[flang] add a static assert in CheckUnitNumberInRangeImpl

Add a check that CheckUnitNumberInRangeImpl is not needlessly instantiated.

Differential Revision: https://reviews.llvm.org/D123285

flang/runtime/io-api.cpp

@@ -1280,6 +1280,9 @@ template <typename INT>
 static enum Iostat CheckUnitNumberInRangeImpl(INT unit, bool handleError,
     char *ioMsg, std::size_t ioMsgLength, const char *sourceFile,
     int sourceLine) {
+  static_assert(sizeof(INT) >= sizeof(ExternalUnit),
+      "only intended to be used when the INT to ExternalUnit conversion is "
+      "narrowing");
   if (unit != static_cast<ExternalUnit>(unit)) {
     Terminator oom{sourceFile, sourceLine};
     IoErrorHandler errorHandler{oom};