Report one-character non-numeric array indices like longer ones (#5800)

* Report one-character non-numeric array indices like longer ones

A JSON pointer reference token that is not a number but has only one
character (e.g. "/a/x") was reported as out_of_range.404 ("unresolved
reference token"), because the "is not a number" check only ran for
tokens longer than one character; "/a/xy" got parse_error.109. Both now
throw parse_error.109. "-" and the empty token are still reported as
out_of_range.404. As a consequence, value(json_pointer, default) on an
array now throws for "/x" as it already did for "/xy".

Also document why ordered_map::erase's destroy/placement-new loop on
pair<const Key, T> is kept despite [basic.life]/8 before C++20.

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

* Document the one-character array index change

Add 3.13.0 version-history entries to at, operator[], value, patch,
patch_inplace, and unflatten, and describe in exceptions.md which array
indices throw parse_error.109 and which out_of_range.404.

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

---------

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

tests/src/unit-json_pointer.cpp

@@ -421,6 +421,27 @@ TEST_CASE("JSON pointers")
             CHECK_THROWS_WITH_AS(json({{"/list/0", 1}, {"/list/1", 2}, {"/list/three", 3}}).unflatten(),
             "[json.exception.parse_error.109] parse error: array index 'three' is not a number", json::parse_error&);
 
+            // a single-character token that is not a digit is reported like a
+            // longer one (parse_error.109), not as unresolved (out_of_range.404)
+            CHECK_THROWS_WITH_AS(j["/x"_json_pointer] = 1,
+                                 "[json.exception.parse_error.109] parse error: array index 'x' is not a number", json::parse_error&);
+            CHECK_THROWS_WITH_AS(j_const["/x"_json_pointer] == 1,
+                                 "[json.exception.parse_error.109] parse error: array index 'x' is not a number", json::parse_error&);
+            CHECK_THROWS_WITH_AS(j.at("/x"_json_pointer) = 1,
+                                 "[json.exception.parse_error.109] parse error: array index 'x' is not a number", json::parse_error&);
+            CHECK_THROWS_WITH_AS(j_const.at("/x"_json_pointer) == 1,
+                                 "[json.exception.parse_error.109] parse error: array index 'x' is not a number", json::parse_error&);
+            CHECK_THROWS_WITH_AS(j.at("/+"_json_pointer),
+                                 "[json.exception.parse_error.109] parse error: array index '+' is not a number", json::parse_error&);
+            CHECK(!j.contains("/x"_json_pointer));
+            CHECK(!j_const.contains("/x"_json_pointer));
+            CHECK_THROWS_WITH_AS(json({{"/list/0", 1}, {"/list/x", 2}}).unflatten(),
+            "[json.exception.parse_error.109] parse error: array index 'x' is not a number", json::parse_error&);
+
+            // "-" is a valid reference token, so it is still reported as unresolved
+            CHECK_THROWS_WITH_AS(json({{"/list/0", 1}, {"/list/-", 2}}).unflatten(),
+            "[json.exception.out_of_range.404] unresolved reference token '-'", json::out_of_range&);
+
             // assign to "-"
             j["/-"_json_pointer] = 99;
             CHECK(j == json({1, 13, 3, 33, nullptr, 55, 99}));