[clang] [Clang] fix -Wimplicit-fallthrough false positive after try CFG changes (PR #183194)

[Oleksandr Tarasiuk via cfe-commits]

https://github.com/a-tarasyuk created https://github.com/llvm/llvm-project/pull/183194

Fixes #182822

--- 

This patch fixes a _false positive_ regression of `-Wimplicit-fallthrough`, caused by a CFG `try` change in 41c566e, which led to `CXXTryStmt` predecessor edges being considered in `switch` _fallthrough_ analysis and incorrectly treated as normal _fallthrough_.

>From a9055f54e6408f4534ec4612c3a2f3b1acb66577 Mon Sep 17 00:00:00 2001
From: Oleksandr Tarasiuk <oleksandr.tarasiuk at outlook.com>
Date: Wed, 25 Feb 2026 00:34:47 +0200
Subject: [PATCH] [Clang] fix -Wimplicit-fallthrough false positive after try
 CFG changes

---
 clang/lib/Sema/AnalysisBasedWarnings.cpp      |  4 ++--
 .../SemaCXX/switch-implicit-fallthrough.cpp   | 19 +++++++++++++++++--
 2 files changed, 19 insertions(+), 4 deletions(-)

diff --git a/clang/lib/Sema/AnalysisBasedWarnings.cpp b/clang/lib/Sema/AnalysisBasedWarnings.cpp
index 5c49e601840cf..1e604b6b6e26f 100644
--- a/clang/lib/Sema/AnalysisBasedWarnings.cpp
+++ b/clang/lib/Sema/AnalysisBasedWarnings.cpp
@@ -1252,8 +1252,8 @@ class FallthroughMapper : public DynamicRecursiveASTVisitor {
         continue;
 
       const Stmt *Term = P->getTerminatorStmt();
-      if (isa_and_nonnull<SwitchStmt>(Term))
-        continue; // Switch statement, good.
+      if (isa_and_nonnull<SwitchStmt, CXXTryStmt>(Term))
+        continue; // switch/try terminators are not switch fallthrough edges
 
       const SwitchCase *SW = dyn_cast_or_null<SwitchCase>(P->getLabel());
       if (SW && SW->getSubStmt() == B.getLabel() && P->begin() == P->end())
diff --git a/clang/test/SemaCXX/switch-implicit-fallthrough.cpp b/clang/test/SemaCXX/switch-implicit-fallthrough.cpp
index 750663717b66a..8b26d7766ca79 100644
--- a/clang/test/SemaCXX/switch-implicit-fallthrough.cpp
+++ b/clang/test/SemaCXX/switch-implicit-fallthrough.cpp
@@ -1,5 +1,4 @@
-// RUN: %clang_cc1 -fsyntax-only -verify -std=c++11 -Wimplicit-fallthrough -Wunreachable-code-fallthrough %s
-
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++11 -Wimplicit-fallthrough -Wunreachable-code-fallthrough -fcxx-exceptions %s
 
 int fallthrough(int n) {
   switch (n / 10) {
@@ -292,6 +291,22 @@ void fallthrough_in_lambda() {
   };
 }
 
+namespace GH182822 {
+  void test(int n) {
+    switch (n) {
+    case 0:
+      try {
+        if (n) {
+          try {
+          } catch (int) {
+          }
+        }
+      } catch (...) {
+      }
+    }
+  }
+}
+
 namespace PR18983 {
   void fatal() __attribute__((noreturn));
   int num();