[flang] Fix source line continuation in potential macro calls (bugzilla 46768)

The prescanner looks for implicit continuation lines when
there are unclosed parentheses at the end of a line, so that
source preprocessing macro references with arguments that span
lines are recognized.  The condition that determines this
implicit continuation has been put into a predicate member
function and corrected to apply only when the following line
is source (not a preprocessing directive, comment, &c.).

Fixes bugzilla #46768.

Reviewed By: sscalpone

Differential Revision: https://reviews.llvm.org/D84280

flang/lib/Parser/prescan.cpp

@@ -887,10 +887,8 @@ const char *Prescanner::FixedFormContinuationLine(bool mightNeedSpace) {
         return nextLine_ + 6;
       }
     }
-    if (delimiterNesting_ > 0) {
-      if (!IsFixedFormCommentChar(col1)) {
-        return nextLine_;
-      }
+    if (IsImplicitContinuation()) {
+      return nextLine_;
     }
   }
   return nullptr; // not a continuation line
@@ -927,7 +925,7 @@ const char *Prescanner::FreeFormContinuationLine(bool ampersand) {
       return p + 1;
     } else if (*p == '!' || *p == '\n' || *p == '#') {
       return nullptr;
-    } else if (ampersand || delimiterNesting_ > 0) {
+    } else if (ampersand || IsImplicitContinuation()) {
       if (p > nextLine_) {
         --p;
       } else {
@@ -981,6 +979,14 @@ bool Prescanner::FreeFormContinuation() {
   return false;
 }
 
+// Implicit line continuation allows a preprocessor macro call with
+// arguments to span multiple lines.
+bool Prescanner::IsImplicitContinuation() const {
+  return !inPreprocessorDirective_ && !inCharLiteral_ &&
+      delimiterNesting_ > 0 && nextLine_ < limit_ &&
+      ClassifyLine(nextLine_).kind == LineClassification::Kind::Source;
+}
+
 bool Prescanner::Continuation(bool mightNeedFixedFormSpace) {
   if (*at_ == '\n' || *at_ == '&') {
     if (inFixedForm_) {

flang/lib/Parser/prescan.h

@@ -166,6 +166,7 @@ private:
   const char *IsPreprocessorDirectiveLine(const char *) const;
   const char *FixedFormContinuationLine(bool mightNeedSpace);
   const char *FreeFormContinuationLine(bool ampersand);
+  bool IsImplicitContinuation() const;
   bool FixedFormContinuation(bool mightNeedSpace);
   bool FreeFormContinuation();
   bool Continuation(bool mightNeedFixedFormSpace);

flang/test/Parser/continuation-in-if.f

@@ -0,0 +1,9 @@
+! RUN: %f18 -funparse %s 2>&1 | FileCheck %s
+! CHECK: CALL foo("N","N")
+#ifdef transpose
+      call foo('T',
+#else
+      call foo('N',
+#endif
+     $     'N')
+      end