Quick A: To allow overloading with the underlying types of uint_least16_t and uint_least32_t

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If nullptr_t isn't a keyword, why are char16_t and char32_t?

The proposal itself explains why: to allow overloading with the underlying types of uint_least16_t and uint_least32_t. If they were typedefed this wouldn't be possible.

Define char16_t to be a distinct new type, that has the same size and representation as uint_least16_t. Likewise, define char32_t to be a distinct new type, that has the same size and representation as uint_least32_t.

[N1040 defined char16_t and char32_t as typedefs to uint_least16_t and uint_least32_t, which make overloading on these characters impossible.]

As for why they aren't in the std namespace, this is for compatibility with the original C proposal. C++ prohibits the C definitions from appearing in its own version of <cuchar>

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The headers shall not define the types char16_t, char32_t, and wchar_t (2.11).

The types then would need to be global typedefs, which carries its own set of issues such as

typedef decltype(u'q') char16_t;

namespace foo {
  typedef int char16_t;
}

The reason for std::nullptr_t not being a keyword can be found in the question you linked

We do not expect to see much direct use of nullptr_t in real programs.

making nullptr_t the real exception here.