[clang][Interp] Fix value truncation when casting int128 to smaller size (#67961)

Before this patch, we would run into an assertion in
`APInt::get{S,Z}ExtValue()` because the `AllOnes` value had more than 64
active bits.

Author: tbaederr

clang/lib/AST/Interp/IntegralAP.h

@@ -32,6 +32,12 @@ template <unsigned Bits, bool Signed> class Integral;
 class Boolean;
 
 template <bool Signed> class IntegralAP final {
+private:
+  template <typename T> static T truncateCast(const APSInt &V) {
+    return std::is_signed_v<T> ? V.trunc(sizeof(T) * 8).getSExtValue()
+                               : V.trunc(sizeof(T) * 8).getZExtValue();
+  }
+
 public:
   APSInt V;
 
@@ -55,14 +61,14 @@ template <bool Signed> class IntegralAP final {
   bool operator<=(IntegralAP RHS) const { return V <= RHS.V; }
 
   explicit operator bool() const { return !V.isZero(); }
-  explicit operator int8_t() const { return V.getSExtValue(); }
-  explicit operator uint8_t() const { return V.getZExtValue(); }
-  explicit operator int16_t() const { return V.getSExtValue(); }
-  explicit operator uint16_t() const { return V.getZExtValue(); }
-  explicit operator int32_t() const { return V.getSExtValue(); }
-  explicit operator uint32_t() const { return V.getZExtValue(); }
-  explicit operator int64_t() const { return V.getSExtValue(); }
-  explicit operator uint64_t() const { return V.getZExtValue(); }
+  explicit operator int8_t() const { return truncateCast<int8_t>(V); }
+  explicit operator uint8_t() const { return truncateCast<uint8_t>(V); }
+  explicit operator int16_t() const { return truncateCast<int16_t>(V); }
+  explicit operator uint16_t() const { return truncateCast<uint16_t>(V); }
+  explicit operator int32_t() const { return truncateCast<int32_t>(V); }
+  explicit operator uint32_t() const { return truncateCast<uint32_t>(V); }
+  explicit operator int64_t() const { return truncateCast<int64_t>(V); }
+  explicit operator uint64_t() const { return truncateCast<uint64_t>(V); }
 
   template <typename T> static IntegralAP from(T Value, unsigned NumBits = 0) {
     assert(NumBits > 0);

clang/test/AST/Interp/literals.cpp

@@ -53,7 +53,7 @@ namespace i128 {
   static_assert(Two == 2, "");
 
   constexpr uint128_t AllOnes = ~static_cast<uint128_t>(0);
-  static_assert(AllOnes == static_cast<uint128_t>(-1), "");
+  static_assert(AllOnes == UINT128_MAX, "");
 
 #if __cplusplus >= 201402L
   template <typename T>
@@ -70,6 +70,14 @@ namespace i128 {
   static_assert(CastFrom<double>(12) == 12, "");
   static_assert(CastFrom<long double>(12) == 12, "");
 
+  static_assert(CastFrom<char>(AllOnes) == -1, "");
+  static_assert(CastFrom<unsigned char>(AllOnes) == 0xFF, "");
+  static_assert(CastFrom<long>(AllOnes) == -1, "");
+  static_assert(CastFrom<unsigned short>(AllOnes) == 0xFFFF, "");
+  static_assert(CastFrom<int>(AllOnes) == -1, "");
+  static_assert(CastFrom<int128_t>(AllOnes) == -1, "");
+  static_assert(CastFrom<uint128_t>(AllOnes) == AllOnes, "");
+
   template <typename T>
   constexpr __int128 CastTo(T A) {
     int128_t B = (int128_t)A;