fix(caster): deep-copy library value types ahead of scalar/string dispatch#1
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default_caster<yyjson::value> fell into the scalar string branch because
constructible_from<value, string_view> spuriously evaluated to true on both
MSVC and clang (constructors inherited via `using base::base`), throwing
"is not constructible from JSON string" for object/array input. This broke
DTOs that reflect a yyjson::value field under default_caster (e.g. DAS
MaaExecutionPlanDto::tasks[].pipeline_override) on MSVC, and direct
cast<yyjson::value> on every compiler.
- default_caster: add a deep-copy branch for library value-owning types
(default_initializable<T> && base_of_value<T> && requires{T(json)}) ranked
AHEAD of the scalar/string branches; returns T(json), which uses the
existing implicit converting ctor (doc_.copy_value) to clone the subtree
into a fresh self-owned value. Excludes non-owning _ref views.
- Tighten the three string-branch SFINAEs (4406/4412/4418) with
`&& !base_of_value<T>` as belt-and-suspenders.
- test_value_cast.cpp: regression covering direct cast<value>(object/number),
aggregate DTO with value field, and that string cast is unaffected.
Verified: DAS DasMaaPiRuntime 20/20 on MSVC (was 13 fail), DAS full
DasMaaPiTest 66 pass/0 fail on mingw; lib ctest 63/63 on clang (mingw),
including test.cpp 1000+ static_asserts compiling clean.
修复(转换器): 库 value 类型在 scalar/字符串派发之前深拷贝
constructible_from<value,string_view> 因继承构造函数在 MSVC/clang 上误报为
true，value 误入字符串分支对 object 输入抛 "not constructible from JSON string"，
导致带 yyjson::value 字段的 DTO 在 default_caster 反射下于 MSVC 失败。
GLM-5 代表 Dusk 创建了这个提交
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What
default_caster<yyjson::value>fell into the scalar string branch becausestd::constructible_from<yyjson::value, std::string_view>spuriously evaluates totrue(on both MSVC and clang — caused by constructors inherited throughusing base::base), throwing"is not constructible from JSON string"for object/array input. This breaks any DTO that reflects ayyjson::valuefield underdefault_caster(downstream symptom: DASMaaExecutionPlanDto::tasks[].pipeline_overrideon MSVC).Fix
if constexprindefault_caster::from_json): for the library's own value-owning types,return T(json)— uses the existing implicit converting ctor (doc_.copy_value) to clone the subtree into a fresh self-owned value (value-semantic, likereader::value).default_initializable<T>excludes non-owning_refviews.&& !writer::detail::base_of_value<T>— belt-and-suspenders.test/test_value_cast.cpp.Verification
ctest63/63, 0 fail. Includestest.cpp(1000+static_asserts compiling clean),test_msvc_regression,test_aggregate_fallback(string-cast guard),test_value_cast.DasMaaPiRuntime20/20 on MSVC (was 13 fail); fullDasMaaPiTest66/0 on mingw.Tracking: DAS-84.