[clang][Sema] Diagnose exceptions only in non-dependent context in discarded try/catch/throw blocks

Resolves #138939

When enabling `--fno-exceptions` flag, discarded statements containing
`try/catch/throw` in an independent context can be avoided from being
rejected.

Author: Rajveer100

clang/include/clang/Sema/Sema.h

@@ -11154,6 +11154,8 @@ class Sema final : public SemaBase {
   StmtResult ActOnCXXTryBlock(SourceLocation TryLoc, Stmt *TryBlock,
                               ArrayRef<Stmt *> Handlers);
 
+  void DiagnoseExceptionUse(SourceLocation Loc, bool IsTry);
+
   StmtResult ActOnSEHTryBlock(bool IsCXXTry, // try (true) or __try (false) ?
                               SourceLocation TryLoc, Stmt *TryBlock,
                               Stmt *Handler);

clang/lib/Sema/SemaExprCXX.cpp

@@ -856,7 +856,7 @@ ExprResult Sema::BuildCXXThrow(SourceLocation OpLoc, Expr *Ex,
   if (!IsOpenMPGPUTarget && !getLangOpts().CXXExceptions &&
       !getSourceManager().isInSystemHeader(OpLoc) && !getLangOpts().CUDA) {
     // Delay error emission for the OpenMP device code.
-    targetDiag(OpLoc, diag::err_exceptions_disabled) << "throw";
+    DiagnoseExceptionUse(OpLoc, /* IsTry */ false);
   }
 
   // In OpenMP target regions, we replace 'throw' with a trap on GPU targets.

clang/lib/Sema/SemaStmt.cpp

@@ -4307,7 +4307,7 @@ StmtResult Sema::ActOnCXXTryBlock(SourceLocation TryLoc, Stmt *TryBlock,
   if (!IsOpenMPGPUTarget && !getLangOpts().CXXExceptions &&
       !getSourceManager().isInSystemHeader(TryLoc) && !getLangOpts().CUDA) {
     // Delay error emission for the OpenMP device code.
-    targetDiag(TryLoc, diag::err_exceptions_disabled) << "try";
+    DiagnoseExceptionUse(TryLoc, /* IsTry */ true);
   }
 
   // In OpenMP target regions, we assume that catch is never reached on GPU
@@ -4410,6 +4410,12 @@ StmtResult Sema::ActOnCXXTryBlock(SourceLocation TryLoc, Stmt *TryBlock,
                             Handlers);
 }
 
+void Sema::DiagnoseExceptionUse(SourceLocation Loc, bool IsTry) {
+  if (!CurContext->isDependentContext()) {
+    targetDiag(Loc, diag::err_exceptions_disabled) << (IsTry ? "try" : "throw");
+  }
+}
+
 StmtResult Sema::ActOnSEHTryBlock(bool IsCXXTry, SourceLocation TryLoc,
                                   Stmt *TryBlock, Stmt *Handler) {
   assert(TryBlock && Handler);

clang/lib/Sema/TreeTransform.h

@@ -9162,6 +9162,8 @@ StmtResult TreeTransform<Derived>::TransformCXXTryStmt(CXXTryStmt *S) {
     Handlers.push_back(Handler.getAs<Stmt>());
   }
 
+  getSema().DiagnoseExceptionUse(S->getTryLoc(), /* IsTry */ true);
+
   if (!getDerived().AlwaysRebuild() && TryBlock.get() == S->getTryBlock() &&
       !HandlerChanged)
     return S;
@@ -14384,6 +14386,8 @@ TreeTransform<Derived>::TransformCXXThrowExpr(CXXThrowExpr *E) {
   if (SubExpr.isInvalid())
     return ExprError();
 
+  getSema().DiagnoseExceptionUse(E->getThrowLoc(), /* IsTry */ false);
+
   if (!getDerived().AlwaysRebuild() &&
       SubExpr.get() == E->getSubExpr())
     return E;

clang/test/SemaCXX/no-exceptions.cpp

@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -fsyntax-only -verify %s
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++20 %s
 
 // Various tests for -fno-exceptions
 
@@ -30,5 +30,28 @@ void g() {
   } catch (...) {
   }
 }
+}
+
+namespace test2 {
+template <auto enable> void foo(auto &&Fnc) {
+  if constexpr (enable)
+    try {
+      Fnc();
+    } catch (...) {
+    }
+  else
+    Fnc();
+}
+
+void bar1() {
+  foo<false>([] {});
+}
 
+template <typename T> void foo() {
+  try { // expected-error {{cannot use 'try' with exceptions disabled}}
+  } catch (...) {
+  }
+  throw 1; // expected-error {{cannot use 'throw' with exceptions disabled}}
+}
+void bar2() { foo<int>(); } // expected-note {{in instantiation of function template specialization 'test2::foo<int>' requested here}}
 }