That's because goto
skips the shadowing variable i
's initialization.
This is one of the minor nuances of the differences between C and C++. In strict C++ go to crossing variable initialization is an error, while in C it's not. GCC also confirms this, when you compile with -std=c11 it allows while with std=c++11 it complains: jump to label 'f' crosses initialization of 'int i'.
From C99:
A goto statement shall not jump from outside the scope of an identifier having a variably modified type to inside the scope of that identifier.
VLAs are of variably modified type. Jumps inside a scope not containing VM types are allowed.
From C++11 (emphasis mine):
A program that jumps from a point where a variable with automatic storage duration is not in scope to a point where it is in scope is ill-formed unless the variable has scalar type, class type with a trivial default constructor and a trivial destructor, a cv-qualified version of one of these types, or an array of one of the preceding types and is declared without an initializer.