[clang-tools-extra] [clang-tidy] Add `readability-redundant-nested-if` check (PR #181558)

[Daniil Dudkin via cfe-commits]

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+// RUN: %check_clang_tidy -check-suffixes=BASE -std=c++11 %s readability-redundant-nested-if %t -- -- -fno-delayed-template-parsing
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unterumarmung wrote:

I've tried but didn't succeed. I wrote justification in the comment. Maybe I didn't quite get how to apply this.

https://github.com/llvm/llvm-project/pull/181558