[fpc-devel] Dangerous optimization in CASE..OF

[Martok]

Addendum to this:

> This was also always my intuition that the else block is also triggered for
> invalid enum values (the docs even literally say that, "If none of the case
> constants match the expression value") - and it *is* true in Delphi.
There is a reason why this is true in Delphi: because this is the way it has
been documented in Borland products for at least 25 years!

I have checked with the TP7 language reference (it pays to keep books around),
which defines the following things:
 - Enumeration element names are implicitly defined as typed constants of their
enum type
 - The enum type is either Byte (<=256 elements) or Word.
 - Subrange types are defined as the smallest type that can contain their range
 - Case statements execute the statements of the matching case label, or the
else block otherwise

Note that they actually defined enumerations as what I called 'fancy constants'
before.


The Delphi 4 language reference (also in book form, which is a bit more detailed
than what is in the .hlp files) uses more precise language:
 - Enumeration element names are implicitly defined as typed constants of their
enum type
 - The enum type is either Byte, Word, or Longword, depending on $Z and element
count
 - Subrange types are defined as the smallest type that can contain their range
 - it is legal to inc/dec outside of a subrange, example from the book:
   type Percentile = 1..99;
   var I: Percentile;
   begin
     I:= 99;
     inc(I);   // I is now 100
   So if this is a legal statement, subrange types can contain values outside of
their range. The description in the German version is "Die Variable wird in
ihren Basistyp umgewandelt", the variable becomes its base type.
 - Case statements execute *precisely one* of their branches: the statements of
the matching case label, or the else block otherwise

So, in D4, we have enums as fancy constants, subrange-types are not safe (so
enums can also never be), and case statements cannot fail.


FPC's language reference has no formal definition of what enums or subranges
really are, and the same language as TP7 regarding case statements.


So at least in modes TP and DELPHI, the optimisation in question is formally wrong.