[coroutines] Fix fallthrough warning on try/catch

[modocache]

Summary

The test case added in this diff would incorrectly warn that control
flow may fall through without returning. Here's a standalone example:
https://godbolt.org/z/dCwXEi

The same program, but using return instead of co_return, does not
produce a warning: https://godbolt.org/z/mVldqQ

The issue was in how Clang analysis would structure its representation
of the control-flow graph. Specifically, when constructing the CFG,
CFGBuilder::Visit had special handling of a ReturnStmt, in which it
would place object destructors in the same CFG block as a return statement,
immediately after it. Doing so would allow the logic in
lib/Sema/AnalysisBasedWarning.cpp CheckFallThrough to work properly in the
program that used return, correctly determining that no "plain edges" preceded
the exit block of the function.

Because a co_return statement would not enjoy the same treatment when
it was being built into the control-flow graph, object destructors
would not be placed in the same CFG block as the co_return, thus
resulting in a "plain edge" preceding the exit block of the function,
and so the warning logic would be triggered.

Add special casing for co_return to Clang analysis, thereby
remedying the mistaken warning.

Test Plan: check-clang

[GorNishanov]

LGTM!

[modocache]

Great, thanks!

lib/Analysis/CFG.cpp

private:
   CFGBlock *VisitObjCAtCatchStmt(ObjCAtCatchStmt *S);
   CFGBlock *VisitObjCAtSynchronizedStmt(ObjCAtSynchronizedStmt *S);
   CFGBlock *VisitObjCAtThrowStmt(ObjCAtThrowStmt *S);
   CFGBlock *VisitObjCAtTryStmt(ObjCAtTryStmt *S);
   CFGBlock *VisitObjCAutoreleasePoolStmt(ObjCAutoreleasePoolStmt *S);
   CFGBlock *VisitObjCForCollectionStmt(ObjCForCollectionStmt *S);
   CFGBlock *VisitObjCMessageExpr(ObjCMessageExpr *E, AddStmtChoice asc);
   CFGBlock *VisitPseudoObjectExpr(PseudoObjectExpr *E);
-  CFGBlock *VisitReturnStmt(ReturnStmt *R);
+  CFGBlock *VisitReturnStmt(Stmt *S);
   CFGBlock *VisitSEHExceptStmt(SEHExceptStmt *S);
   CFGBlock *VisitSEHFinallyStmt(SEHFinallyStmt *S);
   CFGBlock *VisitSEHLeaveStmt(SEHLeaveStmt *S);
   CFGBlock *VisitSEHTryStmt(SEHTryStmt *S);
   CFGBlock *VisitStmtExpr(StmtExpr *S, AddStmtChoice asc);
   CFGBlock *VisitSwitchStmt(SwitchStmt *S);
   CFGBlock *VisitUnaryExprOrTypeTraitExpr(UnaryExprOrTypeTraitExpr *E,
                                           AddStmtChoice asc);
switch (S->getStmtClass()) {
 
     case Stmt::OpaqueValueExprClass:
       return Block;
 
     case Stmt::PseudoObjectExprClass:
       return VisitPseudoObjectExpr(cast<PseudoObjectExpr>(S));
 
     case Stmt::ReturnStmtClass:
-      return VisitReturnStmt(cast<ReturnStmt>(S));
+    case Stmt::CoreturnStmtClass:
+      return VisitReturnStmt(S);
 
     case Stmt::SEHExceptStmtClass:
       return VisitSEHExceptStmt(cast<SEHExceptStmt>(S));
 
     case Stmt::SEHFinallyStmtClass:
       return VisitSEHFinallyStmt(cast<SEHFinallyStmt>(S));
 
     case Stmt::SEHLeaveStmtClass:
CFGBlock *CFGBuilder::VisitIfStmt(IfStmt *I) {
   if (Stmt *Init = I->getInit()) {
     autoCreateBlock();
     LastBlock = addStmt(Init);
   }
 
   return LastBlock;
 }
 
-CFGBlock *CFGBuilder::VisitReturnStmt(ReturnStmt *R) {
+CFGBlock *CFGBuilder::VisitReturnStmt(Stmt *S) {
   // If we were in the middle of a block we stop processing that block.
   //
-  // NOTE: If a "return" appears in the middle of a block, this means that the
-  //       code afterwards is DEAD (unreachable).  We still keep a basic block
-  //       for that code; a simple "mark-and-sweep" from the entry block will be
-  //       able to report such dead blocks.
+  // NOTE: If a "return" or "co_return" appears in the middle of a block, this
+  //       means that the code afterwards is DEAD (unreachable).  We still keep
+  //       a basic block for that code; a simple "mark-and-sweep" from the entry
+  //       block will be able to report such dead blocks.
+  assert(isa<ReturnStmt>(S) || isa<CoreturnStmt>(S));
 
   // Create the new block.
   Block = createBlock(false);
 
-  addAutomaticObjHandling(ScopePos, LocalScope::const_iterator(), R);
+  addAutomaticObjHandling(ScopePos, LocalScope::const_iterator(), S);
 
+  if (auto *R = dyn_cast<ReturnStmt>(S))
     findConstructionContexts(
         ConstructionContextLayer::create(cfg->getBumpVectorContext(), R),
         R->getRetValue());
 
   // If the one of the destructors does not return, we already have the Exit
   // block as a successor.
   if (!Block->hasNoReturnElement())
     addSuccessor(Block, &cfg->getExit());
 
   // Add the return statement to the block.  This may create new blocks if R
   // contains control-flow (short-circuit operations).
-  return VisitStmt(R, AddStmtChoice::AlwaysAdd);
+  return VisitStmt(S, AddStmtChoice::AlwaysAdd);
 }
 
 CFGBlock *CFGBuilder::VisitSEHExceptStmt(SEHExceptStmt *ES) {
   // SEHExceptStmt are treated like labels, so they are the first statement in a
   // block.
 
   // Save local scope position because in case of exception variable ScopePos
   // won't be restored when traversing AST.

test/SemaCXX/coreturn-eh.cpp

+// RUN: %clang_cc1 -triple x86_64-apple-darwin9 %s -std=c++14 -fcoroutines-ts -fcxx-exceptions -fsyntax-only -Wignored-qualifiers -Wno-error=return-type -verify -fblocks -Wall -Wextra -Wno-error=unreachable-code
+// expected-no-diagnostics
+
+#include "Inputs/std-coroutine.h"
+
+using std::experimental::suspend_always;
+using std::experimental::suspend_never;
+
+struct awaitable {
+  bool await_ready();
+  void await_suspend(std::experimental::coroutine_handle<>); // FIXME: coroutine_handle
+  void await_resume();
+} a;
+
+struct object { ~object() {} };
+
+struct promise_void_return_value {
+  void get_return_object();
+  suspend_always initial_suspend();
+  suspend_always final_suspend();
+  void unhandled_exception();
+  void return_value(object);
+};
+
+struct VoidTagReturnValue {
+  struct promise_type {
+    VoidTagReturnValue get_return_object();
+    suspend_always initial_suspend();
+    suspend_always final_suspend();
+    void unhandled_exception();
+    void return_value(object);
+  };
+};
+
+template <typename T1>
+struct std::experimental::coroutine_traits<void, T1> { using promise_type = promise_void_return_value; };
+
+VoidTagReturnValue test() {
+  object x = {};
+  try {
+    co_return {};
+  } catch (...) {
+    throw;
+  }
+}