RequirementMachine: Fix assertion failure when conflicting rules are also redundant

We can end up with two redundant concrete type rules on the same
term, but we would crash in homotopy reduction because the rules
were incomparable due to the concrete type symbol at the end.

If one of them is conflicting though, we don't really care about
the homotopy reduction order, so skip the check if the other
rule was already marked conflicting.

Author: slavapestov

lib/AST/RequirementMachine/HomotopyReduction.cpp

@@ -386,9 +386,13 @@ findRuleToDelete(llvm::function_ref<bool(unsigned)> isRedundantRuleFn,
 
     const auto &otherRule = getRule(found->second);
 
-    // Prefer to delete "less canonical" rules.
-    if (rule.compare(otherRule, Context) > 0)
+    // If the new rule is conflicting, don't compare the rules at all
+    // and prefer to delete the new rule. Otherwise, prefer to delete
+    // the less canonical of the two rules.
+    if (rule.isConflicting() ||
+        rule.compare(otherRule, Context) > 0) {
       found = pair;
+    }
   }
 
   if (!found)

test/Generics/concrete_conflict_in_protocol.swift

@@ -0,0 +1,32 @@
+// RUN: %target-typecheck-verify-swift -debug-generic-signatures -requirement-machine-protocol-signatures=verify
+
+protocol P1 {
+  associatedtype T where T == Int
+}
+
+protocol P2 {
+  associatedtype T where T == String
+}
+
+protocol P3 {
+  associatedtype T where T == Float
+}
+
+// We end up with two redundant rules:
+//
+// [P4:T].[P1:T].[concrete: String]
+// [P4:T].[P1:T].[concrete: Float]
+//
+// These rules are unordered with respect to each other because concrete type
+// symbols are incomparable, but they conflict so we should not compare them;
+// just make sure we don't crash.
+
+// CHECK-LABEL: concrete_conflict_in_protocol.(file).P4@
+// CHECK-LABEL: Requirement signature: <Self where Self.[P4]T : P1, Self.[P4]T : P2, Self.[P4]T : P3>
+
+protocol P4 {
+  associatedtype T : P1 & P2 & P3
+  // expected-note@-1 2{{same-type constraint 'Self.T.T' == 'Int' implied here}}
+  // expected-error@-2 {{'Self.T.T' cannot be equal to both 'String' and 'Int'}}
+  // expected-error@-3 {{'Self.T.T' cannot be equal to both 'Float' and 'Int'}}
+}