[flang] Refine checks for NULL() in expressions

Fix a false positive "NULL can't be an operand here" error message
arising in a defined generic interface for an intrinsic operator (==)
with multiple spellings.

Fixes #163255.

Author: klausler

flang/lib/Semantics/expression.cpp

@@ -176,8 +176,8 @@ class ArgumentAnalyzer {
 
   // Find and return a user-defined operator or report an error.
   // The provided message is used if there is no such operator.
-  MaybeExpr TryDefinedOp(
-      const char *, parser::MessageFixedText, bool isUserOp = false);
+  MaybeExpr TryDefinedOp(const char *, parser::MessageFixedText,
+      bool isUserOp = false, bool checkForNullPointer = true);
   template <typename E>
   MaybeExpr TryDefinedOp(E opr, parser::MessageFixedText msg) {
     return TryDefinedOp(
@@ -211,7 +211,8 @@ class ArgumentAnalyzer {
   void SayNoMatch(
       const std::string &, bool isAssignment = false, bool isAmbiguous = false);
   std::string TypeAsFortran(std::size_t);
-  bool AnyUntypedOrMissingOperand() const;
+  bool AnyUntypedOperand() const;
+  bool AnyMissingOperand() const;
 
   ExpressionAnalyzer &context_;
   ActualArguments actuals_;
@@ -3788,7 +3789,7 @@ MaybeExpr ExpressionAnalyzer::Analyze(const parser::Expr::DefinedUnary &x) {
   ArgumentAnalyzer analyzer{*this, name.source};
   analyzer.Analyze(std::get<1>(x.t));
   return analyzer.TryDefinedOp(name.source.ToString().c_str(),
-      "No operator %s defined for %s"_err_en_US, true);
+      "No operator %s defined for %s"_err_en_US, /*isUserOp=*/true);
 }
 
 // Binary (dyadic) operations
@@ -4176,15 +4177,23 @@ MaybeExpr ExpressionAnalyzer::IterativelyAnalyzeSubexpressions(
   } while (!queue.empty());
   // Analyze the collected subexpressions in bottom-up order.
   // On an error, bail out and leave partial results in place.
-  MaybeExpr result;
-  for (auto riter{finish.rbegin()}; riter != finish.rend(); ++riter) {
-    const parser::Expr &expr{**riter};
-    result = ExprOrVariable(expr, expr.source);
-    if (!result) {
-      return result;
+  if (finish.size() == 1) {
+    const parser::Expr &expr{DEREF(finish.front())};
+    return ExprOrVariable(expr, expr.source);
+  } else {
+    // NULL() operand catching is deferred to operation analysis so
+    // that they can be accepted by defined operators.
+    auto restorer{AllowNullPointer()};
+    MaybeExpr result;
+    for (auto riter{finish.rbegin()}; riter != finish.rend(); ++riter) {
+      const parser::Expr &expr{**riter};
+      result = ExprOrVariable(expr, expr.source);
+      if (!result) {
+        return result;
+      }
     }
+    return result; // last value was from analysis of "top"
   }
-  return result; // last value was from analysis of "top"
 }
 
 MaybeExpr ExpressionAnalyzer::Analyze(const parser::Expr &expr) {
@@ -4681,7 +4690,7 @@ bool ArgumentAnalyzer::AnyCUDADeviceData() const {
 // attribute.
 bool ArgumentAnalyzer::HasDeviceDefinedIntrinsicOpOverride(
     const char *opr) const {
-  if (AnyCUDADeviceData() && !AnyUntypedOrMissingOperand()) {
+  if (AnyCUDADeviceData() && !AnyUntypedOperand() && !AnyMissingOperand()) {
     std::string oprNameString{"operator("s + opr + ')'};
     parser::CharBlock oprName{oprNameString};
     parser::Messages buffer;
@@ -4709,9 +4718,9 @@ bool ArgumentAnalyzer::HasDeviceDefinedIntrinsicOpOverride(
   return false;
 }
 
-MaybeExpr ArgumentAnalyzer::TryDefinedOp(
-    const char *opr, parser::MessageFixedText error, bool isUserOp) {
-  if (AnyUntypedOrMissingOperand()) {
+MaybeExpr ArgumentAnalyzer::TryDefinedOp(const char *opr,
+    parser::MessageFixedText error, bool isUserOp, bool checkForNullPointer) {
+  if (AnyMissingOperand()) {
     context_.Say(error, ToUpperCase(opr), TypeAsFortran(0), TypeAsFortran(1));
     return std::nullopt;
   }
@@ -4790,7 +4799,9 @@ MaybeExpr ArgumentAnalyzer::TryDefinedOp(
     context_.Say(
         "Operands of %s are not conformable; have rank %d and rank %d"_err_en_US,
         ToUpperCase(opr), actuals_[0]->Rank(), actuals_[1]->Rank());
-  } else if (CheckForNullPointer() && CheckForAssumedRank()) {
+  } else if (!CheckForAssumedRank()) {
+  } else if (checkForNullPointer && !CheckForNullPointer()) {
+  } else { // use the supplied error
     context_.Say(error, ToUpperCase(opr), TypeAsFortran(0), TypeAsFortran(1));
   }
   return result;
@@ -4808,15 +4819,16 @@ MaybeExpr ArgumentAnalyzer::TryDefinedOp(
     for (std::size_t i{0}; i < oprs.size(); ++i) {
       parser::Messages buffer;
       auto restorer{context_.GetContextualMessages().SetMessages(buffer)};
-      if (MaybeExpr thisResult{TryDefinedOp(oprs[i], error)}) {
+      if (MaybeExpr thisResult{TryDefinedOp(oprs[i], error, /*isUserOp=*/false,
+              /*checkForNullPointer=*/false)}) {
         result = std::move(thisResult);
         hit.push_back(oprs[i]);
         hitBuffer = std::move(buffer);
       }
     }
   }
-  if (hit.empty()) { // for the error
-    result = TryDefinedOp(oprs[0], error);
+  if (hit.empty()) { // run TryDefinedOp() again just to emit errors
+    CHECK(!TryDefinedOp(oprs[0], error).has_value());
   } else if (hit.size() > 1) {
     context_.Say(
         "Matching accessible definitions were found with %zd variant spellings of the generic operator ('%s', '%s')"_err_en_US,
@@ -5232,10 +5244,19 @@ std::string ArgumentAnalyzer::TypeAsFortran(std::size_t i) {
   }
 }
 
-bool ArgumentAnalyzer::AnyUntypedOrMissingOperand() const {
+bool ArgumentAnalyzer::AnyUntypedOperand() const {
+  for (const auto &actual : actuals_) {
+    if (actual && !actual->GetType() &&
+        !IsBareNullPointer(actual->UnwrapExpr())) {
+      return true;
+    }
+  }
+  return false;
+}
+
+bool ArgumentAnalyzer::AnyMissingOperand() const {
   for (const auto &actual : actuals_) {
-    if (!actual ||
-        (!actual->GetType() && !IsBareNullPointer(actual->UnwrapExpr()))) {
+    if (!actual) {
       return true;
     }
   }

flang/test/Semantics/bug163255.f90

@@ -0,0 +1,21 @@
+!RUN: %flang_fc1 -fdebug-unparse %s | FileCheck %s
+module m
+  type t
+  end type
+  interface operator (==)
+    module procedure equal
+  end interface
+ contains
+  logical function equal(b1, b2)
+    class(t), pointer, intent(in) :: b1, b2
+    equal = associated(b1, b2)
+  end
+end module
+
+program test
+  use m
+  type(t), target :: target
+  class(t), pointer :: p => target
+  !CHECK: IF (equal(p,null(p))) STOP
+  if (p == null(p)) stop
+end

flang/test/Semantics/resolve63.f90

@@ -165,13 +165,12 @@ subroutine s1(x, y)
     logical :: l
     complex :: z
     y = y + z'1'  !OK
-    !ERROR: Operands of + must be numeric; have untyped and COMPLEX(4)
+    !ERROR: No intrinsic or user-defined OPERATOR(+) matches operand types untyped and COMPLEX(4)
     z = z'1' + z
     y = +z'1'  !OK
     !ERROR: Operand of unary - must be numeric; have untyped
     y = -z'1'
-    !ERROR: Operands of + must be numeric; have LOGICAL(4) and untyped
-    y = x + z'1'
+    y = x + z'1' ! matches "add" with conversion of untyped to integer
     !ERROR: A NULL() pointer is not allowed as an operand here
     l = x /= null()
     !ERROR: A NULL() pointer is not allowed as a relational operand