[Clang] add -Wshift-bool warning to handle shifting of bool

clang/docs/ReleaseNotes.rst

@@ -147,6 +147,7 @@ Improvements to Clang's diagnostics
 - The ``-Wunsafe-buffer-usage`` warning has been updated to warn
   about unsafe libc function calls.  Those new warnings are emitted
   under the subgroup ``-Wunsafe-buffer-usage-in-libc-call``.
+- The ``-Wshift-bool`` warning has been added to warn about shifting a boolean. (#GH28334)
 
 Improvements to Clang's time-trace
 ----------------------------------

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -7106,6 +7106,9 @@ def warn_shift_result_sets_sign_bit : Warning<
   "signed shift result (%0) sets the sign bit of the shift expression's "
   "type (%1) and becomes negative">,
   InGroup<DiagGroup<"shift-sign-overflow">>, DefaultIgnore;
+def warn_shift_bool : Warning<
+  "right shifting a `bool` implicitly converts it to 'int'">,
+  InGroup<DiagGroup<"shift-bool">>, DefaultIgnore;
 
 def warn_precedence_bitwise_rel : Warning<
   "%0 has lower precedence than %1; %1 will be evaluated first">,

clang/lib/Sema/SemaExpr.cpp

@@ -11250,6 +11250,12 @@ static void DiagnoseBadShiftValues(Sema& S, ExprResult &LHS, ExprResult &RHS,
   if (S.getLangOpts().OpenCL)
     return;
 
+  if (Opc == BO_Shr &&
+      LHS.get()->IgnoreParenImpCasts()->getType()->isBooleanType()) {
+    S.Diag(Loc, diag::warn_shift_bool) << LHS.get()->getSourceRange();
+    return;
+  }
+
   // Check right/shifter operand
   Expr::EvalResult RHSResult;
   if (RHS.get()->isValueDependent() ||

clang/test/Sema/shift-bool.cpp

@@ -0,0 +1,12 @@
+// RUN: %clang_cc1 -fsyntax-only -Wshift-bool -verify %s
+
+void t() {
+  int x = 10;
+  bool y = true;
+
+  bool a = y >> x;     // expected-warning {{right shifting a `bool` implicitly converts it to 'int'}}
+  bool b = false >> x; // expected-warning {{right shifting a `bool` implicitly converts it to 'int'}}
+  bool c = y >> 5;     // expected-warning {{right shifting a `bool` implicitly converts it to 'int'}}
+  bool d = y >> -1;    // expected-warning {{right shifting a `bool` implicitly converts it to 'int'}}
+  bool e = y >> 0;     // expected-warning {{right shifting a `bool` implicitly converts it to 'int'}}
+}