[flang] remove sequences of duplicate messages (#161916)

Fixes bug exposed by #161915 by keeping a cache of messages printed at a given location.

Author: akuhlens

flang/include/flang/Parser/message.h

@@ -307,9 +307,9 @@ class Message : public common::ReferenceCounted<Message> {
   bool Merge(const Message &);
   bool operator==(const Message &that) const;
   bool operator!=(const Message &that) const { return !(*this == that); }
+  bool AtSameLocation(const Message &) const;
 
 private:
-  bool AtSameLocation(const Message &) const;
   std::variant<ProvenanceRange, CharBlock> location_;
   std::variant<MessageFixedText, MessageFormattedText, MessageExpectedText>
       text_;

flang/lib/Parser/message.cpp

@@ -477,15 +477,31 @@ void Messages::Emit(llvm::raw_ostream &o, const AllCookedSources &allCooked,
   }
   std::stable_sort(sorted.begin(), sorted.end(),
       [](const Message *x, const Message *y) { return x->SortBefore(*y); });
-  const Message *lastMsg{nullptr};
+  std::vector<const Message *> msgsWithLastLocation;
   std::size_t errorsEmitted{0};
   for (const Message *msg : sorted) {
-    if (lastMsg && *msg == *lastMsg) {
-      // Don't emit two identical messages for the same location
+    bool shouldSkipMsg{false};
+    // Don't emit two identical messages for the same location.
+    // At the same location, messages are sorted by the order they were
+    // added to the Messages buffer, which is a decent proxy for the
+    // causality of the messages.
+    if (!msgsWithLastLocation.empty()) {
+      if (msgsWithLastLocation[0]->AtSameLocation(*msg)) {
+        for (const Message *msgAtThisLocation : msgsWithLastLocation) {
+          if (*msg == *msgAtThisLocation) {
+            shouldSkipMsg = true; // continue loop over sorted messages
+            break;
+          }
+        }
+      } else {
+        msgsWithLastLocation.clear();
+      }
+    }
+    if (shouldSkipMsg) {
       continue;
     }
+    msgsWithLastLocation.push_back(msg);
     msg->Emit(o, allCooked, echoSourceLines, hintFlagPtr);
-    lastMsg = msg;
     if (warningsAreErrors || msg->IsFatal()) {
       ++errorsEmitted;
     }

flang/test/Semantics/associated.f90

@@ -253,8 +253,6 @@ subroutine test(assumedRank)
     lvar = associated(intPointerVar1, targetIntCoarray[1])
     !ERROR: 'neverdeclared' is not a procedure
     !ERROR: Could not characterize intrinsic function actual argument 'badpointer'
-    !ERROR: 'neverdeclared' is not a procedure
-    !ERROR: Could not characterize intrinsic function actual argument 'badpointer'
     lvar = associated(badPointer)
   end subroutine test
 end subroutine assoc