[PATCH] D114105: [clang-tidy] Ignore narrowing conversions in case of bitfields

[Balázs Benics via Phabricator via cfe-commits]

steakhal created this revision.
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Consider this example:

  struct SmallBitfield { unsigned int id : 4; } x;
  x.id << 1;

The corresponding AST looks like this:

  BinaryOperator 'int' '<<'
  |-ImplicitCastExpr 'int' <IntegralCast>
  | `-ImplicitCastExpr 'unsigned int' <LValueToRValue>
  |   `-MemberExpr 'unsigned int' lvalue bitfield .id
  |     `-DeclRefExpr 'SmallBitfield' lvalue ParmVar 'x' 'SmallBitfield'
  `-IntegerLiteral 'int' 1

There are two implicit casts:

1. LValueToRValue, loading from the bitfield
2. IntegralCast, casting the 'unsigned int' to 'int' to process the bitshift operation

This patch aims to detect this case and ignore it.

The patch might be too restrictive in terms of matching for only this
exact pattern, but I did not find any other similar occurrences of this
kind.


https://reviews.llvm.org/D114105

Files:
  clang-tools-extra/clang-tidy/cppcoreguidelines/NarrowingConversionsCheck.cpp
  clang-tools-extra/test/clang-tidy/checkers/cppcoreguidelines-narrowing-conversions-bitfields.cpp

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