RequirementMachine: Relax assertion in verifyRewriteSystem()

This invariant is no longer required with the new minimal conformances
algorithm.

Fixes rdar://problem/94746399.

Author: slavapestov

lib/AST/RequirementMachine/RewriteSystem.cpp

@@ -556,7 +556,8 @@ void RewriteSystem::verifyRewriteRules(ValidityPolicy policy) const {
         ASSERT_RULE(symbol.getKind() != Symbol::Kind::GenericParam);
       }
 
-      if (index != 0 && index != lhs.size() - 1) {
+      if (index != 0 && index != lhs.size() - 1 &&
+          !rule.isLHSSimplified()) {
         ASSERT_RULE(symbol.getKind() != Symbol::Kind::Protocol);
       }
     }
@@ -585,13 +586,18 @@ void RewriteSystem::verifyRewriteRules(ValidityPolicy policy) const {
       }
 
       ASSERT_RULE(symbol.getKind() != Symbol::Kind::Layout);
-      ASSERT_RULE(!symbol.hasSubstitutions());
+
+      if (!rule.isRHSSimplified() &&
+          index != rhs.size() - 1) {
+        ASSERT_RULE(!symbol.hasSubstitutions());
+      }
 
       if (index != 0) {
         ASSERT_RULE(symbol.getKind() != Symbol::Kind::GenericParam);
       }
 
-      if (index != 0 && !rule.isRHSSimplified()) {
+      if (!rule.isRHSSimplified() &&
+          index != 0) {
         ASSERT_RULE(symbol.getKind() != Symbol::Kind::Protocol);
       }
     }

test/Generics/rdar9474639.swift

@@ -0,0 +1,27 @@
+// RUN: %target-swift-frontend -typecheck %s -debug-generic-signatures 2>&1 | %FileCheck %s
+
+// CHECK-LABEL: .P1@
+// CHECK-NEXT: Requirement signature: <Self where Self == Self.[P1]T.[P2]T, Self.[P1]T : P2>
+protocol P1 {
+  associatedtype T: P2 where T.T == Self
+}
+
+// CHECK-LABEL: .P2@
+// CHECK-NEXT: Requirement signature: <Self where Self == Self.[P2]T.[P1]T, Self.[P2]T : P1>
+protocol P2 {
+  associatedtype T: P1 where T.T == Self
+}
+
+class SomeClass {}
+
+// CHECK-LABEL: .P3@
+// CHECK-NEXT: Requirement signature: <Self where Self : P2, Self.[P2]T : SomeClass>
+protocol P3: P2 where T: SomeClass {}
+
+protocol P4 {
+  associatedtype T
+}
+
+// CHECK-LABEL: .foo@
+// CHECK-NEXT: Generic signature: <T where T : P4, T.[P4]T : P1, T.[P4]T.[P1]T : P3>
+func foo<T: P4>(_: T) where T.T: P1, T.T.T: P3 {}