18.2: Clarify that some constraints should never be encountered

For completeness, 18.2 prescribes a "false" result for the reduction of certain constraints that should never be encountered by a compiler. These all involve non-proper types appearing in certain parts of a lambda's or method ref's target type.

In these cases, a clarifying comment would be useful.

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18.2.1

If the lambda expression is implicitly typed and one or more of the function type's parameter types is not a proper type, the constraint reduces to false.

***This condition never arises in practice, due to the handling of implicitly-typed lambda expressions in 18.5.1 and the substitution applied to the target type in 18.5.2.***

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If one or more of the function type's parameter types is not a proper type, the constraint reduces to false.

***This condition never arises in practice, due to the handling of inexact method references in 18.5.1 and the substitution applied to the target type in 18.5.2.***

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18.2.5

Otherwise, if the lambda expression is implicitly typed, and one or more of the function type's parameter types is not a proper type, the constraint reduces to false.

Otherwise, if the function type's return type is neither void nor a proper type, the constraint reduces to false.

***The previous two conditions never arise in practice, due to the substitution applied to the target type in 18.5.2.***