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A _cast statement changes the type of an expression or an l-lvalue
to a given type. Beware that the type-checker is forced to accept the
new type as is, and must trust the programmer about the new semantics of
the expression/l-value. Bad casts can raise errors in the next stages of
the back-end, or even lead to invalid output code.
Casts work both on expressions and l-values. For instance, these are valid casts:
_cast("a", int)
_cast(a_string, int) := 42
(Although these examples could produce code with a strange behavior at execution time.)
Casts are currently only used in concrete syntax transformations inside the bounds checking extension and, as any language extension, are forbidden in standard Tiger programs.