[flang] Check for misplaced labels

[klausler]

Summary

In fixed form source, complain when a label digit appears
outside the label field & when a non-digit appears in the label
field.

flang/lib/Parser/prescan.h

private:
   // line in an include file.
   bool omitNewline_{false};
   bool skipLeadingAmpersand_{false};
 
   const Provenance spaceProvenance_{
       cooked_.allSources().CompilerInsertionProvenance(' ')};
   const Provenance backslashProvenance_{
       cooked_.allSources().CompilerInsertionProvenance('\\')};
-  const ProvenanceRange sixSpaceProvenance_{
-      cooked_.allSources().AddCompilerInsertion("      "s)};
 
   // To avoid probing the set of active compiler directive sentinel strings
   // on every comment line, they're checked first with a cheap Bloom filter.
   static const int prime1{1019}, prime2{1021};
   std::bitset<prime2> compilerDirectiveBloomFilter_; // 128 bytes
   std::unordered_set<std::string> compilerDirectiveSentinels_;
 };
 } // namespace Fortran::parser
 #endif // FORTRAN_PARSER_PRESCAN_H_

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, int outCol) {
+  bool badLabel{false};
   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_);
+      if (!IsDecimalDigit(*at_) && !badLabel) {
+        Say(GetProvenance(at_),
+            "Character in fixed-form label field must be a digit"_en_US);
+        badLabel = true;
+      }
       ++outCol;
     }
     ++column_;
   }
   if (outCol > 1) {
     token.CloseToken();
   }
-  if (outCol < 7) {
-    if (outCol == 1) {
-      token.Put("      ", 6, sixSpaceProvenance_.start());
-    } else {
-      for (; outCol < 7; ++outCol) {
-        token.PutNextTokenChar(' ', spaceProvenance_);
-      }
-      token.CloseToken();
-    }
-  }
   SkipToNextSignificantCharacter();
+  if (IsDecimalDigit(*at_)) {
+    Say(GetProvenance(at_),
+        "Label digit is not in fixed-form label field"_en_US);
+  }
 }
 
 void Prescanner::SkipToEndOfLine() {
   while (*at_ != '\n') {
     ++at_, ++column_;
   }
 }
 

flang/test/Parser/badlabel.f

+! RUN: %f18 -E %s 2>&1 | FileCheck %s
+! CHECK: Label digit is not in fixed-form label field
+      1 continue
+! CHECK: Label digit is not in fixed-form label field
+ 1    2 continue
+! CHECK-NOT: Label is not in fixed-form label field
+      con
+     3 tinue
+! CHECK: Character in fixed-form label field must be a digit
+end
+! CHECK: 1continue
+! CHECK: 12continue
+! CHECK: continue
+! CHECK: end