The code is a simplified version of code that used to work in dmd 2.055. I just tried it with 2.058 and received
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dmd: interpret.c:100: void CtfeStack::setValue(VarDeclaration*, Expression*): Assertion `v->ctfeAdrOnStack >= 0 && v->ctfeAdrOnStack < stackPointer()' failed.
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import std.conv;

// Create a string that unrolls the given expression N times replacing the
// idx char ('i' by default) with the loop number in the expression each time
string unroll(size_t N, size_t i = 0)(string expr, char idx='i') {
    static if (i < N) {
        string subs_expr;
        foreach (c; expr) {
            if (c==idx) { 
                subs_expr ~= to!string(i); 
            } else {
                subs_expr ~= c;
            }
        }
        return subs_expr ~ "\n" ~ unroll!(N,i+1)(expr,idx);
    }
    return "";
}

struct VectorT(T, int N)
{
    T[N] values_;

    // triggers CTFE error
    bool opEquals(const ref VectorT!(T, N) _rhs) const {
        const string expr = "if(values_[z]!=_rhs.values_[z]) return 0;";
        mixin( unroll!(N)(expr,'z') );
        return 1;
    }
}

void main(string[] args)
{
    alias VectorT!(double,3) Vec3; // triggers CTFE error
}