Index: clang-tools-extra/clang-tidy/cppcoreguidelines/NarrowingConversionsCheck.cpp
===================================================================
--- clang-tools-extra/clang-tidy/cppcoreguidelines/NarrowingConversionsCheck.cpp
+++ clang-tools-extra/clang-tidy/cppcoreguidelines/NarrowingConversionsCheck.cpp
@@ -83,6 +83,28 @@
             binaryOperator(hasOperands(IsConversionFromIgnoredType,
                                        hasType(isInteger()))));
 
+  // Reading a small unsigned bitfield member will be wrapped by an implicit
+  // cast to 'int' triggering this checker. But this is known to be safe by the
+  // compiler since it chose 'int' instead of 'unsigned int' as the type of the
+  // whole expression, so let's ignore this case.
+  //
+  // struct SmallBitfield { unsigned int id : 4; };
+  // x.id << 1;
+  //
+  // BinaryOperator 'int' '<<'
+  // |-ImplicitCastExpr 'int' <IntegralCast>
+  // | `-ImplicitCastExpr 'unsigned int' <LValueToRValue>
+  // |   `-MemberExpr 'unsigned int' lvalue bitfield .id
+  // |     `-DeclRefExpr 'SmallBitfield' lvalue ParmVar 'x' 'SmallBitfield'
+  // `-IntegerLiteral 'int' 1
+  const auto ShiftingWidenedBitfieldValue = castExpr(
+      hasCastKind(CK_IntegralCast), hasType(asString("int")),
+      has(castExpr(
+          castExpr(hasCastKind(CK_LValueToRValue),
+                   has(memberExpr(hasDeclaration(fieldDecl(isBitField()))))))),
+      hasParent(
+          binaryOperator(anyOf(hasOperatorName("<<"), hasOperatorName(">>")))));
+
   // Casts:
   //   i = 0.5;
   //   void f(int); f(0.5);
@@ -100,7 +122,8 @@
                             IgnoreConversionFromTypes.empty()
                                 ? castExpr()
                                 : castExpr(unless(hasSourceExpression(
-                                      IsIgnoredTypeTwoLevelsDeep))))
+                                      IsIgnoredTypeTwoLevelsDeep))),
+                            unless(ShiftingWidenedBitfieldValue))
                             .bind("cast")),
       this);
 
Index: clang-tools-extra/test/clang-tidy/checkers/cppcoreguidelines-narrowing-conversions-bitfields.cpp
===================================================================
--- /dev/null
+++ clang-tools-extra/test/clang-tidy/checkers/cppcoreguidelines-narrowing-conversions-bitfields.cpp
@@ -0,0 +1,107 @@
+// RUN: %check_clang_tidy %s cppcoreguidelines-narrowing-conversions %t \
+// RUN:   -std=c++17 -- -target x86_64-unknown-linux
+
+#define CHAR_BITS 8
+static_assert(sizeof(unsigned int) == 32 / CHAR_BITS);
+
+template <typename T, typename U>
+struct is_same {
+  static constexpr bool value = false;
+};
+template <typename T>
+struct is_same<T, T> {
+  static constexpr bool value = true;
+};
+
+template <typename T, typename U>
+static constexpr bool is_same_v = is_same<T, U>::value;
+
+struct NoBitfield {
+  unsigned int id;
+};
+struct SmallBitfield {
+  unsigned int id : 4;
+};
+struct BigBitfield {
+  unsigned int id : 32;
+};
+
+void test_binary_and(SmallBitfield x) {
+  // CHECK-MESSAGES: [[@LINE+1]]:36: warning: narrowing conversion from 'unsigned int' to signed type 'int' is implementation-defined [cppcoreguidelines-narrowing-conversions]
+  static_assert(is_same_v<decltype(x.id & 1), int>);
+  static_assert(is_same_v<decltype(x.id & 1u), unsigned>); // no-warning
+
+  // CHECK-MESSAGES: [[@LINE+1]]:3: warning: narrowing conversion from 'unsigned int' to signed type 'int' is implementation-defined [cppcoreguidelines-narrowing-conversions]
+  x.id & 1;
+  x.id & 1u; // no-warning
+
+  // CHECK-MESSAGES: [[@LINE+1]]:7: warning: narrowing conversion from 'unsigned int' to signed type 'int' is implementation-defined [cppcoreguidelines-narrowing-conversions]
+  1 & x.id;
+  1u & x.id; // no-warning
+}
+
+void test_binary_or(SmallBitfield x) {
+  // CHECK-MESSAGES: [[@LINE+1]]:36: warning: narrowing conversion from 'unsigned int' to signed type 'int' is implementation-defined [cppcoreguidelines-narrowing-conversions]
+  static_assert(is_same_v<decltype(x.id | 1), int>);
+  static_assert(is_same_v<decltype(x.id | 1u), unsigned>); // no-warning
+
+  // CHECK-MESSAGES: [[@LINE+1]]:3: warning: narrowing conversion from 'unsigned int' to signed type 'int' is implementation-defined [cppcoreguidelines-narrowing-conversions]
+  x.id | 1;
+  x.id | 1u; // no-warning
+
+  // CHECK-MESSAGES: [[@LINE+1]]:7: warning: narrowing conversion from 'unsigned int' to signed type 'int' is implementation-defined [cppcoreguidelines-narrowing-conversions]
+  1 | x.id;
+  1u | x.id; // no-warning
+}
+
+void test(NoBitfield x) {
+  // no-warning in this function
+  static_assert(is_same_v<decltype(x.id << 1), unsigned>);
+  static_assert(is_same_v<decltype(x.id << 1u), unsigned>);
+
+  x.id << 1;
+  x.id << 1u;
+  x.id >> 1;
+  x.id >> 1u;
+
+  1 << x.id;
+  1u << x.id;
+  1 >> x.id;
+  1u >> x.id;
+}
+
+void test(SmallBitfield x) {
+  // no-warning in this function: bitfield reads should be ignored within shift operations
+
+  // 'int' is large enough to hold the value of 'x.id', since that could at
+  // most populate 4 bits and an 'int' has 32.
+  static_assert(is_same_v<decltype(x.id << 1), int>);
+  static_assert(is_same_v<decltype(x.id << 1u), int>);
+
+  x.id << 1;
+  x.id << 1u;
+  x.id >> 1;
+  x.id >> 1u;
+
+  1 << x.id;
+  1u << x.id;
+  1 >> x.id;
+  1u >> x.id;
+}
+
+void test(BigBitfield x) {
+  // no-warning in this function: there is no int -> unsigned conversion here
+
+  static_assert(is_same_v<decltype(x.id << 1), unsigned>);
+  static_assert(is_same_v<decltype(x.id << 1u), unsigned>);
+
+  x.id << 1;
+  x.id << 1u;
+  x.id >> 1;
+  x.id >> 1u;
+
+  1 << x.id;
+  1u << x.id;
+  1 >> x.id;
+  1u >> x.id;
+}