[clang] Suppress noreturn warning if last statement in a function is a throw (PR #145166)

[Samarth Narang via cfe-commits]

https://github.com/snarang181 updated https://github.com/llvm/llvm-project/pull/145166

>From 359dfb1a835617a83ae13865817480e68aa67750 Mon Sep 17 00:00:00 2001
From: Samarth Narang <snarang at umass.edu>
Date: Sat, 21 Jun 2025 08:42:00 -0400
Subject: [PATCH 1/2] Suppress noreturn warning if last statement in a function
 is a throw

---
 clang/lib/Sema/AnalysisBasedWarnings.cpp     | 45 ++++++++++++++++++++
 clang/test/SemaCXX/wreturn-always-throws.cpp | 22 ++++++++++
 2 files changed, 67 insertions(+)
 create mode 100644 clang/test/SemaCXX/wreturn-always-throws.cpp

diff --git a/clang/lib/Sema/AnalysisBasedWarnings.cpp b/clang/lib/Sema/AnalysisBasedWarnings.cpp
index 2a107a36e24b4..85a5d99c710d5 100644
--- a/clang/lib/Sema/AnalysisBasedWarnings.cpp
+++ b/clang/lib/Sema/AnalysisBasedWarnings.cpp
@@ -626,6 +626,31 @@ struct CheckFallThroughDiagnostics {
 
 } // anonymous namespace
 
+static bool isKnownToAlwaysThrow(const FunctionDecl *FD) {
+  if (!FD->hasBody())
+    return false;
+  const Stmt *Body = FD->getBody();
+  const Stmt *OnlyStmt = nullptr;
+
+  if (const auto *Compound = dyn_cast<CompoundStmt>(Body)) {
+    if (Compound->size() != 1)
+      return false; // More than one statement, can't be known to always throw.
+    OnlyStmt = *Compound->body_begin();
+  } else {
+    OnlyStmt = Body;
+  }
+
+  // Unwrap ExprWithCleanups if necessary.
+  if (const auto *EWC = dyn_cast<ExprWithCleanups>(OnlyStmt)) {
+    OnlyStmt = EWC->getSubExpr();
+  }
+  // Check if the only statement is a throw expression.
+  if (isa<CXXThrowExpr>(OnlyStmt)) {
+    return true; // Known to always throw.
+  }
+  return false; // Not known to always throw.
+}
+
 /// CheckFallThroughForBody - Check that we don't fall off the end of a
 /// function that should return a value.  Check that we don't fall off the end
 /// of a noreturn function.  We assume that functions and blocks not marked
@@ -681,6 +706,26 @@ static void CheckFallThroughForBody(Sema &S, const Decl *D, const Stmt *Body,
       if (CD.diag_FallThrough_HasNoReturn)
         S.Diag(RBrace, CD.diag_FallThrough_HasNoReturn) << CD.FunKind;
     } else if (!ReturnsVoid && CD.diag_FallThrough_ReturnsNonVoid) {
+      // If the final statement is a call to an always-throwing function,
+      // don't warn about the fall-through.
+      if (const auto *FD = dyn_cast<FunctionDecl>(D)) {
+        if (const auto *CS = dyn_cast<CompoundStmt>(Body)) {
+          if (!CS->body_empty()) {
+            const Stmt *LastStmt = CS->body_back();
+            // Unwrap ExprWithCleanups if necessary.
+            if (const auto *EWC = dyn_cast<ExprWithCleanups>(LastStmt)) {
+              LastStmt = EWC->getSubExpr();
+            }
+            if (const auto *CE = dyn_cast<CallExpr>(LastStmt)) {
+              if (const FunctionDecl *Callee = CE->getDirectCallee()) {
+                if (isKnownToAlwaysThrow(Callee)) {
+                  return; // Don't warn about fall-through.
+                }
+              }
+            }
+          }
+        }
+      }
       bool NotInAllControlPaths = FallThroughType == MaybeFallThrough;
       S.Diag(RBrace, CD.diag_FallThrough_ReturnsNonVoid)
           << CD.FunKind << NotInAllControlPaths;
diff --git a/clang/test/SemaCXX/wreturn-always-throws.cpp b/clang/test/SemaCXX/wreturn-always-throws.cpp
new file mode 100644
index 0000000000000..0bbce0f5c1871
--- /dev/null
+++ b/clang/test/SemaCXX/wreturn-always-throws.cpp
@@ -0,0 +1,22 @@
+// RUN: %clang_cc1 -fsyntax-only -fcxx-exceptions -fexceptions -Wreturn-type -verify %s
+// expected-no-diagnostics
+
+namespace std {
+  class string {
+  public:
+    string(const char*); // constructor for runtime_error
+  };
+  class runtime_error {
+  public:
+    runtime_error(const string &); 
+  };
+}
+
+void throwError(const std::string& msg) {
+  throw std::runtime_error(msg);
+}
+
+int ensureZero(const int i) {
+  if (i == 0) return 0;
+  throwError("ERROR"); // no-warning
+}

>From 1009d62354fae31724da3f4d8c44367c7d66281b Mon Sep 17 00:00:00 2001
From: Samarth Narang <70980689+snarang181 at users.noreply.github.com>
Date: Sat, 21 Jun 2025 11:05:17 -0400
Subject: [PATCH 2/2] Update clang/lib/Sema/AnalysisBasedWarnings.cpp

Co-authored-by: Copilot <175728472+Copilot at users.noreply.github.com>
---
 clang/lib/Sema/AnalysisBasedWarnings.cpp | 47 ++++++++++++------------
 1 file changed, 23 insertions(+), 24 deletions(-)

diff --git a/clang/lib/Sema/AnalysisBasedWarnings.cpp b/clang/lib/Sema/AnalysisBasedWarnings.cpp
index 85a5d99c710d5..6d54282c9f0e5 100644
--- a/clang/lib/Sema/AnalysisBasedWarnings.cpp
+++ b/clang/lib/Sema/AnalysisBasedWarnings.cpp
@@ -624,33 +624,32 @@ struct CheckFallThroughDiagnostics {
   }
 };
 
-} // anonymous namespace
-
-static bool isKnownToAlwaysThrow(const FunctionDecl *FD) {
-  if (!FD->hasBody())
-    return false;
-  const Stmt *Body = FD->getBody();
-  const Stmt *OnlyStmt = nullptr;
-
-  if (const auto *Compound = dyn_cast<CompoundStmt>(Body)) {
-    if (Compound->size() != 1)
-      return false; // More than one statement, can't be known to always throw.
-    OnlyStmt = *Compound->body_begin();
-  } else {
-    OnlyStmt = Body;
-  }
+  bool isKnownToAlwaysThrow(const FunctionDecl *FD) {
+    if (!FD->hasBody())
+      return false;
+    const Stmt *Body = FD->getBody();
+    const Stmt *OnlyStmt = nullptr;
+
+    if (const auto *Compound = dyn_cast<CompoundStmt>(Body)) {
+      if (Compound->size() != 1)
+        return false; // More than one statement, can't be known to always throw.
+      OnlyStmt = *Compound->body_begin();
+    } else {
+      OnlyStmt = Body;
+    }
 
-  // Unwrap ExprWithCleanups if necessary.
-  if (const auto *EWC = dyn_cast<ExprWithCleanups>(OnlyStmt)) {
-    OnlyStmt = EWC->getSubExpr();
-  }
-  // Check if the only statement is a throw expression.
-  if (isa<CXXThrowExpr>(OnlyStmt)) {
-    return true; // Known to always throw.
+    // Unwrap ExprWithCleanups if necessary.
+    if (const auto *EWC = dyn_cast<ExprWithCleanups>(OnlyStmt)) {
+      OnlyStmt = EWC->getSubExpr();
+    }
+    // Check if the only statement is a throw expression.
+    if (isa<CXXThrowExpr>(OnlyStmt)) {
+      return true; // Known to always throw.
+    }
+    return false; // Not known to always throw.
   }
-  return false; // Not known to always throw.
-}
 
+} // anonymous namespace
 /// CheckFallThroughForBody - Check that we don't fall off the end of a
 /// function that should return a value.  Check that we don't fall off the end
 /// of a noreturn function.  We assume that functions and blocks not marked