[clang] [CFG] Add a BuildOption to consider default branch of switch on covered enumerations. (PR #161345)

[Balazs Benics via cfe-commits]

================
@@ -93,6 +93,159 @@ TEST(CFG, DependantBaseAddImplicitDtors) {
                 .getStatus());
 }
 
+TEST(CFG, SwitchCoveredEnumNoDefault) {
+  const char *Code = R"(
+    enum class E {E1, E2};
+    int f(E e) {
+      switch(e) {
+        case E::E1:
+          return 1;
+        case E::E2:
+          return 2;
+      }
+      return 0;
+    }
+  )";
+  CFG::BuildOptions Options;
+  Options.AssumeReachableDefaultInSwitchStatements = true;
+  BuildResult B = BuildCFG(Code, Options);
+  EXPECT_EQ(BuildResult::BuiltCFG, B.getStatus());
+
+  // [B5 (ENTRY)]
+  //   Succs (1): B2
+  //
+  // [B1]
+  //   1: 0
+  //   2: return [B1.1];
+  //   Preds (1): B2
+  //   Succs (1): B0
+  //
+  // [B2]
+  //   1: e (ImplicitCastExpr, LValueToRValue, E)
+  //   T: switch [B2.1]
+  //   Preds (1): B5
+  //   Succs (3): B3 B4 B1
+  //
+  // [B3]
+  //  case E::E2:
+  //   1: 2
+  //   2: return [B3.1];
+  //   Preds (1): B2
+  //   Succs (1): B0
+  //
+  // [B4]
+  //  case E::E1:
+  //   1: 1
+  //   2: return [B4.1];
+  //   Preds (1): B2
+  //   Succs (1): B0
+  //
+  // [B0 (EXIT)]
+  //   Preds (3): B1 B3 B4
+
+  auto *CFG = B.getCFG();
+  const auto &Entry = CFG->getEntry();
+  ASSERT_EQ(1u, Entry.succ_size());
+  // First successor of Entry is the switch
+  CFGBlock *SwitchBlock = *Entry.succ_begin();
+  ASSERT_EQ(3u, SwitchBlock->succ_size());
+  // Last successor of the switch is after the switch
+  auto NoCaseSucc = SwitchBlock->succ_rbegin();
+  EXPECT_TRUE(NoCaseSucc->isReachable());
+
+  // Checking that the same node is Unreachable without this setting
+  Options.AssumeReachableDefaultInSwitchStatements = false;
+  B = BuildCFG(Code, Options);
+  EXPECT_EQ(BuildResult::BuiltCFG, B.getStatus());
+
+  const auto &Entry2 = B.getCFG()->getEntry();
+  ASSERT_EQ(1u, Entry2.succ_size());
+  CFGBlock *SwitchBlock2 = *Entry2.succ_begin();
+  ASSERT_EQ(3u, SwitchBlock2->succ_size());
+  auto NoCaseSucc2 = SwitchBlock2->succ_rbegin();
+  EXPECT_FALSE(NoCaseSucc2->isReachable());
+}
+
+TEST(CFG, SwitchCoveredEnumWithDefault) {
+  const char *Code = R"(
+    enum class E {E1, E2};
+    int f(E e) {
+      switch(e) {
+        case E::E1:
+          return 1;
+        case E::E2:
+          return 2;
+        default:
+          return 0;
+      }
+      return -1;
+    }
+  )";
+  CFG::BuildOptions Options;
+  Options.AssumeReachableDefaultInSwitchStatements = true;
+  BuildResult B = BuildCFG(Code, Options);
+  EXPECT_EQ(BuildResult::BuiltCFG, B.getStatus());
+
+  // [B6 (ENTRY)]
+  //   Succs (1): B2
+  //
+  // [B1]
+  //   1: -1
+  //   2: return [B1.1];
+  //   Succs (1): B0
+  //
+  // [B2]
+  //   1: e (ImplicitCastExpr, LValueToRValue, E)
+  //   T: switch [B2.1]
+  //   Preds (1): B6
+  //   Succs (3): B4 B5 B3
+  //
+  // [B3]
+  //  default:
+  //   1: 0
+  //   2: return [B3.1];
+  //   Preds (1): B2
+  //   Succs (1): B0
+  //
+  // [B4]
+  //  case E::E2:
+  //   1: 2
+  //   2: return [B4.1];
+  //   Preds (1): B2
+  //   Succs (1): B0
+  //
+  // [B5]
+  //  case E::E1:
+  //   1: 1
+  //   2: return [B5.1];
+  //   Preds (1): B2
+  //   Succs (1): B0
+  //
+  // [B0 (EXIT)]
+  //   Preds (4): B1 B3 B4 B5
+
+  const auto &Entry = B.getCFG()->getEntry();
+  ASSERT_EQ(1u, Entry.succ_size());
+  // First successor of Entry is the switch
+  CFGBlock *SwitchBlock = *Entry.succ_begin();
+  ASSERT_EQ(3u, SwitchBlock->succ_size());
+  // Last successor of the switch is the default branch
+  auto defaultBlock = SwitchBlock->succ_rbegin();
+  EXPECT_TRUE(defaultBlock->isReachable());
+
+  // Checking that the same node is Unreachable without this setting
+  Options.AssumeReachableDefaultInSwitchStatements = false;
+  B = BuildCFG(Code, Options);
+  EXPECT_EQ(BuildResult::BuiltCFG, B.getStatus());
----------------
steakhal wrote:

```suggestion
  ASSERT_EQ(BuildResult::BuiltCFG, B.getStatus());
```

https://github.com/llvm/llvm-project/pull/161345