The language definition classifies errors into several different categories:
These errors correspond to any violation of a rule given in this reference manual, other than the violations that correspond to (b) or (c) below. In particular, violation of any rule that uses the terms must, allowed, legal, or illegal belongs to this category. Any program that contains such an error is not a legal Ada program; on the other hand, the fact that a program is legal does not mean, per se, that the program is free from other forms of error.
The corresponding error situations are associated with the names of the predefined exceptions. Every Ada compiler is required to generate code that raises the corresponding exception if such an error situation arises during program execution. If an exception is certain to be raised in every execution of a program, then compilers are allowed (although not required) to report this fact at compilation time.
The language rules specify certain rules to be obeyed by Ada programs, although there is no requirement on Ada compilers to provide either a compilation-time or a run-time detection of the violation of such rules. The errors of this category are indicated by the use of the word erroneous to qualify the execution of the corresponding constructs. The effect of erroneous execution is unpredictable.
Whenever the reference manual specifies that different parts of a given construct are to be executed in some order that is not defined by the language, this means that the implementation is allowed to execute these parts in any given order, following the rules that result from that given order, but not in parallel. Furthermore, the construct is incorrect if execution of these parts in a different order would have a different effect. Compilers are not required to provide either compilation-time or run-time detection of incorrect order dependences. The foregoing is expressed in terms of the process that is called execution; it applies equally to the processes that are called evaluation and elaboration.