Make comparisons with scalars noexcept only when the conversion is (#5751)

The comparison operators taking a scalar (==, !=, <, <=, >, >=, and
C++20's <=>) convert the scalar to a basic_json and compare, but were
unconditionally noexcept. When that conversion throws, the program
called std::terminate instead of propagating the exception, e.g. when
comparing a json with a string literal under memory pressure
(std::bad_alloc) or with an enum value not mapped by
NLOHMANN_JSON_SERIALIZE_ENUM_STRICT (out_of_range.410). clang-tidy
22.1 reports the latter as bugprone-exception-escape.

Declare the 16 scalar overloads
noexcept(std::is_nothrow_constructible<basic_json, ScalarType>::value):
they stay noexcept for numbers, Booleans, nullptr, and plain enums,
and are noexcept(false) for strings and enums whose to_json may throw.
The comparisons of two basic_json values are unchanged.

Restore the strict-enum comparisons removed from unit-conversions.cpp
in the previous PR, check that comparing an unmapped strict enum now
throws, and pin the new exception specifications in unit-noexcept.cpp.
Document the exception safety of overload (2) on all seven operator
pages. Ran make amalgamate.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/api/basic_json/operator_eq.md

@@ -5,17 +5,17 @@
 bool operator==(const_reference lhs, const_reference rhs) noexcept;   // (1)
 
 template<typename ScalarType>
-bool operator==(const_reference lhs, const ScalarType rhs) noexcept;  // (2)
+bool operator==(const_reference lhs, const ScalarType rhs) noexcept(/* see below */); // (2)
 
 template<typename ScalarType>
-bool operator==(ScalarType lhs, const const_reference rhs) noexcept;  // (2)
+bool operator==(ScalarType lhs, const const_reference rhs) noexcept(/* see below */); // (2)
 
 // since C++20
 class basic_json {
     bool operator==(const_reference rhs) const noexcept;              // (1)
 
     template<typename ScalarType>
-    bool operator==(ScalarType rhs) const noexcept;                   // (2)
+    bool operator==(ScalarType rhs) const noexcept(/* see below */); // (2)
 };
 ```
 
@@ -46,7 +46,12 @@ whether the values `lhs`/`*this` and `rhs` are equal
 
 ## Exception safety
 
-No-throw guarantee: this function never throws exceptions.
+1. No-throw guarantee: this function never throws exceptions.
+2. No-throw guarantee if converting the scalar to a JSON value cannot throw, as for numbers, Booleans, and
+   `#!cpp nullptr`; the function is `#!cpp noexcept` exactly in that case. Otherwise, it throws what the conversion
+   throws, for example `std::bad_alloc` when converting a string, or
+   [`out_of_range.410`](../../home/exceptions.md#jsonexceptionout_of_range410) for an enum value not mapped by
+   [`NLOHMANN_JSON_SERIALIZE_ENUM_STRICT`](../macros/nlohmann_json_serialize_enum_strict.md).
 
 ## Complexity
 
@@ -171,3 +176,4 @@ Linear.
 
 1. Added in version 1.0.0. Added C++20 member functions in version 3.11.0.
 2. Added in version 1.0.0. Added C++20 member functions in version 3.11.0.
+   Made conditionally `#!cpp noexcept` in version 3.13.0; before, a throwing conversion called `std::terminate`.