approach-3 -- modified

import std.traits    : isIntegral, isFloatingPoint, isCallable,
                       PIT=ParameterIdentifierTuple, PTT=ParameterTypeTuple;
import std.conv      : to ;
import std.algorithm : startsWith ;
import std.array     : split ;

//import std.stdio     : writeln ;

template GenStructs(uint count, U...)
{
    static if (U.length == 0)
    {
        enum GenStructs = "\n";
    }
    else
    {
        enum GenStructs = "\n" ~ (U[0]).stringof ~ " _s" ~    to!string(count) ~ ";" ~
                          GenStructs!(1+count, U[1..$]);
    }

    //pragma(msg, GenStructs);
}

/*
struct test0    {      }

struct test1(T)
{
    T    A;
         this(T A_) { A = A_; }
    void setA(T A_) { A = A_; }
    T    getA()     { return A; }
    void empty()    { writeln("empty() called"); }
         ~this()    { }
}
*/

//pragma(msg, GenStructs!(0, test0, test1!(int)) );


//pragma(msg, __traits(allMembers, test1!(int)));

template Filter(U...)
{
    static if (U.length == 0)
    {
        enum Filter = "";
    }
    else
    {
        static if ( (U[0]).startsWith("__") || (U[0]).startsWith("op") )
        {
            enum Filter = Filter!(U[1..$]); //skip U[0]
        }
        else
        {
            enum Filter = U[0] ~ "," ~ Filter!(U[1..$]);
        }
    }
}

//pragma(msg, Filter!(__traits(allMembers, test1!(int)))[0..$-1].split(",") );

template StringOf(TS...)
{
    static if(TS.length == 0)
    {
        enum StringOf = "";
    }
    else static if(TS.length == 1)
    {
        enum StringOf = (TS[0]).stringof;
    }
    else
    {
        enum StringOf = (TS[0]).stringof ~ "," ~ StringOf!(TS[1..$]) ;
    }
}

template ArgStringOf(TS...)
{
    static if(TS.length == 0)
    {
        enum ArgStringOf = "";
    }
    else static if(TS.length == 1)
    {
        enum ArgStringOf = TS[0];
    }
    else
    {
        enum ArgStringOf = TS[0] ~ "," ~ ArgStringOf!(TS[1..$]);
    }
}

string combine(string[] types, string[] members)
{
    assert(types.length == members.length);

    string combined = "";

    for(int i=0; i < (types.length) ; ++i)
    {
        combined ~= types[i] ~ " " ~ members[i] ~ ", ";
    }

    if(combined != "") combined = combined[0..$-2] ; //trim end ", "

    return combined;
}

template GenProperty(string M, string N, alias SN)
{
    enum GenProperty = `
                        @property auto ref ` ~ N ~ `(` ~
                        combine( StringOf!(PTT!(SN)).split(","),
                                 ArgStringOf!(PIT!(SN)).split(",") )~ `)
                        { return ` ~ M ~ `(` ~ ArgStringOf!(PIT!(SN)) ~ `); }`;

    //pragma(msg, GenProperty);
}

string genProperty(string mem, string name, string s_name)
{
    return `static if ( !__traits(compiles, ` ~ name ~ `) )
            {    static if (isCallable!(` ~ s_name ~ `.` ~ name ~ `))
                {
                    mixin (GenProperty!( "`~ mem ~`", "`~ name ~`", `~ s_name ~`.`~ name ~`));
                }
                else
                {
                    @property auto ref ` ~ name ~ `() { return ` ~ mem ~ `; };
                    @property ` ~ name ~ `(typeof(` ~ s_name ~ `.` ~ name ~ `) _` ~ name ~ `)
                    { ` ~ mem ~ ` = _` ~ name ~ `; }
                }
            }
            else
            {
            }
            `;
}

string getAlias(uint id, string[] members, string s_name)
{
    string output;

    foreach(m ; members)
    {
        output ~= genProperty("this._s" ~ to!string(id) ~ "." ~ m, m, s_name);
    }

    return output;
}

template GenAliases(uint count, U...)
{
    static if (U.length == 0)
    {
        enum GenAliases = "";
    }
    else
    {
        enum GenAliases = getAlias(count,
                                   Filter!(__traits(allMembers, U[0]))[0..$-1].split(","),
                                   U[0].stringof) ~ GenAliases!(1+count, U[1..$]);
    }
    //pragma(msg, GenAliases);
}

/*
struct test2
{
    int X;
    int sumX(int val) { return (cast(int)(X + val)); }
}
*/

//pragma(msg, GenAliases!(0, test1!(int), test2) );

template Gen(string name, U...)
{
    static assert(U.length != 0); //otherwise no use for this template

    enum Gen = `struct ` ~ name ~ ` { ` ~  GenStructs!(0, U) ~
                                           GenAliases!(0, U) ~ ` }`;

    //pragma(msg, Gen);
}

//pragma(msg, Gen!("T1", test1!(int)));

//mixin (Gen!("T1", test1!(int)));


////////////////////////////////////////////////////////////

struct S_A { int x; int y; void func() { x = 2*x; } ; void funcA() { } ; }
struct S_B { int x; int z; void func() { x = 3*x; } ; void funcB() { } ; }

void main()
{
    import std.stdio: writeln;

    mixin (Gen!("S_AB", S_A, S_B));
    pragma(msg, __traits(allMembers, S_AB));

    S_AB s_ab;
    s_ab.x = 10;
    s_ab.func();
    assert(s_ab.x == 20);
}