from_json: number-first dispatch for arithmetic types, loop for std::array (#5891)

from_json(ArithmeticType&) now reads numbers through get_number_value() and
handles a boolean on a cold path, which puts the three number types first in
the per-element type dispatch.

get<std::array<T, N>>() always took the identity_tag overload, a pack
expansion of N calls j.at(i).get<T>() that clang unrolls into straight-line
code (about 28,000 instructions for N = 1000), so its speed depended on
inlining decisions in that body. For trivial element types, which cannot tell
default construction plus assignment from construction in place, it now fills
a std::array in a loop. That loop, shared with get_to(), checks the size once
and reads the inner array directly; a short array still converts the elements
it has and then throws the out_of_range error at() would throw.

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

tests/src/unit-constructor1.cpp

@@ -361,6 +361,28 @@ TEST_CASE("constructors")
             CHECK_THROWS_WITH_AS((j.get<std::pair<int, int>>()), "[json.exception.out_of_range.401] array index 1 is out of range", json::out_of_range&);
             CHECK_THROWS_WITH_AS((j.get<std::tuple<int, int>>()), "[json.exception.out_of_range.401] array index 1 is out of range", json::out_of_range&);
             CHECK_THROWS_WITH_AS((j.get<std::array<int, 3>>()), "[json.exception.out_of_range.401] array index 1 is out of range", json::out_of_range&);
+            CHECK_THROWS_WITH_AS((j.get<std::array<std::string, 3>>()), "[json.exception.type_error.302] type must be string, but is number", json::type_error&);
+
+            // the elements are converted in order, so a type error comes before a missing element
+            json const j2{1, "two"};
+            CHECK_THROWS_WITH_AS((j2.get<std::array<int, 3>>()), "[json.exception.type_error.302] type must be number, but is string", json::type_error&);
+            std::array<int, 3> a{{0, 0, 0}};
+            CHECK_THROWS_WITH_AS(j2.get_to(a), "[json.exception.type_error.302] type must be number, but is string", json::type_error&);
+            CHECK_THROWS_WITH_AS(j.get_to(a), "[json.exception.out_of_range.401] array index 1 is out of range", json::out_of_range&);
+            CHECK(a[0] == 1);
+
+            json const j3{"one", "two"};
+            CHECK_THROWS_WITH_AS((j3.get<std::array<std::string, 3>>()), "[json.exception.out_of_range.401] array index 2 is out of range", json::out_of_range&);
+        }
+
+        SECTION("std::array from a longer array or from booleans")
+        {
+            json const j{1, 2.5, true, 4u};
+            CHECK(j.get<std::array<int, 3>>() == (std::array<int, 3> {{1, 2, 1}}));
+            std::array<int, 4> a{{0, 0, 0, 0}};
+            j.get_to(a);
+            CHECK(a == (std::array<int, 4> {{1, 2, 1, 4}}));
+            CHECK(j.get<std::array<int, 0>>().empty());
         }
 
         SECTION("std::forward_list<json>")