template parameter "T" is not used in declaring the parameter types of function template

[MedicineYeh]

Hello,
I tried different compilers with this super great repo. Both GCC and Clang can compile it without any warning and error. However, due to some unknown issue, Intel ICC seems to implement this template declaration check differently.
Here is the warning message I got when I compile with the icpc (ICC) 16.0.3 20160415.

In file included from ./json.cc(1):
./json.hpp(1335): warning #488: template parameter "T" is not used in declaring the parameter types of function template "nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType>::basic_json<T,<unnamed>>(NumberIntegerType)"
      template<typename T, typename std::enable_if<
                        ^

In file included from ./json.cc(1):
./json.hpp(1430): warning #488: template parameter "T" is not used in declaring the parameter types of function template "nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType>::basic_json<T,<unnamed>>(NumberUnsignedType)"
      template<typename T, typename std::enable_if<

I found a way to walk around it, which is using the template type instead of using the specific type on line 1338 and 1433.
Here is my walk around (I only change number_integer_t to T):

 1335     template<typename T, typename std::enable_if<
 1336                  not (std::is_same<T, int>::value) and
 1337                  std::is_same<T, number_integer_t>::value, int>::type = 0>
 1338     basic_json(const T val) noexcept
 1339         : m_type(value_t::number_integer), m_value(val)
 1340     {
 1341         assert_invariant();
 1342     }

Line 1433 is the same thing.

[nlohmann]

Hey @MedicineYeh, thanks for the report. I don't exactly remember why I had to add T in this indirect style, but there is a comment:

    @tparam T A helper type to remove this function via SFINAE in case @ref
    number_integer_t is the same as `int`. In this case, this constructor
    would have the same signature as @ref basic_json(const int value). Note
    the helper type @a T is not visible in this constructor's interface.

It may be possible to change the code, but I think that this may break the code on MSVC or any other compiler which made me code it this way in the first place.

[gregmarr]

@nlohmann I think changing number_integer_t to T in the function parameter list should be a safe change.

[MedicineYeh]

Hi @nlohmann @gregmarr ,
Thanks for your replies. I think using T should be safe to any compiler since it's basically the same thing of written number_integer since you have a type checking on the constructor. It depends on whether you want to make this function a real template function or a function with template type check. I usually prefer the later since it's more clear on types. However, the chain constructor tells the types already, so it doesn't really matter to me.
I don't have a MSVC compiler. It's worthy to try if there's any problem. Thanks.

[nlohmann]

When I make this change

diff --git a/src/json.hpp b/src/json.hpp
index 6fed0a12..c0535cea 100644
--- a/src/json.hpp
+++ b/src/json.hpp
@@ -1335,7 +1335,7 @@ class basic_json
     template<typename T, typename std::enable_if<
                  not (std::is_same<T, int>::value) and
                  std::is_same<T, number_integer_t>::value, int>::type = 0>
-    basic_json(const number_integer_t val) noexcept
+    basic_json(const T val) noexcept
         : m_type(value_t::number_integer), m_value(val)
     {
         assert_invariant();

I get this compiler error:

src/unit-constructor1.cpp:368:18: error: call to constructor of 'json' (aka 'basic_json<>') is ambiguous
            json j(n);
                 ^ ~
../src/json.hpp:1338:5: note: candidate constructor [with T = long long, $1 = 0]
    basic_json(const T val) noexcept
    ^
../src/json.hpp:1406:5: note: candidate constructor [with CompatibleNumberIntegerType = long long, $1 = 0]
    basic_json(const CompatibleNumberIntegerType val) noexcept
    ^
../src/json.hpp:1306:5: note: candidate constructor
    basic_json(boolean_t val) noexcept
    ^
../src/json.hpp:1369:5: note: candidate constructor
    basic_json(const int val) noexcept
    ^
../src/json.hpp:1495:5: note: candidate constructor
    basic_json(const number_float_t val) noexcept
    ^
../src/json.hpp:1961:5: note: candidate constructor
    basic_json(const basic_json& other)
    ^
../src/json.hpp:2038:5: note: candidate constructor
    basic_json(basic_json&& other) noexcept
    ^

Tried with Apple LLVM version 8.0.0 (clang-800.0.42.1) and g++-6 (Homebrew gcc 6.2.0) 6.2.0.

[MedicineYeh]

Cool, so this work around do make some ambiguity... Seems like my work around is not a good way to fix the warning. Still, the warning is not harmful so I would like to conclude with don't change the type. I have no better idea on fixing this warning. I would consider to send a report to Intel so that they might fix this in the future version. It's really non sense of reporting warning on this peice of code.
By the way, my Linux compiler versions are (GCC) 6.2.1 20160830 and clang version 3.9.0.

[nlohmann]

I think we can close this issue, right?

[TurpentineDistillery]

To follow-up,
ICC has #pragma warning disable facilities, so this can be silenced in compiler-specific manner.

[MedicineYeh]

Cool. Thanks everyone.