Question

We’ve all heard the warnings that if you invoke undefined behaviour in C or C++, anything at all can happen.

Is this limited to any runtime behaviour at all, or does this also include any compile-time behaviour? In particular, is a compiler, upon encountering a construct that invokes undefined behaviour, allowed to reject the code (in the absence of other requirements in the standard to do so), or even to crash?

Answer 1

"You’re all ignoring the actual definition and focusing on the note, The standard imposes no requirements." – @R.MartinhoFernandes

^{The message above was written by the given user in Lounge<C++> and makes a very valid argument; the standard doesn’t impose any requirements when it comes to code that invokes undefined behavior.}

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! ! !

undefined-behavior stretches even to the far corner of parsing the input data (ie. code) by the compiler, as verified with the below quotations from both the C++11 and C99 standards.

To answer your question with one sentence;

• undefined behavior is not limited to runtime execution, and it’s permissible to crash during compilation "in a documented manner characteristic of the environment" ¹

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^{"in a documented manner characteristic of the environment" is a kind of odd statement, you could pretty much write a compiler documenting that it might crash upon any given code (that’s invalid) to grant it the possibility to crash whenever it wants to.}

^1. quote from the C++11/C99 standards

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###c++11

###1.3.24 [defns.undefined]

Undefined behavior; behavior for which this International Standard
imposes no requirements

[ Note:

Undefined behavior may be expected
when this International Standard omits any explicit definition of
behavior or when a program uses an erroneous construct or erroneous
data.

Permissible undefined behavior ranges from ignoring the situation completely with unpredictable results, to behaving during translation
or program execution in a documented manner characteristic of the
environment (with or without the issuance of a diagnostic message), to
terminating a translation or execution (with the issuance of a
diagnostic message).

Many erroneous program constructs do not engender undefined behavior;
they are required to be diagnosed.

— end note ]

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###c99

3.4.3 – Undefined Behavior

1. behavior, upon use of a nonportable or erroneous program construct or of erroneous data, for which this >International Standard imposes no requirements

2. NOTE Possible undefined behavior ranges from ignoring the situation completely with unpredictable results, to behaving during translation
   or program execution in a documented manner characteristic of the
   environment (with or without the issuance of a diagnostic message), to
   terminating a translation or execution (with the issuance of a
   diagnostic message).