[flang] Diagnose fixed form statement that begins with continuation line

[klausler]

Summary

A fixed form continuation line should not be the first line of a statement;
emit a warning if it looks like one is so that the programmer can look
for a missing card.

flang/lib/Parser/prescan.cpp

if (!v) {
     nextLine_ = limit_;
   } else {
     const char *nl{const_cast<const char *>(static_cast<char *>(v))};
     nextLine_ = nl + 1;
   }
 }
 
 void Prescanner::LabelField(TokenSequence &token) {
-  const char *bad{nullptr};
   int outCol{1};
   const char *start{at_};
+  std::optional<int> badColumn;
   for (; *at_ != '\n' && column_ <= 6; ++at_) {
     if (*at_ == '\t') {
       ++at_;
       column_ = 7;
       break;
     }
     if (*at_ != ' ' &&
         !(*at_ == '0' && column_ == 6)) { // '0' in column 6 becomes space
       EmitChar(token, *at_);
       ++outCol;
-      if (!bad && !IsDecimalDigit(*at_)) {
-        bad = at_;
+      if (!badColumn && (column_ == 6 || !IsDecimalDigit(*at_))) {
+        badColumn = column_;
       }
     }
     ++column_;
   }
-  if (bad && !preprocessor_.IsNameDefined(token.CurrentOpenToken())) {
-    Say(GetProvenance(bad),
-        "Character in fixed-form label field must be a digit"_warn_en_US);
+  if (badColumn && !preprocessor_.IsNameDefined(token.CurrentOpenToken())) {
+    Say(GetProvenance(start + *badColumn - 1),
+        *badColumn == 6
+            ? "Statement should not begin with a continuation line"_warn_en_US
+            : "Character in fixed-form label field must be a digit"_warn_en_US);
     token.clear();
+    if (*badColumn < 6) {
       at_ = start;
+      column_ = 1;
       return;
     }
+    outCol = 1;
+  }
   if (outCol == 1) { // empty label field
     // Emit a space so that, if the line is rescanned after preprocessing,
     // a leading 'C' or 'D' won't be left-justified and then accidentally
     // misinterpreted as a comment card.
     EmitChar(token, ' ');
     ++outCol;
   }
   token.CloseToken();

flang/test/Preprocessing/pp044.F

 ! RUN: %flang -E %s 2>&1 | FileCheck %s
 ! CHECK-NOT:z = 111
-* #define directive amid continuations
-      integer, parameter :: KWM = 222, KWM111 = 333, KWM222 = 555
-      integer, parameter :: KWMKWM = 333
+! CHECK:warning: Statement should not begin with a continuation line
+! CHECK:j=111+444
+* #define directive amid continuations.
+      integer, parameter :: KWM = 222
       integer, parameter :: z = KWM
 #define KWM 111
-     +KWM+444
-      if (z .EQ. 777) then
+     ,j=KWM+444
+      if (z .EQ. 222 .AND. j .EQ. 555) then
         print *, 'yes'
       else
-        print *, 'no', z
+        print *, 'no', z, _4
       end if
       end