Needs (in principle) C++ parenthesis around if statement condition?

In current C++ when body of if statements contain only one command then:

Parenthesis around if condition are mandatory but block are optional. So, both examples are OK:

if ( condition ) { return 0; }
if ( condition ) return 0;

But is it theoretically possible to do it also oppositely?:

Blocks mandatory and parenthesis around if condition optional:

if ( condition ) { return 0; }
if condition { return 0; }

Is it teoretically possible to extend C++ syntax this way? (for example as extension in some C++ compiler or theoretically in some future C++ standard). Or collides this hypothetical extension with some other C++ syntactic rule?