
Compilers already know what you want to do as it will print an error such as: "unknown type name ‘Vec’; use ‘struct’ keyword to refer to the type".
struct S { ... };
typedef int S;
I don't really think changing this in C would break all that much code, but it's definitely not backwards compatible.