import std.stdio, std.conv, std.exception;

​

struct Checked( alias proof ) {

​

    size_t value;

    alias value this;

    auto opBinary( string operation )( size_t sub )

    if( operation == "-" )

    {

        enforce( this.value >= sub );

        auto val = this.value - sub;

        return Checked!proof( val );

    }

}

​

auto indexFrom( alias list )( size_t index )

{

    enforce( index < list.length );

    return Checked!list( index );

}

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auto itemFrom( alias list )( Checked!list index )

{

    return list[ index ];

}

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unittest

{

    auto list1 = [ 1 , 2 , 3 ];

    auto list2 = [ 4 , 5 ];

    auto index = 2.indexFrom!list1;

    assert( ( index - 1 ).itemFrom!list1 == 2 );

    assert( ( index - 1 ).indexFrom!list2.itemFrom!list2 == 5 );

    static assert( !__traits( compiles , 5.itemFrom!list2 ) );

    static assert( !__traits( compiles , ( index + 1 ).itemFrom!list1 ) );

    static assert( !__traits( compiles , index.itemFrom!list2 ) );

}

​