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

Fixes https://github.com/llvm/llvm-project/issues/144952

clang/docs/ReleaseNotes.rst

@@ -649,6 +649,13 @@ Improvements to Clang's diagnostics
   #GH69470, #GH59391, #GH58172, #GH46215, #GH45915, #GH45891, #GH44490,
   #GH36703, #GH32903, #GH23312, #GH69874.
 
+- Clang now avoids issuing `-Wreturn-type` warnings in some cases where
+  the final statement of a non-void function is a `throw` expression, or
+  a call to a function that is trivially known to always throw (i.e., its
+  body consists solely of a `throw` statement). This avoids certain
+  false positives in exception-heavy code, though only simple patterns
+  are currently recognized.
+
   
 Improvements to Clang's time-trace
 ----------------------------------

clang/include/clang/Basic/Attr.td

@@ -965,6 +965,13 @@ def AnalyzerNoReturn : InheritableAttr {
   let Documentation = [Undocumented];
 }
 
+def InferredNoReturn : InheritableAttr {
+  let Spellings = []; 
+  let SemaHandler = 0;
+  let Subjects = SubjectList<[Function], ErrorDiag>;
+  let Documentation = [InternalOnly];
+}
+
 def Annotate : InheritableParamOrStmtAttr {
   let Spellings = [Clang<"annotate">];
   let Args = [StringArgument<"Annotation">, VariadicExprArgument<"Args">];

clang/include/clang/Sema/Sema.h

@@ -834,6 +834,8 @@ enum class CCEKind {
                            ///< message.
 };
 
+void inferNoReturnAttr(Sema &S, const Decl *D);
+
 /// Sema - This implements semantic analysis and AST building for C.
 /// \nosubgrouping
 class Sema final : public SemaBase {

clang/lib/Sema/AnalysisBasedWarnings.cpp

@@ -643,7 +643,7 @@ static void CheckFallThroughForBody(Sema &S, const Decl *D, const Stmt *Body,
       ReturnsVoid = CBody->getFallthroughHandler() != nullptr;
     else
       ReturnsVoid = FD->getReturnType()->isVoidType();
-    HasNoReturn = FD->isNoReturn();
+    HasNoReturn = FD->isNoReturn() || FD->hasAttr<InferredNoReturnAttr>();
   }
   else if (const auto *MD = dyn_cast<ObjCMethodDecl>(D)) {
     ReturnsVoid = MD->getReturnType()->isVoidType();
@@ -681,6 +681,28 @@ 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 = D->getAsFunction()) {
+        if (const auto *CS = dyn_cast<CompoundStmt>(Body);
+            CS && !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();
+                Callee && Callee->hasAttr<InferredNoReturnAttr>()) {
+              return; // Don't warn about fall-through.
+            }
+          }
+          // Direct throw.
+          if (isa<CXXThrowExpr>(LastStmt)) {
+            return; // Don't warn about fall-through.
+          }
+        }
+      }
       bool NotInAllControlPaths = FallThroughType == MaybeFallThrough;
       S.Diag(RBrace, CD.diag_FallThrough_ReturnsNonVoid)
           << CD.FunKind << NotInAllControlPaths;

clang/lib/Sema/Sema.cpp

@@ -2434,9 +2434,10 @@ Sema::PopFunctionScopeInfo(const AnalysisBasedWarnings::Policy *WP,
     OpenMP().popOpenMPFunctionRegion(Scope.get());
 
   // Issue any analysis-based warnings.
-  if (WP && D)
+  if (WP && D) {
+    inferNoReturnAttr(*this, D);
     AnalysisWarnings.IssueWarnings(*WP, Scope.get(), D, BlockType);
-  else
+  } else
     for (const auto &PUD : Scope->PossiblyUnreachableDiags)
       Diag(PUD.Loc, PUD.PD);
 

clang/lib/Sema/SemaDeclAttr.cpp

@@ -40,6 +40,7 @@
 #include "clang/Sema/ParsedAttr.h"
 #include "clang/Sema/Scope.h"
 #include "clang/Sema/ScopeInfo.h"
+#include "clang/Sema/Sema.h"
 #include "clang/Sema/SemaAMDGPU.h"
 #include "clang/Sema/SemaARM.h"
 #include "clang/Sema/SemaAVR.h"
@@ -1938,6 +1939,49 @@ static void handleNakedAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
   D->addAttr(::new (S.Context) NakedAttr(S.Context, AL));
 }
 
+// FIXME: This is a best-effort heuristic.
+// Currently only handles single throw expressions (optionally with
+// ExprWithCleanups). We could expand this to perform control-flow analysis for
+// more complex patterns.
+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.
+  return isa<CXXThrowExpr>(OnlyStmt);
+}
+
+void clang::inferNoReturnAttr(Sema &S, const Decl *D) {
+  auto *FD = dyn_cast<FunctionDecl>(D);
+  if (!FD)
+    return;
+
+  auto *NonConstFD = const_cast<FunctionDecl *>(FD);
+  DiagnosticsEngine &Diags = S.getDiagnostics();
+  if (Diags.isIgnored(diag::warn_falloff_nonvoid, FD->getLocation()) &&
+      Diags.isIgnored(diag::warn_suggest_noreturn_function, FD->getLocation()))
+    return;
+
+  if (!FD->hasAttr<NoReturnAttr>() && !FD->hasAttr<InferredNoReturnAttr>() &&
+      isKnownToAlwaysThrow(FD)) {
+    NonConstFD->addAttr(InferredNoReturnAttr::CreateImplicit(S.Context));
+  }
+}
+
 static void handleNoReturnAttr(Sema &S, Decl *D, const ParsedAttr &Attrs) {
   if (hasDeclarator(D)) return;
 

clang/test/SemaCXX/wreturn-always-throws.cpp

@@ -0,0 +1,46 @@
+// 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 &); 
+  };
+}
+
+// Non-template version.
+
+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
+}
+
+int alwaysThrows() {
+  throw std::runtime_error("This function always throws"); // no-warning
+}
+
+// Template version.
+
+template<typename T> 
+void throwErrorTemplate(const T& msg) {
+  throw msg;
+}
+
+template <typename T>
+int ensureZeroTemplate(T i) {
+  if (i == 0) return 0;
+  throwErrorTemplate("ERROR"); // no-warning
+}
+
+void testTemplates() {
+  throwErrorTemplate("ERROR");
+  (void)ensureZeroTemplate(42);
+}