In the C++ standard, what is specified to occur when a “shall” requirement is violated?

Question

For example, the famous words (§3.2/1)

No translation unit shall contain more than one definition of any variable, function, class type, enumeration

Answer 1

In addition to @JerryCoffin's answer, there is also ISO/IEC Directives Part 2 (that governs all ISO/IEC documents, including the C++ Standard), in particular Annex H Verbal forms for the expression of provisions

The verbal forms shown in Table H.1 shall be used to indicate requirements strictly to be followed in order to conform to the document and from which no deviation is permitted.

shall:

• is to,
• is required to,
• it is required that,
• has to,
• only … is permitted,
• it is necessary

shall not:

• is not allowed [permitted] [acceptable] [permissible],
• is required to be not
• is required that … be not
• is not to be

So a violation of a "shall" requirement makes a program ill-formed. The diagnostic issues have been answered elsewhere.