[ADT] Introduce Casting function objects (llvm#165803)

Adding casting function objects as a convenience for expressing e.g. `auto AsDoubles = map_range(RangeOfInts, StaticCastTo<double>)`​

Author: mtrofin

llvm/include/llvm/Support/Casting.h

@@ -816,6 +816,42 @@ template <typename... Types> struct IsaAndPresentCheckPredicate {
     return isa_and_present<Types...>(Val);
   }
 };
+
+//===----------------------------------------------------------------------===//
+// Casting Function Objects
+//===----------------------------------------------------------------------===//
+
+/// Usable in generic algorithms like map_range
+template <typename U> struct StaticCastFunc {
+  template <typename T> decltype(auto) operator()(T &&Val) const {
+    return static_cast<U>(Val);
+  }
+};
+
+template <typename U> struct DynCastFunc {
+  template <typename T> decltype(auto) operator()(T &&Val) const {
+    return dyn_cast<U>(Val);
+  }
+};
+
+template <typename U> struct CastFunc {
+  template <typename T> decltype(auto) operator()(T &&Val) const {
+    return cast<U>(Val);
+  }
+};
+
+template <typename U> struct CastIfPresentFunc {
+  template <typename T> decltype(auto) operator()(T &&Val) const {
+    return cast_if_present<U>(Val);
+  }
+};
+
+template <typename U> struct DynCastIfPresentFunc {
+  template <typename T> decltype(auto) operator()(T &&Val) const {
+    return dyn_cast_if_present<U>(Val);
+  }
+};
+
 } // namespace detail
 
 /// Function object wrapper for the `llvm::isa` type check. The function call
@@ -841,6 +877,20 @@ template <typename... Types>
 inline constexpr detail::IsaAndPresentCheckPredicate<Types...>
     IsaAndPresentPred{};
 
+/// Function objects corresponding to the Cast types defined above.
+template <typename From>
+inline constexpr detail::StaticCastFunc<From> StaticCastTo{};
+
+template <typename From> inline constexpr detail::CastFunc<From> CastTo{};
+
+template <typename From>
+inline constexpr detail::CastIfPresentFunc<From> CastIfPresentTo{};
+
+template <typename From>
+inline constexpr detail::DynCastIfPresentFunc<From> DynCastIfPresentTo{};
+
+template <typename From> inline constexpr detail::DynCastFunc<From> DynCastTo{};
+
 } // end namespace llvm
 
 #endif // LLVM_SUPPORT_CASTING_H

llvm/unittests/Support/Casting.cpp

@@ -561,6 +561,47 @@ TEST(CastingTest, assertion_check_unique_ptr) {
       << "Invalid cast of const ref did not cause an abort()";
 }
 
+TEST(Casting, StaticCastPredicate) {
+  uint32_t Value = 1;
+
+  static_assert(
+      std::is_same_v<decltype(StaticCastTo<uint64_t>(Value)), uint64_t>);
+}
+
+TEST(Casting, LLVMRTTIPredicates) {
+  struct Base {
+    enum Kind { BK_Base, BK_Derived };
+    const Kind K;
+    Base(Kind K = BK_Base) : K(K) {}
+    Kind getKind() const { return K; }
+    virtual ~Base() = default;
+  };
+
+  struct Derived : Base {
+    Derived() : Base(BK_Derived) {}
+    static bool classof(const Base *B) { return B->getKind() == BK_Derived; }
+    bool Field = false;
+  };
+
+  Base B;
+  Derived D;
+  Base *BD = &D;
+  Base *Null = nullptr;
+
+  // Pointers.
+  EXPECT_EQ(DynCastTo<Derived>(BD), &D);
+  EXPECT_EQ(CastTo<Derived>(BD), &D);
+  EXPECT_EQ(DynCastTo<Derived>(&B), nullptr);
+  EXPECT_EQ(CastIfPresentTo<Derived>(BD), &D);
+  EXPECT_EQ(CastIfPresentTo<Derived>(Null), nullptr);
+  EXPECT_EQ(DynCastIfPresentTo<Derived>(BD), &D);
+  EXPECT_EQ(DynCastIfPresentTo<Derived>(Null), nullptr);
+
+  Base &R = D;
+  CastTo<Derived>(R).Field = true;
+  EXPECT_TRUE(D.Field);
+}
+
 } // end namespace assertion_checks
 #endif
 } // end namespace