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Suppress noreturn warning if last statement in a function is a throw #145166

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@snarang181 snarang181 commented Jun 21, 2025

Fixes #144952

@snarang181 snarang181 force-pushed the clang-no-return-fix branch 7 times, most recently from 8a51a2f to 2c2324a Compare June 21, 2025 14:41
@snarang181 snarang181 force-pushed the clang-no-return-fix branch from 2c2324a to 359dfb1 Compare June 21, 2025 14:50
@snarang181 snarang181 marked this pull request as ready for review June 21, 2025 15:00
@llvmbot llvmbot added clang Clang issues not falling into any other category clang:frontend Language frontend issues, e.g. anything involving "Sema" labels Jun 21, 2025
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llvmbot commented Jun 21, 2025

@llvm/pr-subscribers-clang

Author: Samarth Narang (snarang181)

Changes

Full diff: https://github.com/llvm/llvm-project/pull/145166.diff

2 Files Affected:

  • (modified) clang/lib/Sema/AnalysisBasedWarnings.cpp (+45)
  • (added) clang/test/SemaCXX/wreturn-always-throws.cpp (+22)
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
+}

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Pull Request Overview

This PR adds support to suppress “fall-through” return-type warnings when a function’s last statement always throws, along with a verifying test.

  • Introduce isKnownToAlwaysThrow helper to detect single-statement throw functions
  • Update CheckFallThroughForBody to skip warnings for calls to those functions
  • Add a new test (wreturn-always-throws.cpp) to verify suppression

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File Description
clang/test/SemaCXX/wreturn-always-throws.cpp New test covering a non-void function that ends in a throw
clang/lib/Sema/AnalysisBasedWarnings.cpp Helper and logic to suppress return-type warnings on throwers

Comment on lines 716 to 718
if (const auto *EWC = dyn_cast<ExprWithCleanups>(LastStmt)) {
LastStmt = EWC->getSubExpr();
}
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Copilot AI Jun 21, 2025

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[nitpick] The unwrap of ExprWithCleanups is duplicated in two places. Consider extracting a helper function to reduce duplication and centralize cleanup unwrapping logic.

Suggested change
if (const auto *EWC = dyn_cast<ExprWithCleanups>(LastStmt)) {
LastStmt = EWC->getSubExpr();
}
LastStmt = unwrapExprWithCleanups(LastStmt);

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github-actions bot commented Jun 21, 2025

✅ With the latest revision this PR passed the C/C++ code formatter.

@snarang181 snarang181 force-pushed the clang-no-return-fix branch from 1009d62 to b148279 Compare June 21, 2025 15:09
@snarang181 snarang181 self-assigned this Jun 21, 2025
@snarang181 snarang181 force-pushed the clang-no-return-fix branch from 5e2fc5f to 4340fec Compare June 21, 2025 15:31
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Thank you for the patch! Please add a release note in clang/docs/ReleaseNotes.rst.

I'm not sure if this case should be handled:

if (condition) {
  throw 1;
} else {
  throw 2;
}

Maybe we need to open an issue for it.

@zwuis zwuis requested a review from shafik June 21, 2025 16:18
Add a note in the release notes about the fix for the no-return function warning
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Thank you for the patch! Please add a release note in clang/docs/ReleaseNotes.rst.

@zwuis, added a description of the patch in the release notes and addressed your review comments.

@snarang181 snarang181 requested a review from zwuis June 21, 2025 16:36
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False positive Wreturn-type warning when a function either returns or throws
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