Copy a pair-shaped array value under JSON_BRACE_INIT_COPY_SEMANTICS

With JSON_BRACE_INIT_COPY_SEMANTICS enabled, single-element brace
initialization from a JSON value decided whether to copy the value or
build an object by inspecting the value's runtime shape: a two-element
array whose first element is a string, such as ["key", 42], was turned
into an object instead of being copied. This made the behavior depend
on the element's content, and it did not distinguish an existing value
of this shape from a nested braced pair written in the source, such as
the inner {"key", "value"} of {{"key", "value"}}.

json_ref now records whether it was constructed from a braced list
(true only for the std::initializer_list<json_ref> constructor used
for nested braced lists) or from a value. The initializer-list
constructor uses this to copy or move a single non-braced-list element
before deciding whether the list describes an object, so a JSON value
is always copied regardless of its shape, while a braced pair written
in the source still creates an object.

Fixes #5662.

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

tests/src/unit-brace-init-copy-semantics.cpp

@@ -72,6 +72,28 @@ TEST_CASE("JSON_BRACE_INIT_COPY_SEMANTICS")
         CHECK(j7 == json::array({1, 2}));
     }
 
+    SECTION("single-element brace initialization copies a pair-shaped array value (#5662)")
+    {
+        // a JSON value that happens to be a 2-element array whose first
+        // element is a string must still be copied, not turned into an
+        // object; only a braced list written in the source, such as the
+        // inner {"key", "value"} of {{"key", "value"}}, describes an object
+        json const pair_shaped = json::array({"key", 42});
+
+        json const j1{pair_shaped};
+        CHECK(j1.is_array());
+        CHECK(j1 == pair_shaped);
+
+        json const j2 = {pair_shaped};
+        CHECK(j2.is_array());
+        CHECK(j2 == pair_shaped);
+
+        // the same holds for an rvalue of the same shape
+        json const j3{json::array({"key", 42})};
+        CHECK(j3.is_array());
+        CHECK(j3 == pair_shaped);
+    }
+
     SECTION("what the macro does not change")
     {
         // lists with more than one element are unaffected