[clang][diagnostics] Don't warn about unreachable code in constexpr if

The point of a constexpr if statement is to determine which branch to
take at compile time, so warning on unreachable code is meaningless in
these situations.

Fixes #57123.

Reviewed By: thakis

Differential Revision: https://reviews.llvm.org/D131818

1 files changed, 6 insertions, 0 deletions

diff --git a/clang/lib/Analysis/ReachableCode.cpp b/clang/lib/Analysis/ReachableCode.cpp
index 652bef3..ce0f4f8 100644
--- a/clang/lib/Analysis/ReachableCode.cpp
+++ b/clang/lib/Analysis/ReachableCode.cpp
@@ -299,6 +299,12 @@ static bool shouldTreatSuccessorsAsReachable(const CFGBlock *B,
     if (isa<BinaryOperator>(Term)) {
       return isConfigurationValue(Term, PP);
     }
+    // Do not treat constexpr if statement successors as unreachable in warnings
+    // since the point of these statements is to determine branches at compile
+    // time.
+    if (const auto *IS = dyn_cast<IfStmt>(Term);
+        IS != nullptr && IS->isConstexpr())
+      return true;
   }
 
   const Stmt *Cond = B->getTerminatorCondition(/* stripParens */ false);