Some legal Ada 83 programs are illegal (i.e., they will fail to compile) in Ada 95 and later versions of the standard:
Some uses of character literals are ambiguous. Since Ada 95 has introduced
Wide_Character as a new predefined character type, some uses of
character literals that were legal in Ada 83 are illegal in Ada 95.
for Char in 'A' .. 'Z' loop ... end loop;
The problem is that ’A’ and ’Z’ could be from either
Wide_Character. The simplest correction
is to make the type explicit; e.g.:
for Char in Character range 'A' .. 'Z' loop ... end loop;
until are reserved in Ada 95.
Existing Ada 83 code using any of these identifiers must be edited to
use some alternative name.
The rules in Ada 95 are slightly different with regard to the point at which entities are frozen, and representation pragmas and clauses are not permitted past the freeze point. This shows up most typically in the form of an error message complaining that a representation item appears too late, and the appropriate corrective action is to move the item nearer to the declaration of the entity to which it refers.
A particular case is that representation pragmas cannot be applied to a subprogram body. If necessary, a separate subprogram declaration must be introduced to which the pragma can be applied.
In Ada 83, a package that did not require a package body was nevertheless
allowed to have one. This lead to certain surprises in compiling large
systems (situations in which the body could be unexpectedly ignored by the
binder). In Ada 95, if a package does not require a body then it is not
permitted to have a body. To fix this problem, simply remove a redundant
body if it is empty, or, if it is non-empty, introduce a dummy declaration
into the spec that makes the body required. One approach is to add a private
part to the package declaration (if necessary), and define a parameterless
Requires_Body, which must then be given a dummy
procedure body in the package body, which then becomes required.
Another approach (assuming that this does not introduce elaboration
circularities) is to add an
Elaborate_Body pragma to the package spec,
since one effect of this pragma is to require the presence of a package body.
In Ada 95, the exception
Numeric_Error is a renaming of
This means that it is illegal to have separate exception handlers for
the two exceptions. The fix is simply to remove the handler for the
Numeric_Error case (since even in Ada 83, a compiler was free to raise
Constraint_Error in place of
Numeric_Error in all cases).
In Ada 83, it was permissible to pass an indefinite type (e.g,
as the actual for a generic formal private type, but then the instantiation
would be illegal if there were any instances of declarations of variables
of this type in the generic body. In Ada 95, to avoid this clear violation
of the methodological principle known as the ’contract model’,
the generic declaration explicitly indicates whether
or not such instantiations are permitted. If a generic formal parameter
has explicit unknown discriminants, indicated by using
(<>) after the
subtype name, then it can be instantiated with indefinite types, but no
stand-alone variables can be declared of this type. Any attempt to declare
such a variable will result in an illegality at the time the generic is
declared. If the
(<>) notation is not used, then it is illegal
to instantiate the generic with an indefinite type.
This is the potential incompatibility issue when porting Ada 83 code to Ada 95.
It will show up as a compile time error, and
the fix is usually simply to add the
(<>) to the generic declaration.