[coroutines][PR40978] Emit error for co_yield within catch block

Summary:
As reported in https://bugs.llvm.org/show_bug.cgi?id=40978, it's an
error to use the `co_yield` or `co_await` keywords outside of a valid
"suspension context" as defined by [expr.await]p2 of
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/n4775.pdf.

Whether or not the current scope was in a function-try-block's
(https://en.cppreference.com/w/cpp/language/function-try-block) handler
could be determined using scope flag `Scope::FnTryCatchScope`. No
such flag existed for a simple C++ catch statement, so this commit adds
one.

Reviewers: GorNishanov, tks2103, rsmith

Reviewed By: GorNishanov

Subscribers: EricWF, jdoerfert, cfe-commits, lewissbaker

Tags: #clang

Differential Revision: https://reviews.llvm.org/D59076

llvm-svn: 356296

Author: modocache

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -9251,6 +9251,8 @@ def err_coroutine_objc_method : Error<
   "Objective-C methods as coroutines are not yet supported">;
 def err_coroutine_unevaluated_context : Error<
   "'%0' cannot be used in an unevaluated context">;
+def err_coroutine_within_handler : Error<
+  "'%0' cannot be used in the handler of a try block">;
 def err_coroutine_outside_function : Error<
   "'%0' cannot be used outside a function">;
 def err_coroutine_invalid_func_context : Error<

clang/include/clang/Sema/Scope.h

@@ -131,6 +131,9 @@ class Scope {
 
     /// We are between inheritance colon and the real class/struct definition scope.
     ClassInheritanceScope = 0x800000,
+
+    /// This is the scope of a C++ catch statement.
+    CatchScope = 0x1000000,
   };
 
 private:

clang/lib/Parse/ParseStmt.cpp

@@ -2260,8 +2260,10 @@ StmtResult Parser::ParseCXXCatchBlock(bool FnCatch) {
   // C++ 3.3.2p3:
   // The name in a catch exception-declaration is local to the handler and
   // shall not be redeclared in the outermost block of the handler.
-  ParseScope CatchScope(this, Scope::DeclScope | Scope::ControlScope |
-                          (FnCatch ? Scope::FnTryCatchScope : 0));
+  unsigned ScopeFlags = Scope::DeclScope | Scope::ControlScope |
+                        Scope::CatchScope |
+                        (FnCatch ? Scope::FnTryCatchScope : 0);
+  ParseScope CatchScope(this, ScopeFlags);
 
   // exception-declaration is equivalent to '...' or a parameter-declaration
   // without default arguments.
@@ -2290,7 +2292,7 @@ StmtResult Parser::ParseCXXCatchBlock(bool FnCatch) {
     return StmtError(Diag(Tok, diag::err_expected) << tok::l_brace);
 
   // FIXME: Possible draft standard bug: attribute-specifier should be allowed?
-  StmtResult Block(ParseCompoundStatement());
+  StmtResult Block(ParseCompoundStatement(/*isStmtExpr=*/false, ScopeFlags));
   if (Block.isInvalid())
     return Block;
 

clang/lib/Sema/Scope.cpp

@@ -166,7 +166,9 @@ void Scope::dumpImpl(raw_ostream &OS) const {
       {SEHExceptScope, "SEHExceptScope"},
       {SEHFilterScope, "SEHFilterScope"},
       {CompoundStmtScope, "CompoundStmtScope"},
-      {ClassInheritanceScope, "ClassInheritanceScope"}};
+      {ClassInheritanceScope, "ClassInheritanceScope"},
+      {CatchScope, "CatchScope"},
+  };
 
   for (auto Info : FlagInfo) {
     if (Flags & Info.first) {

clang/lib/Sema/SemaCoroutine.cpp

@@ -185,21 +185,8 @@ static QualType lookupCoroutineHandleType(Sema &S, QualType PromiseType,
 
 static bool isValidCoroutineContext(Sema &S, SourceLocation Loc,
                                     StringRef Keyword) {
-  // 'co_await' and 'co_yield' are not permitted in unevaluated operands,
-  // such as subexpressions of \c sizeof.
-  //
-  // [expr.await]p2, emphasis added: "An await-expression shall appear only in
-  // a *potentially evaluated* expression within the compound-statement of a
-  // function-body outside of a handler [...] A context within a function where
-  // an await-expression can appear is called a suspension context of the
-  // function." And per [expr.yield]p1: "A yield-expression shall appear only
-  // within a suspension context of a function."
-  if (S.isUnevaluatedContext()) {
-    S.Diag(Loc, diag::err_coroutine_unevaluated_context) << Keyword;
-    return false;
-  }
-
-  // Per [expr.await]p2, any other usage must be within a function.
+  // [expr.await]p2 dictates that 'co_await' and 'co_yield' must be used within
+  // a function body.
   // FIXME: This also covers [expr.await]p2: "An await-expression shall not
   // appear in a default argument." But the diagnostic QoI here could be
   // improved to inform the user that default arguments specifically are not
@@ -668,12 +655,57 @@ bool Sema::ActOnCoroutineBodyStart(Scope *SC, SourceLocation KWLoc,
   return true;
 }
 
+// Recursively walks up the scope hierarchy until either a 'catch' or a function
+// scope is found, whichever comes first.
+static bool isWithinCatchScope(Scope *S) {
+  // 'co_await' and 'co_yield' keywords are disallowed within catch blocks, but
+  // lambdas that use 'co_await' are allowed. The loop below ends when a
+  // function scope is found in order to ensure the following behavior:
+  //
+  // void foo() {      // <- function scope
+  //   try {           //
+  //     co_await x;   // <- 'co_await' is OK within a function scope
+  //   } catch {       // <- catch scope
+  //     co_await x;   // <- 'co_await' is not OK within a catch scope
+  //     []() {        // <- function scope
+  //       co_await x; // <- 'co_await' is OK within a function scope
+  //     }();
+  //   }
+  // }
+  while (S && !(S->getFlags() & Scope::FnScope)) {
+    if (S->getFlags() & Scope::CatchScope)
+      return true;
+    S = S->getParent();
+  }
+  return false;
+}
+
+// [expr.await]p2, emphasis added: "An await-expression shall appear only in
+// a *potentially evaluated* expression within the compound-statement of a
+// function-body *outside of a handler* [...] A context within a function
+// where an await-expression can appear is called a suspension context of the
+// function."
+static void checkSuspensionContext(Sema &S, SourceLocation Loc,
+                                   StringRef Keyword) {
+  // First emphasis of [expr.await]p2: must be a potentially evaluated context.
+  // That is, 'co_await' and 'co_yield' cannot appear in subexpressions of
+  // \c sizeof.
+  if (S.isUnevaluatedContext())
+    S.Diag(Loc, diag::err_coroutine_unevaluated_context) << Keyword;
+
+  // Second emphasis of [expr.await]p2: must be outside of an exception handler.
+  if (isWithinCatchScope(S.getCurScope()))
+    S.Diag(Loc, diag::err_coroutine_within_handler) << Keyword;
+}
+
 ExprResult Sema::ActOnCoawaitExpr(Scope *S, SourceLocation Loc, Expr *E) {
   if (!ActOnCoroutineBodyStart(S, Loc, "co_await")) {
     CorrectDelayedTyposInExpr(E);
     return ExprError();
   }
 
+  checkSuspensionContext(*this, Loc, "co_await");
+
   if (E->getType()->isPlaceholderType()) {
     ExprResult R = CheckPlaceholderExpr(E);
     if (R.isInvalid()) return ExprError();
@@ -771,6 +803,8 @@ ExprResult Sema::ActOnCoyieldExpr(Scope *S, SourceLocation Loc, Expr *E) {
     return ExprError();
   }
 
+  checkSuspensionContext(*this, Loc, "co_yield");
+
   // Build yield_value call.
   ExprResult Awaitable = buildPromiseCall(
       *this, getCurFunction()->CoroutinePromise, Loc, "yield_value", E);

clang/test/SemaCXX/coroutines.cpp

@@ -314,20 +314,71 @@ struct CtorDtor {
   }
 };
 
+namespace std { class type_info; }
+
 void unevaluated() {
-  decltype(co_await a); // expected-error {{cannot be used in an unevaluated context}}
-  sizeof(co_await a); // expected-error {{cannot be used in an unevaluated context}}
-  typeid(co_await a); // expected-error {{cannot be used in an unevaluated context}}
-  decltype(co_yield a); // expected-error {{cannot be used in an unevaluated context}}
-  sizeof(co_yield a); // expected-error {{cannot be used in an unevaluated context}}
-  typeid(co_yield a); // expected-error {{cannot be used in an unevaluated context}}
+  decltype(co_await a); // expected-error {{'co_await' cannot be used in an unevaluated context}}
+                        // expected-warning@-1 {{declaration does not declare anything}}
+  sizeof(co_await a); // expected-error {{'co_await' cannot be used in an unevaluated context}}
+                      // expected-error@-1 {{invalid application of 'sizeof' to an incomplete type 'void'}}
+  typeid(co_await a); // expected-error {{'co_await' cannot be used in an unevaluated context}}
+                      // expected-warning@-1 {{expression with side effects has no effect in an unevaluated context}}
+                      // expected-warning@-2 {{expression result unused}}
+  decltype(co_yield 1); // expected-error {{'co_yield' cannot be used in an unevaluated context}}
+                        // expected-warning@-1 {{declaration does not declare anything}}
+  sizeof(co_yield 2); // expected-error {{'co_yield' cannot be used in an unevaluated context}}
+                      // expected-error@-1 {{invalid application of 'sizeof' to an incomplete type 'void'}}
+  typeid(co_yield 3); // expected-error {{'co_yield' cannot be used in an unevaluated context}}
+                      // expected-warning@-1 {{expression with side effects has no effect in an unevaluated context}}
+                      // expected-warning@-2 {{expression result unused}}
 }
 
 // [expr.await]p2: "An await-expression shall not appear in a default argument."
 // FIXME: A better diagnostic would explicitly state that default arguments are
 // not allowed. A user may not understand that this is "outside a function."
 void default_argument(int arg = co_await 0) {} // expected-error {{'co_await' cannot be used outside a function}}
 
+void await_in_catch_coroutine() {
+  try {
+  } catch (...) { // FIXME: Emit a note diagnostic pointing out the try handler on this line.
+    []() -> void { co_await a; }(); // OK
+    co_await a; // expected-error {{'co_await' cannot be used in the handler of a try block}}
+  }
+}
+
+void await_nested_in_catch_coroutine() {
+  try {
+  } catch (...) { // FIXME: Emit a note diagnostic pointing out the try handler on this line.
+    try {
+      co_await a; // expected-error {{'co_await' cannot be used in the handler of a try block}}
+      []() -> void { co_await a; }(); // OK
+    } catch (...) {
+      co_return 123;
+    }
+  }
+}
+
+void await_in_lambda_in_catch_coroutine() {
+  try {
+  } catch (...) {
+    []() -> void { co_await a; }(); // OK
+  }
+}
+
+void yield_in_catch_coroutine() {
+  try {
+  } catch (...) {
+    co_yield 1; // expected-error {{'co_yield' cannot be used in the handler of a try block}}
+  }
+}
+
+void return_in_catch_coroutine() {
+  try {
+  } catch (...) {
+    co_return 123; // OK
+  }
+}
+
 constexpr auto constexpr_deduced_return_coroutine() {
   co_yield 0; // expected-error {{'co_yield' cannot be used in a constexpr function}}
   // expected-error@-1 {{'co_yield' cannot be used in a function with a deduced return type}}