[clang-tools-extra] [clang-tidy] Add modernize-use-to-underlying check (PR #210459)

[Yanzuo Liu via cfe-commits]

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@@ -0,0 +1,55 @@
+// RUN: %check_clang_tidy -check-suffixes=IGNORE -std=c++23-or-later %s \
+// RUN:   modernize-use-to-underlying %t -- \
+// RUN:   -config="{CheckOptions: {modernize-use-to-underlying.ImpreciseCasts: Ignore}}"
+// RUN: %check_clang_tidy -check-suffixes=WARN -std=c++23-or-later %s \
+// RUN:   modernize-use-to-underlying %t -- \
+// RUN:   -config="{CheckOptions: {modernize-use-to-underlying.ImpreciseCasts: Warn}}"
+// RUN: %check_clang_tidy -check-suffixes=PRESERVE -std=c++23-or-later %s \
+// RUN:   modernize-use-to-underlying %t -- \
+// RUN:   -config="{CheckOptions: {modernize-use-to-underlying.ImpreciseCasts: PreserveType}}"
+// RUN: %check_clang_tidy -check-suffixes=UNDERLYING -std=c++23-or-later %s \
+// RUN:   modernize-use-to-underlying %t -- \
+// RUN:   -config="{CheckOptions: {modernize-use-to-underlying.ImpreciseCasts: UseUnderlyingType}}"
+
+// CHECK-FIXES-WARN: #include <utility>
+// CHECK-FIXES-PRESERVE: #include <utility>
+// CHECK-FIXES-UNDERLYING: #include <utility>
+
+namespace std {
+template <typename T>
+constexpr __underlying_type(T) to_underlying(T value) noexcept {
+  return static_cast<__underlying_type(T)>(value);
+}
+} // namespace std
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zwuis wrote:

Move this to `clang-tools-extra/test/clang-tidy/checkers/Inputs/Headers/std/utility`.

https://github.com/llvm/llvm-project/pull/210459