Solidity provides the var keyword to declare variables. The type of the variable in this case is decided dynamically depending on the first value assigned to it. Once a value is assigned, the type is fixed, so if you assign another type to it, it will cause type conversion.

Here is an example to demonstrate var:

int256 x = 12; 

//y type is int256 
var y = x; 

uint256 z= 9; 

//exception because implicit conversion not possible 
y = z;