narrowing conversion from unsigned to double

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Why is this a narrowing conversion?

Because the definition includes (with my emphasis):

C++11 8.5.4/7 A narrowing conversion is an implicit conversion
[…] from an integer type […] to a floating-point type, except where the source is a constant expression and the actual value after conversion will fit into the target type and will produce the original value when converted back to the original type.

u is not a constant expression, so it’s a narrowing conversion whether or not all possible values of the source type might be representable in the target type.

Isn’t every unsigned perfectly representable as a double?

That’s implementation defined. In the common case of 32-bit unsigned and double with a 52-bit mantissa, that is the case; but some implementations have larger unsigned and/or smaller double representations, so code that depends on that assumption is not portable.

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