Index: lib/Basic/Targets/OSTargets.h
===================================================================
--- lib/Basic/Targets/OSTargets.h
+++ lib/Basic/Targets/OSTargets.h
@@ -133,6 +133,15 @@
   /// is very similar to ELF's "protected";  Darwin requires a "weak"
   /// attribute on declarations that can be dynamically replaced.
   bool hasProtectedVisibility() const override { return false; }
+
+  TargetInfo::IntType getLeastIntTypeByWidth(unsigned BitWidth,
+                                             bool IsSigned) const final {
+    // Darwin uses `long long` for `int_least64_t` and `int_fast64_t`.
+    return BitWidth == 64
+               ? (IsSigned ? TargetInfo::SignedLongLong
+                           : TargetInfo::UnsignedLongLong)
+               : TargetInfo::getLeastIntTypeByWidth(BitWidth, IsSigned);
+  }
 };

 // DragonFlyBSD Target
Index: test/Preprocessor/init.c
===================================================================
--- test/Preprocessor/init.c
+++ test/Preprocessor/init.c
@@ -1331,10 +1331,10 @@
 // AARCH64-DARWIN: #define __INT_FAST32_FMTi__ "i"
 // AARCH64-DARWIN: #define __INT_FAST32_MAX__ 2147483647
 // AARCH64-DARWIN: #define __INT_FAST32_TYPE__ int
-// AARCH64-DARWIN: #define __INT_FAST64_FMTd__ "ld"
-// AARCH64-DARWIN: #define __INT_FAST64_FMTi__ "li"
-// AARCH64-DARWIN: #define __INT_FAST64_MAX__ 9223372036854775807L
-// AARCH64-DARWIN: #define __INT_FAST64_TYPE__ long int
+// AARCH64-DARWIN: #define __INT_FAST64_FMTd__ "lld"
+// AARCH64-DARWIN: #define __INT_FAST64_FMTi__ "lli"
+// AARCH64-DARWIN: #define __INT_FAST64_MAX__ 9223372036854775807LL
+// AARCH64-DARWIN: #define __INT_FAST64_TYPE__ long long int
 // AARCH64-DARWIN: #define __INT_FAST8_FMTd__ "hhd"
 // AARCH64-DARWIN: #define __INT_FAST8_FMTi__ "hhi"
 // AARCH64-DARWIN: #define __INT_FAST8_MAX__ 127
@@ -1347,10 +1347,10 @@
 // AARCH64-DARWIN: #define __INT_LEAST32_FMTi__ "i"
 // AARCH64-DARWIN: #define __INT_LEAST32_MAX__ 2147483647
 // AARCH64-DARWIN: #define __INT_LEAST32_TYPE__ int
-// AARCH64-DARWIN: #define __INT_LEAST64_FMTd__ "ld"
-// AARCH64-DARWIN: #define __INT_LEAST64_FMTi__ "li"
-// AARCH64-DARWIN: #define __INT_LEAST64_MAX__ 9223372036854775807L
-// AARCH64-DARWIN: #define __INT_LEAST64_TYPE__ long int
+// AARCH64-DARWIN: #define __INT_LEAST64_FMTd__ "lld"
+// AARCH64-DARWIN: #define __INT_LEAST64_FMTi__ "lli"
+// AARCH64-DARWIN: #define __INT_LEAST64_MAX__ 9223372036854775807LL
+// AARCH64-DARWIN: #define __INT_LEAST64_TYPE__ long long int
 // AARCH64-DARWIN: #define __INT_LEAST8_FMTd__ "hhd"
 // AARCH64-DARWIN: #define __INT_LEAST8_FMTi__ "hhi"
 // AARCH64-DARWIN: #define __INT_LEAST8_MAX__ 127
@@ -1418,16 +1418,16 @@
 // AARCH64-DARWIN: #define __UINT_FAST16_TYPE__ unsigned short
 // AARCH64-DARWIN: #define __UINT_FAST32_MAX__ 4294967295U
 // AARCH64-DARWIN: #define __UINT_FAST32_TYPE__ unsigned int
-// AARCH64-DARWIN: #define __UINT_FAST64_MAX__ 18446744073709551615UL
-// AARCH64-DARWIN: #define __UINT_FAST64_TYPE__ long unsigned int
+// AARCH64-DARWIN: #define __UINT_FAST64_MAX__ 18446744073709551615ULL
+// AARCH64-DARWIN: #define __UINT_FAST64_TYPE__ long long unsigned int
 // AARCH64-DARWIN: #define __UINT_FAST8_MAX__ 255
 // AARCH64-DARWIN: #define __UINT_FAST8_TYPE__ unsigned char
 // AARCH64-DARWIN: #define __UINT_LEAST16_MAX__ 65535
 // AARCH64-DARWIN: #define __UINT_LEAST16_TYPE__ unsigned short
 // AARCH64-DARWIN: #define __UINT_LEAST32_MAX__ 4294967295U
 // AARCH64-DARWIN: #define __UINT_LEAST32_TYPE__ unsigned int
-// AARCH64-DARWIN: #define __UINT_LEAST64_MAX__ 18446744073709551615UL
-// AARCH64-DARWIN: #define __UINT_LEAST64_TYPE__ long unsigned int
+// AARCH64-DARWIN: #define __UINT_LEAST64_MAX__ 18446744073709551615ULL
+// AARCH64-DARWIN: #define __UINT_LEAST64_TYPE__ long long unsigned int
 // AARCH64-DARWIN: #define __UINT_LEAST8_MAX__ 255
 // AARCH64-DARWIN: #define __UINT_LEAST8_TYPE__ unsigned char
 // AARCH64-DARWIN: #define __USER_LABEL_PREFIX__ _