Incorrect/unclear description of explicit conversions involving user-defined conversions

[Nigel-Ecma]

(This could be tackled alongside #1541.)

The clause §10.5.5 User-defined explicit conversions appears to define the semantics of the application of user-defined explicit conversions, this is incorrect. The semantics described cover explicit conversions, aka casts, which involve user-defined conversions whether declared as explicit or implicit – which is an important distinction!

As with #1541 §10.5.3 should be reviewed in the light of any changes made to §10.5.5.

Note the same does not apply to §10.5.4 User-defined implicit conversions, as that process is not applicable to explicit user-defined conversions.

This inaccuracy has recently caused some confusion “in the wild” however I’ll set the milestone to “C# 9” as there is no “C# 8 if it can be squeezed in” ;-) But feel free to tackled it before 9.