6.30 Mixed Declarations and Code

ISO C99 and ISO C++ allow declarations and code to be freely mixed within compound statements. As an extension, GNU C also allows this in C90 mode. For example, you could do:

int i;
/*  */
i++;
int j = i + 2;

Each identifier is visible from where it is declared until the end of the enclosing block.

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https://gcc.gnu.org/onlinedocs/gcc-7.4.0/gcc/Mixed-Declarations.html