[ADT] Add DefaultUnreachable("msg") to TypeSwitch (llvm#161970)

This is to allow making it explicit that all the cases must be handled.
The error message is customizable.

Something similar was already supported using the conversion operator
for the typed case but less explicit. In the `void` case when
`TypeSwitch` doesn't return anything, this was not possible without a
custom lambda.

Author: kuhar

llvm/include/llvm/ADT/TypeSwitch.h

@@ -17,6 +17,7 @@
 
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/Support/Casting.h"
+#include "llvm/Support/ErrorHandling.h"
 #include <optional>
 
 namespace llvm {
@@ -117,11 +118,16 @@ class TypeSwitch : public detail::TypeSwitchBase<TypeSwitch<T, ResultT>, T> {
     return defaultResult;
   }
 
-  [[nodiscard]] operator ResultT() {
-    assert(result && "Fell off the end of a type-switch");
-    return std::move(*result);
+  /// Declare default as unreachable, making sure that all cases were handled.
+  [[nodiscard]] ResultT DefaultUnreachable(
+      const char *message = "Fell off the end of a type-switch") {
+    if (result)
+      return std::move(*result);
+    llvm_unreachable(message);
   }
 
+  [[nodiscard]] operator ResultT() { return DefaultUnreachable(); }
+
 private:
   /// The pointer to the result of this switch statement, once known,
   /// null before that.
@@ -158,6 +164,13 @@ class TypeSwitch<T, void>
       defaultFn(this->value);
   }
 
+  /// Declare default as unreachable, making sure that all cases were handled.
+  void DefaultUnreachable(
+      const char *message = "Fell off the end of a type-switch") {
+    if (!foundMatch)
+      llvm_unreachable(message);
+  }
+
 private:
   /// A flag detailing if we have already found a match.
   bool foundMatch = false;

llvm/unittests/ADT/TypeSwitchTest.cpp

@@ -114,3 +114,31 @@ TEST(TypeSwitchTest, CasesOptional) {
   EXPECT_EQ(std::nullopt, translate(DerivedC()));
   EXPECT_EQ(-1, translate(DerivedD()));
 }
+
+TEST(TypeSwitchTest, DefaultUnreachableWithValue) {
+  auto translate = [](auto value) {
+    return TypeSwitch<Base *, int>(&value)
+        .Case([](DerivedA *) { return 0; })
+        .DefaultUnreachable("Unhandled type");
+  };
+  EXPECT_EQ(0, translate(DerivedA()));
+
+#if defined(GTEST_HAS_DEATH_TEST) && !defined(NDEBUG)
+  EXPECT_DEATH((void)translate(DerivedD()), "Unhandled type");
+#endif
+}
+
+TEST(TypeSwitchTest, DefaultUnreachableWithVoid) {
+  auto translate = [](auto value) {
+    int result = -1;
+    TypeSwitch<Base *>(&value)
+        .Case([&result](DerivedA *) { result = 0; })
+        .DefaultUnreachable("Unhandled type");
+    return result;
+  };
+  EXPECT_EQ(0, translate(DerivedA()));
+
+#if defined(GTEST_HAS_DEATH_TEST) && !defined(NDEBUG)
+  EXPECT_DEATH((void)translate(DerivedD()), "Unhandled type");
+#endif
+}