Fix Clang deprecation warning for json_pointer operator== with ordered_json

is_comparable used a flat && chain to both exclude json_pointer/string
comparisons (added for #4621) and check whether Compare(A, B) is well-formed.
Naming std::is_constructible<decltype(...)> as a later operand of that chain
still causes the decltype to be substituted regardless of the first
operand's value, since the operands aren't lazily deferred like
std::conjunction would defer them. That instantiates the transparent
std::equal_to<>::operator() used by ordered_json, whose noexcept-specifier
evaluates the deprecated json_pointer/string operator==, which Clang (unlike
GCC in this case) warns about even though the result is discarded.

Split is_comparable so the Compare(A, B) checks live in a separate helper
that is only referenced from the specialization selected when
is_json_pointer_of is false, so the decltype is never written when A/B are
a json_pointer/string pair, regardless of compiler.

Signed-off-by: Niels Lohmann <niels.lohmann@gmail.com>

.github/workflows/codeql-analysis.yml

@@ -38,14 +38,14 @@ jobs:
 
     # Initializes the CodeQL tools for scanning.
     - name: Initialize CodeQL
-      uses: github/codeql-action/init@7188fc363630916deb702c7fdcf4e481b751f97a # v4.37.1
+      uses: github/codeql-action/init@99df26d4f13ea111d4ec1a7dddef6063f76b97e9 # v4.37.0
       with:
         languages: c-cpp
 
     # Autobuild attempts to build any compiled languages  (C/C++, C#, or Java).
     # If this step fails, then you should remove it and run the build manually (see below)
     - name: Autobuild
-      uses: github/codeql-action/autobuild@7188fc363630916deb702c7fdcf4e481b751f97a # v4.37.1
+      uses: github/codeql-action/autobuild@99df26d4f13ea111d4ec1a7dddef6063f76b97e9 # v4.37.0
 
     - name: Perform CodeQL Analysis
-      uses: github/codeql-action/analyze@7188fc363630916deb702c7fdcf4e481b751f97a # v4.37.1
+      uses: github/codeql-action/analyze@99df26d4f13ea111d4ec1a7dddef6063f76b97e9 # v4.37.0

.github/workflows/flawfinder.yml

@@ -43,6 +43,6 @@ jobs:
           output: 'flawfinder_results.sarif'
 
       - name: Upload analysis results to GitHub Security tab
-        uses: github/codeql-action/upload-sarif@7188fc363630916deb702c7fdcf4e481b751f97a # v4.37.1
+        uses: github/codeql-action/upload-sarif@99df26d4f13ea111d4ec1a7dddef6063f76b97e9 # v4.37.0
         with:
           sarif_file: ${{github.workspace}}/flawfinder_results.sarif

.github/workflows/scorecards.yml

@@ -76,6 +76,6 @@ jobs:
 
       # Upload the results to GitHub's code scanning dashboard.
       - name: "Upload to code-scanning"
-        uses: github/codeql-action/upload-sarif@7188fc363630916deb702c7fdcf4e481b751f97a # v4.37.1
+        uses: github/codeql-action/upload-sarif@99df26d4f13ea111d4ec1a7dddef6063f76b97e9 # v4.37.0
         with:
           sarif_file: results.sarif

.github/workflows/semgrep.yml

@@ -61,7 +61,7 @@ jobs:
 
       # Upload SARIF file generated in previous step
       - name: Upload SARIF file
-        uses: github/codeql-action/upload-sarif@7188fc363630916deb702c7fdcf4e481b751f97a # v4.37.1
+        uses: github/codeql-action/upload-sarif@99df26d4f13ea111d4ec1a7dddef6063f76b97e9 # v4.37.0
         with:
           sarif_file: semgrep.sarif
         if: always()

include/nlohmann/detail/meta/type_traits.hpp

@@ -699,21 +699,35 @@ struct is_json_pointer_of<A, ::nlohmann::json_pointer<A>> : std::true_type {};
 template <typename A>
 struct is_json_pointer_of<A, ::nlohmann::json_pointer<A>&> : std::true_type {};
 
-// checks if A and B are comparable using Compare functor
+// checks if A and B are comparable using Compare functor, assuming that
+// neither A nor B is a json_pointer type (that case is handled by
+// is_comparable below, which never instantiates this helper otherwise)
 template<typename Compare, typename A, typename B, typename = void>
-struct is_comparable : std::false_type {};
+struct is_comparable_no_json_pointer : std::false_type {};
 
-// We exclude json_pointer here, because the checks using Compare(A, B) will
-// use json_pointer::operator string_t() which triggers a deprecation warning
-// for GCC. See https://github.com/nlohmann/json/issues/4621. The call to
-// is_json_pointer_of can be removed once the deprecated function has been
-// removed.
 template<typename Compare, typename A, typename B>
-struct is_comparable < Compare, A, B, enable_if_t < !is_json_pointer_of<A, B>::value
-&& std::is_constructible <decltype(std::declval<Compare>()(std::declval<A>(), std::declval<B>()))>::value
+struct is_comparable_no_json_pointer < Compare, A, B, enable_if_t <
+std::is_constructible <decltype(std::declval<Compare>()(std::declval<A>(), std::declval<B>()))>::value
 && std::is_constructible <decltype(std::declval<Compare>()(std::declval<B>(), std::declval<A>()))>::value
 >> : std::true_type {};
 
+// checks if A and B are comparable using Compare functor
+// We dispatch on is_json_pointer_of as a plain bool (rather than folding it
+// into a single enable_if_t condition together with the checks below) so
+// that the Compare(A, B) checks are only ever written - and thus only ever
+// instantiated - when A/B are not a json_pointer/string pair. Those checks
+// use json_pointer::operator string_t() (GCC, see #4621) resp. the
+// deprecated json_pointer/string operator== (Clang, see #5288), and merely
+// naming them as later operands of a plain && chain is not sufficient to
+// avoid their instantiation on all compilers, even when the first operand
+// is false. The dispatch on is_json_pointer_of can be removed once the
+// deprecated json_pointer comparison operators have been removed.
+template<typename Compare, typename A, typename B, bool = is_json_pointer_of<A, B>::value>
+struct is_comparable : std::false_type {};
+
+template<typename Compare, typename A, typename B>
+struct is_comparable<Compare, A, B, false> : is_comparable_no_json_pointer<Compare, A, B> {};
+
 template<typename T>
 using detect_is_transparent = typename T::is_transparent;
 

single_include/nlohmann/json.hpp

@@ -4462,21 +4462,35 @@ struct is_json_pointer_of<A, ::nlohmann::json_pointer<A>> : std::true_type {};
 template <typename A>
 struct is_json_pointer_of<A, ::nlohmann::json_pointer<A>&> : std::true_type {};
 
-// checks if A and B are comparable using Compare functor
+// checks if A and B are comparable using Compare functor, assuming that
+// neither A nor B is a json_pointer type (that case is handled by
+// is_comparable below, which never instantiates this helper otherwise)
 template<typename Compare, typename A, typename B, typename = void>
-struct is_comparable : std::false_type {};
+struct is_comparable_no_json_pointer : std::false_type {};
 
-// We exclude json_pointer here, because the checks using Compare(A, B) will
-// use json_pointer::operator string_t() which triggers a deprecation warning
-// for GCC. See https://github.com/nlohmann/json/issues/4621. The call to
-// is_json_pointer_of can be removed once the deprecated function has been
-// removed.
 template<typename Compare, typename A, typename B>
-struct is_comparable < Compare, A, B, enable_if_t < !is_json_pointer_of<A, B>::value
-&& std::is_constructible <decltype(std::declval<Compare>()(std::declval<A>(), std::declval<B>()))>::value
+struct is_comparable_no_json_pointer < Compare, A, B, enable_if_t <
+std::is_constructible <decltype(std::declval<Compare>()(std::declval<A>(), std::declval<B>()))>::value
 && std::is_constructible <decltype(std::declval<Compare>()(std::declval<B>(), std::declval<A>()))>::value
 >> : std::true_type {};
 
+// checks if A and B are comparable using Compare functor
+// We dispatch on is_json_pointer_of as a plain bool (rather than folding it
+// into a single enable_if_t condition together with the checks below) so
+// that the Compare(A, B) checks are only ever written - and thus only ever
+// instantiated - when A/B are not a json_pointer/string pair. Those checks
+// use json_pointer::operator string_t() (GCC, see #4621) resp. the
+// deprecated json_pointer/string operator== (Clang, see #5288), and merely
+// naming them as later operands of a plain && chain is not sufficient to
+// avoid their instantiation on all compilers, even when the first operand
+// is false. The dispatch on is_json_pointer_of can be removed once the
+// deprecated json_pointer comparison operators have been removed.
+template<typename Compare, typename A, typename B, bool = is_json_pointer_of<A, B>::value>
+struct is_comparable : std::false_type {};
+
+template<typename Compare, typename A, typename B>
+struct is_comparable<Compare, A, B, false> : is_comparable_no_json_pointer<Compare, A, B> {};
+
 template<typename T>
 using detect_is_transparent = typename T::is_transparent;