[flang] Accomodate historic preprocessing usage

flang/lib/Parser/prescan.cpp

@@ -438,7 +438,8 @@ void Prescanner::NextChar() {
 // character is reached; handles C-style comments in preprocessing
 // directives, Fortran ! comments, stuff after the right margin in
 // fixed form, and all forms of line continuation.
-void Prescanner::SkipToNextSignificantCharacter() {
+bool Prescanner::SkipToNextSignificantCharacter() {
+  auto anyContinuationLine{false};
   if (inPreprocessorDirective_) {
     SkipCComments();
   } else {
@@ -449,6 +450,7 @@ void Prescanner::SkipToNextSignificantCharacter() {
       mightNeedSpace = *at_ == '\n';
     }
     for (; Continuation(mightNeedSpace); mightNeedSpace = false) {
+      anyContinuationLine = true;
       ++continuationLines_;
       if (MustSkipToEndOfLine()) {
         SkipToEndOfLine();
@@ -458,6 +460,7 @@ void Prescanner::SkipToNextSignificantCharacter() {
       tabInCurrentLine_ = true;
     }
   }
+  return anyContinuationLine;
 }
 
 void Prescanner::SkipCComments() {
@@ -625,7 +628,23 @@ bool Prescanner::NextToken(TokenSequence &tokens) {
     }
     preventHollerith_ = false;
   } else if (IsLegalInIdentifier(*at_)) {
-    while (IsLegalInIdentifier(EmitCharAndAdvance(tokens, *at_))) {
+    int parts{1};
+    do {
+      EmitChar(tokens, *at_);
+      ++at_, ++column_;
+      if (SkipToNextSignificantCharacter() && IsLegalIdentifierStart(*at_)) {
+        tokens.CloseToken();
+        ++parts;
+      }
+    } while (IsLegalInIdentifier(*at_));
+    if (parts >= 3) {
+      // Subtlety: When an identifier is split across three or more continuation
+      // lines, its parts are kept as distinct pp-tokens so that macro
+      // operates on them independently.  This trick accommodates the historic
+      // practice of using line continuation for token pasting after
+      // replacement.
+    } else if (parts == 2) {
+      tokens.ReopenLastToken();
     }
     if (InFixedFormSource()) {
       SkipSpaces();

flang/lib/Parser/prescan.h

@@ -159,7 +159,8 @@ class Prescanner {
   void SkipToEndOfLine();
   bool MustSkipToEndOfLine() const;
   void NextChar();
-  void SkipToNextSignificantCharacter();
+  // True when input flowed to a continuation line
+  bool SkipToNextSignificantCharacter();
   void SkipCComments();
   void SkipSpaces();
   static const char *SkipWhiteSpace(const char *);

flang/test/Preprocessing/pp133.F90

@@ -0,0 +1,9 @@
+! RUN: %flang -E %s 2>&1 | FileCheck %s
+! CHECK: print *, ADC
+#define B D
+implicit none
+real ADC
+print *, A&
+  &B&
+  &C
+end