Undefined Behavior vs. \'Ill-formed, No Diagnostic Required\': What\'s the Difference?

Difference between Undefined Behavior and "Ill-formed, No Diagnostic Required"

The C standard classifies program behavior into various categories, including well-formed, ill-formed, unspecified, and undefined. "Undefined behavior" refers to situations where the standard provides no requirements or assumptions about the implementation's response. In contrast, "ill-formed" programs violate the syntax or semantic rules and are not considered well-formed.

The phrase "ill-formed; no diagnostic required" appears in the standard and seemingly contradicts the definition of an ill-formed program. According to the standard, a conforming implementation must accept and execute a well-formed program, while there are no requirements for ill-formed programs except in cases of extension use.

This inconsistency stems from the fact that the C standard draws inspiration from the C standard, which considers all errors as undefined behavior unless explicitly stated otherwise. However, the C standard aimed to require diagnostics for all errors by default.

Ultimately, the presence of "no diagnostics required" indicates that the behavior in question is not considered undefined. It differs from "undefined behavior" only in that the standard explicitly allows the compiler to continue compilation and execution without issuing an error or warning. This distinction is primarily used to describe runtime issues rather than grammatical errors.

In summary, while the term "ill-formed; no diagnostic required" may seem confusing, it essentially means the same as undefined behavior. It represents situations where the program is not well-formed but the compiler is allowed to proceed without providing a diagnostic. The main difference is that "ill-formed; no diagnostic required" typically refers to runtime issues, whereas "undefined behavior" encompasses both grammatical errors and runtime issues.

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