RequirementMachine: Fix subtle bug in Knuth-Bendix algorithm

Filter out trivial overlaps where a rule overlaps entirely with
itself before looking at CheckedOverlaps. Otherwise, we'll miss
overlaps where a rule overlaps with itself at a non-zero
position.

Author: slavapestov

lib/AST/RequirementMachine/KnuthBendix.cpp

@@ -528,11 +528,6 @@ RewriteSystem::computeConfluentCompletion(unsigned maxIterations,
       auto to = lhs.getLHS().end();
       while (from < to) {
         Trie.findAll(from, to, [&](unsigned j) {
-          // We don't have to consider the same pair of rules more than once,
-          // since those critical pairs were already resolved.
-          if (!CheckedOverlaps.insert(std::make_pair(i, j)).second)
-            return;
-
           const auto &rhs = getRule(j);
           if (rhs.isSimplified())
             return;
@@ -554,6 +549,11 @@ RewriteSystem::computeConfluentCompletion(unsigned maxIterations,
               return;
           }
 
+          // We don't have to consider the same pair of rules more than once,
+          // since those critical pairs were already resolved.
+          if (!CheckedOverlaps.insert(std::make_pair(i, j)).second)
+            return;
+
           // Try to repair the confluence violation by adding a new rule.
           if (computeCriticalPair(from, lhs, rhs,
                                   resolvedCriticalPairs,

test/Generics/non_confluent.swift

@@ -1,5 +1,11 @@
 // RUN: %target-typecheck-verify-swift -requirement-machine-protocol-signatures=on -requirement-machine-inferred-signatures=on
 
+protocol ABA // expected-error {{cannot build rewrite system for protocol; depth limit exceeded}}
+    where A.B == A.B.A { // expected-error *{{is not a member type}}
+  associatedtype A : ABA
+  associatedtype B : ABA
+}
+
 protocol Undecidable // expected-error {{cannot build rewrite system for protocol; depth limit exceeded}}
     where A.C == C.A, // expected-error *{{is not a member type}}
           A.D == D.A, // expected-error *{{is not a member type}}