C++11 Can'；在静态断言中不使用constexpr函数

请告诉我，如果字符串包含所需的符号数，为什么我不能使用constexpr递归函数来静态断言？Constexpr函数应该是一个常量值，但VS2015编译器告诉我表达式的计算结果不是常量（错误C2131）

结果（翻译成英文）：

1>d:\cprojects\logo\exchlogo.h（43）：错误C2131:表达式未由常量定义
1> d:\cprojects\logo\exchlogo.h（43）：注意：故障是由解析分配操作引起的
1> d:\cprojects\logo\exchlogo.h（43）：注意：在计算“请求的变量（（（“GETVARS:u local_u=I，I..8:1-1；NI，NI..8:1-1”）+26，0）时）
1> d:\cprojects\logo\exchlogo.h（43）：注意：在计算“请求的变量（（（“GETVARS:u local_u=I，I..8:1-1；NI，NI..8:1-1”）+25，0）时）
1> d:\cprojects\logo\exchlogo.h（43）：注意：在计算“请求的变量（（（“GETVARS:u local_u=I，I..8:1-1；NI，NI..8:1-1”）+24，0）时）
1> d:\cprojects\logo\exchlogo.h（43）：注意：在计算“请求的变量（（（“GETVARS:_local_u=I，I..8:1-1；NI，NI..8:1-1”）+23，0）时）
1> d:\cprojects\logo\exchlogo.h（43）：注意：在计算“请求的变量（（（“GETVARS:_local_u=I，I..8:1-1；NI，NI..8:1-1”）+22，0）时）
1> d:\cprojects\logo\exchlogo.h（43）：注意：跳过的中间调用：16（/constexpr:backtrace）
1> d:\cprojects\logo\exchlogo.h（43）：注意：在计算“请求的变量（（（“GETVARS:u local_u=I，I..8:1-1；NI，NI..8:1-1”）+5，0）
1> d:\cprojects\logo\exchlogo.h（43）：注意：在计算“请求的变量（（（“GETVARS:_local_u=I，I..8:1-1；NI，NI..8:1-1”）+4，0）
1> d:\cprojects\logo\exchlogo.h（43）：注意：在计算“请求的变量（（（“GETVARS:_local_u=I，I..8:1-1；NI，NI..8:1-1”）+3，0）时）
1> d:\cprojects\logo\exchlogo.h（43）：注意：在计算“请求的变量（（（“GETVARS:u local_u=I，I..8:1-1；NI，NI..8:1-1”）+2，0）
1> d:\cprojects\logo\exchlogo.h（43）：注意：在计算“请求的变量（（（“GETVARS:u local_u=I，I..8:1-1；NI，NI..8:1-1”）+1，0）
1> d:\cprojects\logo\exchlogo.h（45）：注意：在计算“请求的变量”时（“GETVARS:u local_u=I，I..8:1-1；NI，NI..8:1-1”，0）”
1> d:\cprojects\logo\exchlogo.h（45）：错误C2131:表达式未由常量定义
1> d:\cprojects\logo\exchlogo.h（43）：注意：故障是由解析分配操作引起的
<代码> >看来，VS2015是不可能的。我删除了我的答案。可能有其他选择吗？如果它不支持它，我会考虑升级到VS2019或者删除<代码> CONTXPRPR <代码>要求。
constexpr const char* GetRequestStr = "GETVARS:_local_=I,I..8:1-1;NI,NI..8:1-1";

constexpr unsigned RequestedVars(const char* str, unsigned cnt=0){
    return str[0] == 0 ? cnt+1 : RequestedVars(str+1, str[0] == ';' ? ++cnt : cnt);
};
static_assert(RequestedVars(GetRequestStr) == 2, "Must be 2"); // C2131 here

1> d: \ cprojects \ logo \ exchlogo.h (43): error C2131: expression not defined by constant
1> d: \ cprojects \ logo \ exchlogo.h (43): note: the failure was caused by parsing an assignment operation
1> d: \ cprojects \ logo \ exchlogo.h (43): note: when calculating "RequestedVars ((" GETVARS: _local_ = I, I..8: 1-1; NI, NI..8: 1-1 ") + 26, 0)"
1> d: \ cprojects \ logo \ exchlogo.h (43): note: when calculating "RequestedVars ((" GETVARS: _local_ = I, I..8: 1-1; NI, NI..8: 1-1 ") + 25, 0)"
1> d: \ cprojects \ logo \ exchlogo.h (43): note: when calculating "RequestedVars ((" GETVARS: _local_ = I, I..8: 1-1; NI, NI..8: 1-1 ") + 24, 0)"
1> d: \ cprojects \ logo \ exchlogo.h (43): note: when calculating "RequestedVars ((" GETVARS: _local_ = I, I..8: 1-1; NI, NI..8: 1-1 ") + 23, 0)"
1> d: \ cprojects \ logo \ exchlogo.h (43): note: when calculating "RequestedVars ((" GETVARS: _local_ = I, I..8: 1-1; NI, NI..8: 1-1 ") + 22, 0)"
1> d: \ cprojects \ logo \ exchlogo.h (43): note: skipped intermediate calls: 16 (/ constexpr: backtrace <NUMBER>)
1> d: \ cprojects \ logo \ exchlogo.h (43): note: when calculating "RequestedVars ((" GETVARS: _local_ = I, I..8: 1-1; NI, NI..8: 1-1 ") + 5, 0)"
1> d: \ cprojects \ logo \ exchlogo.h (43): note: when calculating "RequestedVars ((" GETVARS: _local_ = I, I..8: 1-1; NI, NI..8: 1-1 ") + 4, 0)"
1> d: \ cprojects \ logo \ exchlogo.h (43): note: when calculating "RequestedVars ((" GETVARS: _local_ = I, I..8: 1-1; NI, NI..8: 1-1 ") + 3, 0)"
1> d: \ cprojects \ logo \ exchlogo.h (43): note: when calculating "RequestedVars ((" GETVARS: _local_ = I, I..8: 1-1; NI, NI..8: 1-1 ") + 2, 0)"
1> d: \ cprojects \ logo \ exchlogo.h (43): note: when calculating "RequestedVars ((" GETVARS: _local_ = I, I..8: 1-1; NI, NI..8: 1-1 ") + 1, 0)"
1> d: \ cprojects \ logo \ exchlogo.h (45): note: when calculating "RequestedVars (" GETVARS: _local_ = I, I..8: 1-1; NI, NI..8: 1-1 " , 0) "
1> d: \ cprojects \ logo \ exchlogo.h (45): error C2131: expression not defined by constant
1> d: \ cprojects \ logo \ exchlogo.h (43): note: the failure was caused by parsing an assignment operation