[flang] Handle more use cases reported for issues/78797

I implemented legacy "token pasting" via line continuation for

  call prefix&
    &MACRO&
    &suffix(1)

in a recent patch; this patch addresses the related cases

  call prefix&
    &MACRO&
    &(1)

and

  call &
    &MACRO&
    &suffix(1)

and

  call prefix&
    &MACRO

Fixes the latest #79590.

Author: klausler

flang/lib/Parser/prescan.cpp

@@ -630,22 +630,32 @@ bool Prescanner::NextToken(TokenSequence &tokens) {
     preventHollerith_ = false;
   } else if (IsLegalInIdentifier(*at_)) {
     int parts{1};
+    const char *afterLast{nullptr};
     do {
       EmitChar(tokens, *at_);
       ++at_, ++column_;
+      afterLast = at_;
       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.
+      // lines (or two continuation lines, immediately preceded or followed
+      // by '&' free form continuation line markers, 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 ((start > start_ && start[-1] == '&') ||
+          (afterLast < limit_ && (*afterLast == '&' || *afterLast == '\n'))) {
+        // call &                call foo&        call foo&
+        //   &MACRO&      OR       &MACRO&   OR     &MACRO
+        //   &foo(...)             &(...)
+      } else {
+        tokens.ReopenLastToken();
+      }
     }
     if (InFixedFormSource()) {
       SkipSpaces();

flang/test/Preprocessing/pp005.F

@@ -1,11 +1,11 @@
 ! RUN: %flang -E %s 2>&1 | FileCheck %s
-! CHECK: res = 777
+! CHECK: res = (777)
 * KWM split across continuation, implicit padding
       integer, parameter :: KWM = 666
 #define KWM 777
       integer :: res
-      res = KW
-     +M
+      res = (KW
+     +M)
       if (res .eq. 777) then
         print *, 'pp005.F yes'
       else

flang/test/Preprocessing/pp006.F

@@ -1,12 +1,12 @@
 ! RUN: %flang -E %s 2>&1 | FileCheck %s
-! CHECK: res = 777
+! CHECK: res = (777)
 * ditto, but with intervening *comment line
       integer, parameter :: KWM = 666
 #define KWM 777
       integer :: res
-      res = KW
+      res = (KW
 *comment
-     +M
+     +M)
       if (res .eq. 777) then
         print *, 'pp006.F yes'
       else

flang/test/Preprocessing/pp105.F90

@@ -1,11 +1,11 @@
 ! RUN: %flang -E %s 2>&1 | FileCheck %s
-! CHECK: res = 777
+! CHECK: res = (777)
 ! KWM call name split across continuation, with leading &
       integer, parameter :: KWM = 666
 #define KWM 777
       integer :: res
-      res = KW&
-&M
+      res = (KW&
+&M)
       if (res .eq. 777) then
         print *, 'pp105.F90 yes'
       else

flang/test/Preprocessing/pp106.F90

@@ -1,11 +1,11 @@
 ! RUN: %flang -E %s 2>&1 | FileCheck %s
-! CHECK: res = 777
+! CHECK: res = (777)
 ! ditto, with & ! comment
       integer, parameter :: KWM = 666
 #define KWM 777
       integer :: res
-      res = KW& ! comment
-&M
+      res = (KW& ! comment
+&M)
       if (res .eq. 777) then
         print *, 'pp106.F90 yes'
       else

flang/test/Preprocessing/pp134.F90

@@ -1,9 +1,23 @@
 ! RUN: %flang -E %s 2>&1 | FileCheck %s
-! CHECK: print *, ADC
+! CHECK: print *, ADC, 1
+! CHECK: print *, AD, 1
+! CHECK: print *, DC, 1
+! CHECK: print *, AD
+! CHECK: print *, AB
 #define B D
 implicit none
 real ADC
 print *, A&
   &B&
-  &C
+  &C, 1
+print *, A&
+  &B&
+  &, 1
+print *, &
+  &B&
+  &C, 1
+print *, A&
+  &B
+print *, A&
+  &B ! but not this
 end