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Dim value As Int64
Dim i As Long
For i = 0 To 18
    value = (10 ^ i) As Int64 - 1
    debug
Next

今更気付いたのですが，これは単にキャストすれば良かっただけのような気がします。
[code]Dim value As Int64
Dim i As Long
For i = 0 To 18
value = (10 ^ i) As Int64 - 1
debug
Next[/code]

わざわざコードまでありがとうございます。
これを使うかはまだわからないですが、参考にさせていただきます。こちらでもいい方法を模索したいと思います。

Class VARIANT
Public
	vt As Word
	wReserved1 As Word
	wReserved2 As Word
	wReserved3 As Word
	llVal As Int64

	Function bstrVal() As BSTR
		Return GetDWord(VarPtr(llVal))
	End Function
End Class

TypeDef VARTYPE = Word

Declare Function VarDiv Lib "oleaut32.dll" (ByRef lhs As VARIANT, ByRef rhs As VARIANT, ByRef result As VARIANT) As HRESULT
Declare Function VarCat Lib "oleaut32.dll" (ByRef lhs As VARIANT, ByRef rhs As VARIANT, ByRef result As VARIANT) As HRESULT
Declare Function VarInt Lib "oleaut32.dll" (ByRef in As VARIANT, ByRef result As VARIANT) As HRESULT
Declare Function VariantChangeType Lib "oleaut32.dll" (ByRef vargDest As VARIANT, ByRef varSrc As VARIANT, Flags As Word, vt As VARTYPE) As HRESULT
Declare Function VariantClear Lib "oleaut32.dll" (ByRef varg As VARIANT) As HRESULT

Declare Function MessageBoxW Lib "user32" (hWnd As HWND, pText As *WCHAR, pCaption As *WCHAR, uType As DWord) As Long
Declare Function wsprintfW CDECL Lib "user32" (pText As *WCHAR, pFormat As *WCHAR, ...) As Long

Const VT_EMPTY = 0
Const VT_BSTR = 8
Const VT_I8 = 20
Const VT_DECIMAL = 14

Dim n As VARIANT, d As VARIANT, result As VARIANT, empty As VARIANT, tmp As VARIANT
n.vt = VT_I8
n.llVal = 10 ^ 18 - 1

d.vt = VT_I8
d.llVal = 10

result.vt = VT_EMPTY

VariantChangeType(n, n, 0, VT_DECIMAL) ' n = CDec(n) CDecはVBの関数
VariantChangeType(d, d, 0, VT_DECIMAL) ' d = CDec(d)

Dim hr As HRESULT
hr = VarDiv(n, d, result) ' result = n / d

empty.vt = VT_EMPTY
tmp.vt = VT_EMPTY

VariantChangeType(n, n, 0, VT_BSTR) 'n = CStr(n) ABではn = Str$(n)に近い
VariantChangeType(d, d, 0, VT_BSTR)

VarInt(result, result) ' result = Int(result) 'ABのIntと同じ意味
VariantChangeType(result, result, 0, VT_BSTR)

Dim buf[1024] As WCHAR
Dim format[1024] As WCHAR, i As Long, p As *Byte, len As Long
p = "%s / %s = %s"
len = lstrlen(p)
For i = 0 To len
	format = p  ' ワイド文字へ変換
Next

wsprintfW(buf, format, n.bstrVal, d.bstrVal, result.bstrVal)
MessageBoxW(0, buf, 0, MB_OK)

VariantClear(n)
VariantClear(d)
VariantClear(result)
End

試しにOLEオートメーションのVARIANT型（内部形式：Decimal（十進小数）型）に変換し，VARIANT型の演算関数で行わせたところ正しい結果にさせることができました。
ただし，これはもはや64ビット演算ではありません。
[hide][code]Class VARIANT
Public
vt As Word
wReserved1 As Word
wReserved2 As Word
wReserved3 As Word
llVal As Int64

Function bstrVal() As BSTR
Return GetDWord(VarPtr(llVal))
End Function
End Class

TypeDef VARTYPE = Word

Declare Function VarDiv Lib "oleaut32.dll" (ByRef lhs As VARIANT, ByRef rhs As VARIANT, ByRef result As VARIANT) As HRESULT
Declare Function VarCat Lib "oleaut32.dll" (ByRef lhs As VARIANT, ByRef rhs As VARIANT, ByRef result As VARIANT) As HRESULT
Declare Function VarInt Lib "oleaut32.dll" (ByRef in As VARIANT, ByRef result As VARIANT) As HRESULT
Declare Function VariantChangeType Lib "oleaut32.dll" (ByRef vargDest As VARIANT, ByRef varSrc As VARIANT, Flags As Word, vt As VARTYPE) As HRESULT
Declare Function VariantClear Lib "oleaut32.dll" (ByRef varg As VARIANT) As HRESULT

Declare Function MessageBoxW Lib "user32" (hWnd As HWND, pText As *WCHAR, pCaption As *WCHAR, uType As DWord) As Long
Declare Function wsprintfW CDECL Lib "user32" (pText As *WCHAR, pFormat As *WCHAR, ...) As Long

Const VT_EMPTY = 0
Const VT_BSTR = 8
Const VT_I8 = 20
Const VT_DECIMAL = 14

Dim n As VARIANT, d As VARIANT, result As VARIANT, empty As VARIANT, tmp As VARIANT
n.vt = VT_I8
n.llVal = 10 ^ 18 - 1

d.vt = VT_I8
d.llVal = 10

result.vt = VT_EMPTY

VariantChangeType(n, n, 0, VT_DECIMAL) ' n = CDec(n) CDecはVBの関数
VariantChangeType(d, d, 0, VT_DECIMAL) ' d = CDec(d)

Dim hr As HRESULT
hr = VarDiv(n, d, result) ' result = n / d

empty.vt = VT_EMPTY
tmp.vt = VT_EMPTY

VariantChangeType(n, n, 0, VT_BSTR) 'n = CStr(n) ABではn = Str$(n)に近い
VariantChangeType(d, d, 0, VT_BSTR)

VarInt(result, result) ' result = Int(result) 'ABのIntと同じ意味
VariantChangeType(result, result, 0, VT_BSTR)

Dim buf[1024] As WCHAR
Dim format[1024] As WCHAR, i As Long, p As *Byte, len As Long
p = "%s / %s = %s"
len = lstrlen(p)
For i = 0 To len
format[i] = p[i] ' ワイド文字へ変換
Next

wsprintfW(buf, format, n.bstrVal, d.bstrVal, result.bstrVal)
MessageBoxW(0, buf, 0, MB_OK)

VariantClear(n)
VariantClear(d)
VariantClear(result)
End[/code][/hide]
VARIANT型はメンバ変数vtに現在保持しているデータ型を格納し，llValの部分（実際には共用体）に実際のデータを格納すると言う形式です。

また十進型はVBA 6のリファレンスで次のように説明されている型です。
[quote][b]10 進型 (Decimal)[/b]
10 の累乗の指数によって算出される、10 進数を格納するデータ型。小数点以下の桁数が 0 の場合 (小数部分を持たない場合)、絶対値の最大値は 79,228,162,514,264,337,593,543,950,335 です。小数点以下 28 桁の場合、絶対値の最大値は 7.9228162514264337593543950335 です。10 進型 (Decimal) で表される絶対値の最小値は、0 を除いた場合、0.0000000000000000000000000001 です。[/quote]

^演算子の実体は，basic/function.sbpにあるpow関数ではないでしょうか？
そうだとすれば，Double型で演算されて情報落ちになっているのだと思います。

Dim value As Int64
Dim i As Long
For i = 0 To 18
	value = 10^i-1
	debug
Next

64bit変数において、次のような計算をすると、なぜか10^16あたりから正しく計算できなくなってしまいます。本来valueには10^16-1が入りますが、なぜかvalueに10^16が入ってしまいます。
ActiveBasic5.0で確認しました。
[code]Dim value As Int64
Dim i As Long
For i = 0 To 18
value = 10^i-1
debug
Next[/code]