How does C++ realize that the template parameters are at least regular or semi-normal?

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T.46: template parameters are required to be at least normal or semi-normal

Reason (reason)

Readability. Preventing surprises and errors. Most uses support that anyway.

Readability. Prevent unexpected code and errors. Anyway, many uses already support this.

Example (sample)

Class X {

Public:

Explicit X (int)

X (const X &); / / copy

X operator= (const X &)

X (X &) noexcept; / / move

X & operator= (X &) noexcept

~ X ()

/ /... No more constructors...

}

X x {1}; / / fine

X y = x; / / fine

Std::vector v (10); / / error: no default constructorNote (note)

Semiregular requires default constructible.

Semi-formal requirements can be constructed by default.

Enforcement (implementation recommendations)

Flag types that are not at least SemiRegular.

Tags types that are not even implemented by semi-normal.

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