[flang][runtime] EOF is recoverable only with END= or IOSTAT=

[klausler]

Summary

The runtime's I/O error handler was not crashing the program on an
end-of-file condition that arises for a data transfer statement with
at least one of ERR= or IOMSG= and none of END= or IOSTAT= control
items. This turns out to be incorrect (per subclause 12.11);
an EOF is recoverable only for END= and/or IOSTAT=, and an
non-advancing end-of-record is recoverable only for EOR= and/or IOSTAT=.

flang/runtime/io-error.cpp

 #include <cerrno>
 #include <cstdarg>
 #include <cstdio>
 #include <cstring>
 
 namespace Fortran::runtime::io {
 
 void IoErrorHandler::SignalError(int iostatOrErrno, const char *msg, ...) {
-  if (iostatOrErrno == IostatEnd && (flags_ & hasEnd)) {
+  // Note that IOMSG= alone without IOSTAT=/END=/EOR=/ERR= does not suffice
+  // for error recovery (see F'2018 subclause 12.11).
+  switch (iostatOrErrno) {
+  case IostatOk:
+    return;
+  case IostatEnd:
+    if (flags_ & (hasIoStat | hasEnd)) {
       if (ioStat_ == IostatOk || ioStat_ < IostatEnd) {
         ioStat_ = IostatEnd;
       }
-  } else if (iostatOrErrno == IostatEor && (flags_ & hasEor)) {
+      return;
+    }
+    break;
+  case IostatEor:
+    if (flags_ & (hasIoStat | hasEor)) {
       if (ioStat_ == IostatOk || ioStat_ < IostatEor) {
         ioStat_ = IostatEor; // least priority
       }
-  } else if (iostatOrErrno != IostatOk) {
-    if (flags_ & (hasIoStat | hasIoMsg | hasErr)) {
+      return;
+    }
+    break;
+  default:
+    if (flags_ & (hasIoStat | hasErr)) {
       if (ioStat_ <= 0) {
         ioStat_ = iostatOrErrno; // priority over END=/EOR=
         if (msg && (flags_ & hasIoMsg)) {
           char buffer[256];
           va_list ap;
           va_start(ap, msg);
           std::vsnprintf(buffer, sizeof buffer, msg, ap);
           ioMsg_ = SaveDefaultCharacter(buffer, std::strlen(buffer) + 1, *this);
           va_end(ap);
         }
       }
-    } else if (msg) {
-      va_list ap;
-      va_start(ap, msg);
-      CrashArgs(msg, ap);
-      va_end(ap);
-    } else if (const char *errstr{IostatErrorString(iostatOrErrno)}) {
-      Crash(errstr);
-    } else {
-      Crash("I/O error (errno=%d): %s", iostatOrErrno,
-          std::strerror(iostatOrErrno));
-    }
-  }
-}
+      return;
+    }
+    break;
+  }
+  // I/O error not caught!
+  if (msg) {
 
 void IoErrorHandler::SignalError(int iostatOrErrno) {
   SignalError(iostatOrErrno, nullptr);
 }
 
 void IoErrorHandler::Forward(
     int ioStatOrErrno, const char *msg, std::size_t length) {
   if (ioStatOrErrno != IostatOk) {