why mixin template can not inclulde statement;

[learnfirst1 via Digitalmars-d-learn]

mixin template test(A...){
        __gshared a = A;
        a++;
}
extern(C) void main(){
        mixin test!123;
}

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I dont want to use string mixin to intro a lot un want symbol,  
try with mixin template find this not work.

I know if mixin test on global it should not working, but why not 
make the limit for function scope ?


I try use D for a WASM project then find so much limitation with 
no good reason.  or I missing some thing here ?