Index: flang/runtime/command.cpp
===================================================================
--- flang/runtime/command.cpp
+++ flang/runtime/command.cpp
@@ -107,9 +107,13 @@
 }
 
 template <int KIND> struct FitsInIntegerKind {
-  bool operator()(std::int64_t value) {
-    return value <= std::numeric_limits<Fortran::runtime::CppTypeFor<
-                        Fortran::common::TypeCategory::Integer, KIND>>::max();
+  bool operator()([[maybe_unused]] std::int64_t value) {
+    if constexpr (KIND >= 8) {
+      return true;
+    } else {
+      return value <= std::numeric_limits<Fortran::runtime::CppTypeFor<
+                          Fortran::common::TypeCategory::Integer, KIND>>::max();
+    }
   }
 };
 
Index: flang/runtime/io-api.cpp
===================================================================
--- flang/runtime/io-api.cpp
+++ flang/runtime/io-api.cpp
@@ -1483,7 +1483,9 @@
     // Only provide the bad unit number in the message if SignalError can print
     // it accurately. Otherwise, the generic IostatUnitOverflow message will be
     // used.
-    if (static_cast<std::intmax_t>(unit) == unit) {
+    if constexpr (sizeof(INT) > sizeof(std::intmax_t)) {
+      errorHandler.SignalError(IostatUnitOverflow);
+    } else if (static_cast<std::intmax_t>(unit) == unit) {
       errorHandler.SignalError(IostatUnitOverflow,
           "UNIT number %jd is out of range", static_cast<std::intmax_t>(unit));
     } else {
Index: flang/runtime/numeric.cpp
===================================================================
--- flang/runtime/numeric.cpp
+++ flang/runtime/numeric.cpp
@@ -306,7 +306,7 @@
     CppTypeFor<TypeCategory::Real, 4> x) {
   return Ceiling<CppTypeFor<TypeCategory::Integer, 8>>(x);
 }
-#ifdef __SIZEOF_INT128__
+#if defined __SIZEOF_INT128__ && !AVOID_NATIVE_UINT128_T
 CppTypeFor<TypeCategory::Integer, 16> RTNAME(Ceiling4_16)(
     CppTypeFor<TypeCategory::Real, 4> x) {
   return Ceiling<CppTypeFor<TypeCategory::Integer, 16>>(x);
@@ -328,7 +328,7 @@
     CppTypeFor<TypeCategory::Real, 8> x) {
   return Ceiling<CppTypeFor<TypeCategory::Integer, 8>>(x);
 }
-#ifdef __SIZEOF_INT128__
+#if defined __SIZEOF_INT128__ && !AVOID_NATIVE_UINT128_T
 CppTypeFor<TypeCategory::Integer, 16> RTNAME(Ceiling8_16)(
     CppTypeFor<TypeCategory::Real, 8> x) {
   return Ceiling<CppTypeFor<TypeCategory::Integer, 16>>(x);
@@ -351,7 +351,7 @@
     CppTypeFor<TypeCategory::Real, 10> x) {
   return Ceiling<CppTypeFor<TypeCategory::Integer, 8>>(x);
 }
-#ifdef __SIZEOF_INT128__
+#if defined __SIZEOF_INT128__ && !AVOID_NATIVE_UINT128_T
 CppTypeFor<TypeCategory::Integer, 16> RTNAME(Ceiling10_16)(
     CppTypeFor<TypeCategory::Real, 10> x) {
   return Ceiling<CppTypeFor<TypeCategory::Integer, 16>>(x);
@@ -374,7 +374,7 @@
     CppTypeFor<TypeCategory::Real, 16> x) {
   return Ceiling<CppTypeFor<TypeCategory::Integer, 8>>(x);
 }
-#ifdef __SIZEOF_INT128__
+#if defined __SIZEOF_INT128__ && !AVOID_NATIVE_UINT128_T
 CppTypeFor<TypeCategory::Integer, 16> RTNAME(Ceiling16_16)(
     CppTypeFor<TypeCategory::Real, 16> x) {
   return Ceiling<CppTypeFor<TypeCategory::Integer, 16>>(x);
@@ -434,7 +434,7 @@
     CppTypeFor<TypeCategory::Real, 4> x) {
   return Floor<CppTypeFor<TypeCategory::Integer, 8>>(x);
 }
-#ifdef __SIZEOF_INT128__
+#if defined __SIZEOF_INT128__ && !AVOID_NATIVE_UINT128_T
 CppTypeFor<TypeCategory::Integer, 16> RTNAME(Floor4_16)(
     CppTypeFor<TypeCategory::Real, 4> x) {
   return Floor<CppTypeFor<TypeCategory::Integer, 16>>(x);
@@ -456,7 +456,7 @@
     CppTypeFor<TypeCategory::Real, 8> x) {
   return Floor<CppTypeFor<TypeCategory::Integer, 8>>(x);
 }
-#ifdef __SIZEOF_INT128__
+#if defined __SIZEOF_INT128__ && !AVOID_NATIVE_UINT128_T
 CppTypeFor<TypeCategory::Integer, 16> RTNAME(Floor8_16)(
     CppTypeFor<TypeCategory::Real, 8> x) {
   return Floor<CppTypeFor<TypeCategory::Integer, 16>>(x);
@@ -479,7 +479,7 @@
     CppTypeFor<TypeCategory::Real, 10> x) {
   return Floor<CppTypeFor<TypeCategory::Integer, 8>>(x);
 }
-#ifdef __SIZEOF_INT128__
+#if defined __SIZEOF_INT128__ && !AVOID_NATIVE_UINT128_T
 CppTypeFor<TypeCategory::Integer, 16> RTNAME(Floor10_16)(
     CppTypeFor<TypeCategory::Real, 10> x) {
   return Floor<CppTypeFor<TypeCategory::Integer, 16>>(x);
@@ -502,7 +502,7 @@
     CppTypeFor<TypeCategory::Real, 16> x) {
   return Floor<CppTypeFor<TypeCategory::Integer, 8>>(x);
 }
-#ifdef __SIZEOF_INT128__
+#if defined __SIZEOF_INT128__ && !AVOID_NATIVE_UINT128_T
 CppTypeFor<TypeCategory::Integer, 16> RTNAME(Floor16_16)(
     CppTypeFor<TypeCategory::Real, 16> x) {
   return Floor<CppTypeFor<TypeCategory::Integer, 16>>(x);
@@ -710,7 +710,7 @@
     CppTypeFor<TypeCategory::Real, 4> x) {
   return Nint<CppTypeFor<TypeCategory::Integer, 8>>(x);
 }
-#ifdef __SIZEOF_INT128__
+#if defined __SIZEOF_INT128__ && !AVOID_NATIVE_UINT128_T
 CppTypeFor<TypeCategory::Integer, 16> RTNAME(Nint4_16)(
     CppTypeFor<TypeCategory::Real, 4> x) {
   return Nint<CppTypeFor<TypeCategory::Integer, 16>>(x);
@@ -732,7 +732,7 @@
     CppTypeFor<TypeCategory::Real, 8> x) {
   return Nint<CppTypeFor<TypeCategory::Integer, 8>>(x);
 }
-#ifdef __SIZEOF_INT128__
+#if defined __SIZEOF_INT128__ && !AVOID_NATIVE_UINT128_T
 CppTypeFor<TypeCategory::Integer, 16> RTNAME(Nint8_16)(
     CppTypeFor<TypeCategory::Real, 8> x) {
   return Nint<CppTypeFor<TypeCategory::Integer, 16>>(x);
@@ -755,7 +755,7 @@
     CppTypeFor<TypeCategory::Real, 10> x) {
   return Nint<CppTypeFor<TypeCategory::Integer, 8>>(x);
 }
-#ifdef __SIZEOF_INT128__
+#if defined __SIZEOF_INT128__ && !AVOID_NATIVE_UINT128_T
 CppTypeFor<TypeCategory::Integer, 16> RTNAME(Nint10_16)(
     CppTypeFor<TypeCategory::Real, 10> x) {
   return Nint<CppTypeFor<TypeCategory::Integer, 16>>(x);
@@ -778,7 +778,7 @@
     CppTypeFor<TypeCategory::Real, 16> x) {
   return Nint<CppTypeFor<TypeCategory::Integer, 8>>(x);
 }
-#ifdef __SIZEOF_INT128__
+#if defined __SIZEOF_INT128__ && !AVOID_NATIVE_UINT128_T
 CppTypeFor<TypeCategory::Integer, 16> RTNAME(Nint16_16)(
     CppTypeFor<TypeCategory::Real, 16> x) {
   return Nint<CppTypeFor<TypeCategory::Integer, 16>>(x);
Index: flang/runtime/tools.h
===================================================================
--- flang/runtime/tools.h
+++ flang/runtime/tools.h
@@ -131,7 +131,7 @@
       return FUNC<TypeCategory::Integer, 4>{}(std::forward<A>(x)...);
     case 8:
       return FUNC<TypeCategory::Integer, 8>{}(std::forward<A>(x)...);
-#ifdef __SIZEOF_INT128__
+#if defined __SIZEOF_INT128__ && !AVOID_NATIVE_UINT128_T
     case 16:
       return FUNC<TypeCategory::Integer, 16>{}(std::forward<A>(x)...);
 #endif
@@ -230,7 +230,7 @@
     return FUNC<4>{}(std::forward<A>(x)...);
   case 8:
     return FUNC<8>{}(std::forward<A>(x)...);
-#ifdef __SIZEOF_INT128__
+#if defined __SIZEOF_INT128__ && !AVOID_NATIVE_UINT128_T
   case 16:
     return FUNC<16>{}(std::forward<A>(x)...);
 #endif
@@ -310,6 +310,11 @@
       return std::make_pair(TypeCategory::Integer, maxKind);
     case TypeCategory::Real:
     case TypeCategory::Complex:
+#if !(defined __SIZEOF_INT128__ && !AVOID_NATIVE_UINT128_T)
+      if (xKind == 16) {
+        break;
+      }
+#endif
       return std::make_pair(yCat, yKind);
     default:
       break;
@@ -318,6 +323,11 @@
   case TypeCategory::Real:
     switch (yCat) {
     case TypeCategory::Integer:
+#if !(defined __SIZEOF_INT128__ && !AVOID_NATIVE_UINT128_T)
+      if (yKind == 16) {
+        break;
+      }
+#endif
       return std::make_pair(TypeCategory::Real, xKind);
     case TypeCategory::Real:
     case TypeCategory::Complex:
@@ -329,6 +339,11 @@
   case TypeCategory::Complex:
     switch (yCat) {
     case TypeCategory::Integer:
+#if !(defined __SIZEOF_INT128__ && !AVOID_NATIVE_UINT128_T)
+      if (yKind == 16) {
+        break;
+      }
+#endif
       return std::make_pair(TypeCategory::Complex, xKind);
     case TypeCategory::Real:
     case TypeCategory::Complex: