fix #18007: std/json now serializes nan,inf,-inf as strings instead of invalid json (#18026)

* fix #18007: std/json now serializes nan,inf,-inf as raw strings instead of invalid json

* fix roundtrip

* fix tests

* fix changelog

* simplify

* add runnableExamples

* fix typo [skip ci]

tests/stdlib/tjson.nim

@@ -8,14 +8,27 @@ Note: Macro tests are in tests/stdlib/tjsonmacro.nim
 ]#
 
 import std/[json,parsejson,strutils]
+from std/math import isNaN
 when not defined(js):
   import std/streams
 
 proc testRoundtrip[T](t: T, expected: string) =
+  # checks that `T => json => T2 => json2` is such that json2 = json
   let j = %t
   doAssert $j == expected, $j
   doAssert %(j.to(T)) == j
 
+proc testRoundtripVal[T](t: T, expected: string) =
+  # similar to testRoundtrip, but also checks that the `T => json => T2` is such that `T2 == T`
+  # note that this isn't always possible, e.g. for pointer-like types or nans
+  let j = %t
+  doAssert $j == expected, $j
+  let j2 = ($j).parseJson
+  doAssert $j2 == expected, $(j2, t)
+  let t2 = j2.to(T)
+  doAssert t2 == t
+  doAssert $(%* t2) == expected # sanity check, because -0.0 = 0.0 but their json representation differs
+
 let testJson = parseJson"""{ "a": [1, 2, 3, 4], "b": "asd", "c": "\ud83c\udf83", "d": "\u00E6"}"""
 # nil passthrough
 doAssert(testJson{"doesnt_exist"}{"anything"}.isNil)
@@ -301,6 +314,16 @@ block: # bug #17383
     testRoundtrip(int64.high): "9223372036854775807"
     testRoundtrip(uint64.high): "18446744073709551615"
 
+block: # bug #18007
+  testRoundtrip([NaN, Inf, -Inf, 0.0, -0.0, 1.0]): """["nan","inf","-inf",0.0,-0.0,1.0]"""
+  # pending https://github.com/nim-lang/Nim/issues/18025 use:
+  # testRoundtrip([float32(NaN), Inf, -Inf, 0.0, -0.0, 1.0])
+  let inf = float32(Inf)
+  testRoundtrip([float32(NaN), inf, -inf, 0.0, -0.0, 1.0]): """["nan","inf","-inf",0.0,-0.0,1.0]"""
+  when not defined(js): # because of Infinity vs inf
+    testRoundtripVal([inf, -inf, 0.0, -0.0, 1.0]): """["inf","-inf",0.0,-0.0,1.0]"""
+  let a = parseJson($(%NaN)).to(float)
+  doAssert a.isNaN
 
 block:
   let a = "18446744073709551615"

tests/stdlib/tjsonutils.nim

@@ -4,15 +4,28 @@ discard """
 
 import std/jsonutils
 import std/json
+from std/math import isNaN
 
 proc testRoundtrip[T](t: T, expected: string) =
+  # checks that `T => json => T2 => json2` is such that json2 = json
   let j = t.toJson
-  doAssert $j == expected, $j
+  doAssert $j == expected, "\n" & $j & "\n" & expected
   doAssert j.jsonTo(T).toJson == j
   var t2: T
   t2.fromJson(j)
   doAssert t2.toJson == j
 
+proc testRoundtripVal[T](t: T, expected: string) =
+  # similar to testRoundtrip, but also checks that the `T => json => T2` is such that `T2 == T`
+  # note that this isn't always possible, e.g. for pointer-like types.
+  let j = t.toJson
+  let j2 = $j
+  doAssert j2 == expected, j2
+  let j3 = j2.parseJson
+  let t2 = j3.jsonTo(T)
+  doAssert t2 == t
+  doAssert $t2.toJson == j2 # still needed, because -0.0 = 0.0 but their json representation differs
+
 import tables, sets, algorithm, sequtils, options, strtabs
 from strutils import contains
 
@@ -91,6 +104,12 @@ template fn() =
       else:
         testRoundtrip(a): "[9223372036854775807,18446744073709551615]"
 
+  block: # bug #18007
+    testRoundtrip((NaN, Inf, -Inf, 0.0, -0.0, 1.0)): """["nan","inf","-inf",0.0,-0.0,1.0]"""
+    testRoundtrip((float32(NaN), Inf, -Inf, 0.0, -0.0, 1.0)): """["nan","inf","-inf",0.0,-0.0,1.0]"""
+    testRoundtripVal((Inf, -Inf, 0.0, -0.0, 1.0)): """["inf","-inf",0.0,-0.0,1.0]"""
+    doAssert ($NaN.toJson).parseJson.jsonTo(float).isNaN
+
   block: # case object
     type Foo = object
       x0: float