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

flang/include/flang/Parser/char-block.h

@@ -46,6 +46,8 @@ class CharBlock {
   constexpr const char &operator[](std::size_t j) const {
     return interval_.start()[j];
   }
+  constexpr const char &front() const { return (*this)[0]; }
+  constexpr const char &back() const { return (*this)[size() - 1]; }
 
   bool Contains(const CharBlock &that) const {
     return interval_.Contains(that.interval_);

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->back()};
+      char first{after.front()};
+      // 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/lib/Parser/unparse.cpp

@@ -778,7 +778,7 @@ class UnparseVisitor {
   }
   void Unparse(const SubstringInquiry &x) {
     Walk(x.v);
-    Put(x.source.end()[-1] == 'n' ? "%LEN" : "%KIND");
+    Put(x.source.back() == 'n' ? "%LEN" : "%KIND");
   }
   void Unparse(const SubstringRange &x) { // R910
     Walk(x.t, ":");

flang/lib/Semantics/expression.cpp

@@ -1269,7 +1269,7 @@ MaybeExpr ExpressionAnalyzer::Analyze(
 MaybeExpr ExpressionAnalyzer::Analyze(const parser::SubstringInquiry &x) {
   if (MaybeExpr substring{Analyze(x.v)}) {
     CHECK(x.source.size() >= 8);
-    int nameLen{x.source.end()[-1] == 'n' ? 3 /*LEN*/ : 4 /*KIND*/};
+    int nameLen{x.source.back() == 'n' ? 3 /*LEN*/ : 4 /*KIND*/};
     parser::CharBlock name{
         x.source.end() - nameLen, static_cast<std::size_t>(nameLen)};
     CHECK(name == "len" || name == "kind");

flang/lib/Semantics/resolve-names.cpp

@@ -2139,7 +2139,7 @@ bool ImplicitRules::isImplicitNoneExternal() const {
 
 const DeclTypeSpec *ImplicitRules::GetType(
     SourceName name, bool respectImplicitNoneType) const {
-  char ch{name.begin()[0]};
+  char ch{name.front()};
   if (isImplicitNoneType_ && respectImplicitNoneType) {
     return nullptr;
   } else if (auto it{map_.find(ch)}; it != map_.end()) {

flang/lib/Semantics/tools.cpp

@@ -1683,7 +1683,7 @@ std::forward_list<std::string> GetOperatorNames(
 std::forward_list<std::string> GetAllNames(
     const SemanticsContext &context, const SourceName &name) {
   std::string str{name.ToString()};
-  if (!name.empty() && name.end()[-1] == ')' &&
+  if (!name.empty() && name.back() == ')' &&
       name.ToString().rfind("operator(", 0) == 0) {
     for (int i{0}; i != common::LogicalOperator_enumSize; ++i) {
       auto names{GetOperatorNames(context, common::LogicalOperator{i})};

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)