Gcc/10.2.0/gcc/Mixed-Declarations
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6.32 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.