What is the C++ compiler required to do with ill-formed programs according to the Standard?

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Is the C++ implementation required to do something specific when it encounters an ill-formed program or is the C++ implementation behavior just undefined?

If the Standard does not specify otherwise, an implementation should emit a diagnostic message (error or warning). However, for some violations the Standard explicitly specifies that no diagnostic is required. In this case, the program is ill-formed, but implementations are not required to tell the user – usually, because doing so in the general case would be too difficult.

Concerning the One Definition Rule, for example, see Paragraph 3.2/4 of the C++11 Standard:

Every program shall contain exactly one definition of every non-inline function or variable that is odr-used
in that program; no diagnostic required.

Regarding the requirements on implementations when encountering a violation of a rule, Paragraph 1.4/2 specifies:

[…]

— If a program contains no violations of the rules in this International Standard, a conforming implementation
shall, within its resource limits, accept and correctly execute that program.

— If a program contains a violation of any diagnosable rule or an occurrence of a construct described in
this Standard as “conditionally-supported” when the implementation does not support that construct,
a conforming implementation shall issue at least one diagnostic message.

— If a program contains a violation of a rule for which no diagnostic is required, this International
Standard places no requirement on implementations with respect to that program.

Also relevant is Paragraph 1.4/1, which explains what is meant by “diagnosable rules” in the Paragraph quoted above:

The set of diagnosable rules consists of all syntactic and semantic rules in this International Standard except
for those rules containing an explicit notation that “no diagnostic is required” or which are described as
resulting in “undefined behavior.”

So to sum it up: if an ill-formed program contains a diagnosable violation for which the Standard does not explicitly specify “no diagnostic required“, conforming implementations should emit a diagnostic.

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