Given an expression, we have to do computations in some other type than
the type of the expression? Now that's just silly.
Sure, but that's not what I said.
If the expression has some type X, then we should be doing our
computations in type X.
Right.
Let me try again and take a simpler example. If we have
subtype T is Integer range 20..50;
Y: T;
... Y + 1 ...
What the tree looks like is a PLUS_EXPR of type "Integer" (the base type of
T), not T, whose first operand is a NOP_EXPR converting Y to Integer and
whose second operand is the constant 1 also of type Integer, not T.
So the expression is of type Integer and that's what we do the
computation in.
If the context of that expression is
Y := Y + 1;
then there'll be a conversion to type T (and possibly a bounds check)
of the RHS of the MODIFY_EXPR. VRP will know that the results of the
PLUS_EXPR are [21,51] (not [21,50]!). The bounds check will be against
[20,50], so VRP could convert it to a test of "!= 51" if it wanted.
