[flang] Don't create bogus tokens from token pasting (##)

When blank tokens arise from macro replacement in token sequences
with token pasting (##), the preprocessor is producing some bogus
tokens (e.g., "name(") that can lead to subtle bugs later when
macro names are not recognized as such.

The fix is to not paste tokens together when the result would not
be a valid Fortran or C token in the preprocessing context.

Author: klausler

flang/lib/Parser/preprocessor.cpp

@@ -156,23 +156,50 @@ static TokenSequence TokenPasting(TokenSequence &&text) {
   }
   TokenSequence result;
   std::size_t tokens{text.SizeInTokens()};
-  bool pasting{false};
+  std::optional<CharBlock> before; // last non-blank token before ##
   for (std::size_t j{0}; j < tokens; ++j) {
-    if (IsTokenPasting(text.TokenAt(j))) {
-      if (!pasting) {
+    CharBlock after{text.TokenAt(j)};
+    if (!before) {
+      if (IsTokenPasting(after)) {
         while (!result.empty() &&
             result.TokenAt(result.SizeInTokens() - 1).IsBlank()) {
           result.pop_back();
         }
         if (!result.empty()) {
-          result.ReopenLastToken();
-          pasting = true;
+          before = result.TokenAt(result.SizeInTokens() - 1);
         }
+      } else {
+        result.AppendRange(text, j, 1);
+      }
+    } else if (after.IsBlank() || IsTokenPasting(after)) {
+      // drop it
+    } else { // pasting before ## after
+      bool doPaste{false};
+      char last{before->end()[-1]};
+      char first{after.begin()[0]};
+      // Apply basic sanity checking to pasting so avoid constructing a bogus
+      // token that might cause macro replacement to fail, like "macro(".
+      if (IsLegalInIdentifier(last) && IsLegalInIdentifier(first)) {
+        doPaste = true;
+      } else if (IsDecimalDigit(first) &&
+          (last == '.' || last == '+' || last == '-')) {
+        doPaste = true; // 1. ## 0, - ## 1
+      } else if (before->size() == 1 && after.size() == 1) {
+        if (first == last &&
+            (last == '<' || last == '>' || last == '*' || last == '/' ||
+                last == '=' || last == '&' || last == '|' || last == ':')) {
+          // Fortran **, //, ==, ::
+          // C <<, >>, &&, || for use in #if expressions
+          doPaste = true;
+        } else if (first == '=' && (last == '!' || last == '/')) {
+          doPaste = true; // != and /=
+        }
+      }
+      if (doPaste) {
+        result.ReopenLastToken();
       }
-    } else if (pasting && text.TokenAt(j).IsBlank()) {
-    } else {
       result.AppendRange(text, j, 1);
-      pasting = false;
+      before.reset();
     }
   }
   return result;

flang/test/Preprocessing/bug1126.F90

@@ -0,0 +1,20 @@
+! RUN: %flang -E %s 2>&1 | FileCheck %s
+#define STRINGIFY(x) #x
+#define TOSTRING(x) STRINGIFY(x)
+#define PREFIX(x) prefix ## x
+#define NAME(x) PREFIX(foo ## x)
+#define AUGMENT(x) NAME(x ## suffix)
+
+! CHECK: subroutine prefixfoosuffix()
+! CHECK: print *, "prefixfoosuffix"
+! CHECK: end subroutine prefixfoosuffix
+subroutine AUGMENT()()
+  print *, TOSTRING(AUGMENT())
+end subroutine AUGMENT()
+
+! CHECK: subroutine prefixfoobarsuffix()
+! CHECK: print *, "prefixfoobarsuffix"
+! CHECK: end subroutine prefixfoobarsuffix
+subroutine AUGMENT(bar)()
+  print *, TOSTRING(AUGMENT(bar))
+end subroutine AUGMENT(bar)