edit note: originally question said illegal where now it says unspecified.

Thanks to video comment section of Jason Turner video recently I learned that std::complex<int> is unspecified.

But all(AFAIK) implementations seem to happily compile

std::complex<int>

I wonder if there is some reason why detection for "bad" types was never implemented. I know that std lib implementations need to work with older standards so they can not just stuck std::floating_point concept everywhere, but even before C 20 we had ways to constrain templates.

In other words: is this just "would be nice, but we dont have time" issue, or is there some compatibility reason to keep this compiling. Only thing I can think of is that some people are using std::complex and std lib "manufacturers" do not want to obviously break their already broken code.

CodePudding user response：

It is actually not illegal, it is unspecified;

From [complex.numbers]/2

The effect of instantiating the template complex for any type other than float, double, or long double is unspecified.

Unspecified, from [defns.unspecified] means

unspecified behavior behavior, for a well-formed program construct and correct data, that depends on the implementation [Note 1 to entry: The implementation is not required to document which behavior occurs. The range of possible behaviors is usually delineated by this document. —end note]

(references from N4860 (C 20 draft)

CodePudding user response：

I wonder if there is some reason why detection for "bad" types was never implemented.

Both unspecified behavior and undefined behavior allow for revisions in future standards.

If a future C standard were to implement a long long double or decimal type, then std::complex could also be revised to support it.

It it were someday decided that std::complex<int> has great importance, a future standard could promise to implement it.

If, instead, the standard had promised to "detect 'bad' types", these revisions could not happen without putting C standards in conflict.