[flang][runtime] Don't skip input spaces when they are significant

[klausler]

Summary

When formatted input (not list-directed or NAMELIST) is in "BZ" mode,
either because a BZ control edit descriptor appeared in a FORMAT or
BLANK="ZERO" appeared in OPEN or READ, input editing must not skip
over blanks before or within the input field.

flang/include/flang/Runtime/iostat.h

   IostatBadUnformattedRecord,
   IostatUTF8Decoding,
   IostatUnitOverflow,
+  IostatBadRealInput,
 };
 
 const char *IostatErrorString(int);

flang/runtime/edit-input.cpp

 // Returns true if there's a '-' sign.
 static bool ScanNumericPrefix(IoStatementState &io, const DataEdit &edit,
     std::optional<char32_t> &next, std::optional<int> &remaining) {
-  next = io.PrepareInput(edit, remaining);
+  bool bzMode{(edit.modes.editingFlags & blankZero) != 0};
+  next = io.PrepareInput(edit, remaining, !bzMode);
   bool negative{false};
   if (next) {
     negative = *next == '-';
     if (negative || *next == '+') {
-      io.SkipSpaces(remaining);
+      if (!bzMode) {
+        io.SkipSpaces(remaining);
+      }
       next = io.NextInField(remaining, edit);
     }
   }

   if (ScanNumericPrefix(io, edit, next, remaining)) {
     Put('-');
   }
-  if (next.value_or(' ') == ' ') { // empty/blank field means zero
+  bool bzMode{(edit.modes.editingFlags & blankZero) != 0};
+  if (!next || (!bzMode && *next == ' ')) { // empty/blank field means zero
     remaining.reset();
     if (!io.GetConnectionState().IsAtEOF()) {
       Put('0');

     }
     exponent = 0;
   } else if (first == decimal || (first >= '0' && first <= '9') ||
-      first == 'E' || first == 'D' || first == 'Q') {
+      (bzMode && (first == ' ' || first == '\t')) || first == 'E' ||
+      first == 'D' || first == 'Q') {
     Put('.'); // input field is normalized to a fraction
     auto start{got};
-    bool bzMode{(edit.modes.editingFlags & blankZero) != 0};
+    bool anyDigit{false};
     for (; next; next = io.NextInField(remaining, edit)) {
       char32_t ch{*next};
       if (ch == ' ' || ch == '\t') {

         }
       }
       if (ch == '0' && got == start && !decimalPoint) {
+        anyDigit = true;
         // omit leading zeroes before the decimal
       } else if (ch >= '0' && ch <= '9') {
+        anyDigit = true;
         Put(ch);
       } else if (ch == decimal && !decimalPoint) {
         // the decimal point is *not* copied to the buffer

       }
     }
     if (got == start) {
-      Put('0'); // emit at least one digit
+      if (anyDigit) {
+        Put('0'); // emit at least one digit
+      } else {
+        return 0; // no digits, invalid input
+      }
     }
     if (next &&
         (*next == 'e' || *next == 'E' || *next == 'd' || *next == 'D' ||

     return false;
   }
   if (got == 0) {
-    io.GetIoErrorHandler().SignalError("Bad REAL input value");
+    io.GetIoErrorHandler().SignalError(IostatBadRealInput);
     return false;
   }
   bool hadExtra{got > maxDigits};

flang/runtime/io-stmt.h

   // For fixed-width fields, initialize the number of remaining characters.
   // Skip over leading blanks, then return the first non-blank character (if
   // any).
-  std::optional<char32_t> PrepareInput(
-      const DataEdit &edit, std::optional<int> &remaining) {
+  std::optional<char32_t> PrepareInput(const DataEdit &edit,
+      std::optional<int> &remaining, bool skipSpaces = true) {
     remaining.reset();
     if (edit.descriptor == DataEdit::ListDirected) {
       std::size_t byteCount{0};

       if (edit.width.value_or(0) > 0) {
         remaining = *edit.width;
       }
-      SkipSpaces(remaining);
+      if (skipSpaces) {
+        SkipSpaces(remaining);
+      }
     }
     return NextInField(remaining, edit);
   }

flang/runtime/iostat.cpp

     return "UTF-8 decoding error";
   case IostatUnitOverflow:
     return "UNIT number is out of range";
+  case IostatBadRealInput:
+    return "Bad REAL input value";
   default:
     return nullptr;
   }

flang/unittests/Runtime/NumericalFormatTest.cpp

       {"(1P,F18.0)", "               125", 0x4029000000000000}, // 12.5
       {"(BZ,F18.0)", "              125 ", 0x4093880000000000}, // 1250
       {"(BZ,F18.0)", "       125 . e +1 ", 0x42a6bcc41e900000}, // 1.25e13
+      {"(BZ,F18.0)", "           .      ", 0x0},
+      {"(BZ,F18.0)", "           . e +1 ", 0x0},
       {"(DC,F18.0)", "              12,5", 0x4029000000000000},
   };
   for (auto const &[format, data, want] : testCases) {