[TRIVIAL] Simpler CommonType

[andralex]

this is it

[dlang-bot]

Thanks for your pull request, @andralex!

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[nordlow]

What about making the implementation iterative using static foreach and alias assignment? I tried as

    template CommonType(T...)
    {
        static if (T.length == 0)
            alias CommonType = void;
        else
        {
            alias CommonType = T[0];
            static foreach (i; 1 .. T.length)
                static if (is(typeof(true ? CommonType.init : T[i].init) U))
                    CommonType = U;
                else
                    CommonType = void;
        }
    }

but it errors at the line

CommonType = U;

because CommonType is read in the line above. In this case I believe this restriction should be relaxed in the compiler because CommonType is not aliased in the static if expression.

Coulld such an aliasing checking enable more uses cases for alias assignment, @andralex @WalterBright?