RequirementMachine: Always add a rule for the trivial [P].[P] => [P] conformance

Completion adds this rule when a same-type requirement equates the Self of P
with some other type parameter conforming to P; one example is something like this:

  protocol P {
    associatedtype T : P where T.T == Self
  }

The initial rewrite system looks like this:

  (1) [P].T => [P:T]
  (2) [P:T].[P] => [P:T]
  (3) [P:T].[P:T] => [P]

Rules (2) and (3) overlap on the term

  [P:T].[P:T].[P]

Simplifying the overlapped term with rule (2) produces

  [P:T].[P:T]

Which further reduces to

  [P]

Simplifying the overlapped term with rule (3) produces

  [P].[P]

So we get a new rule

  [P].[P] => [P]

This is not a "real" conformance rule, and homotopy reduction wastes work
unraveling it in rewrite loops where this rule occurs. Instead, it's better
to introduce it as a permanent rule that is not subject to homotopy reduction
for every protocol.

Author: slavapestov

lib/AST/RequirementMachine/RequirementMachine.cpp

@@ -275,6 +275,15 @@ void RewriteSystemBuilder::processProtocolDependencies() {
       llvm::dbgs() << "protocol " << proto->getName() << " {\n";
     }
 
+    MutableTerm lhs;
+    lhs.add(Symbol::forProtocol(proto, Context));
+    lhs.add(Symbol::forProtocol(proto, Context));
+
+    MutableTerm rhs;
+    rhs.add(Symbol::forProtocol(proto, Context));
+
+    AssociatedTypeRules.emplace_back(lhs, rhs);
+
     for (auto *assocType : proto->getAssociatedTypeMembers())
       addAssociatedType(assocType, proto);
 

test/Generics/rdar79564324.swift

@@ -23,11 +23,13 @@ public func test<T : P>(_ t: T) where T == T.A {
 
 // CHECK-LABEL: Requirement machine for <τ_0_0, τ_0_1 where τ_0_0 == τ_0_0.A, τ_0_1 : P, τ_0_0.A == τ_0_1.A>
 // CHECK-NEXT: Rewrite system: {
-// CHECK-NEXT: - [P].A => [P:A]
+// CHECK-NEXT: - [P].[P] => [P] [permanent]
+// CHECK-NEXT: - [P].A => [P:A] [permanent]
 // CHECK-NEXT: - [P:A].[P] => [P:A]
 // CHECK-NEXT: - τ_0_0.[P:A] => τ_0_0
 // CHECK-NEXT: - τ_0_1.[P] => τ_0_1
 // CHECK-NEXT: - τ_0_1.[P:A] => τ_0_0
+// CHECK-NEXT: - [P].[P:A] => [P:A]
 // CHECK-NEXT: - [P:A].A => [P:A].[P:A]
 // CHECK-NEXT: - τ_0_0.[P] => τ_0_0
 // CHECK-NEXT: - τ_0_1.A => τ_0_0

test/Generics/unify_superclass_types_1.swift

@@ -15,6 +15,7 @@ extension P where Self : Derived {
 
 // CHECK-LABEL: Requirement machine for <τ_0_0 where τ_0_0 : Derived, τ_0_0 : P>
 // CHECK-NEXT: Rewrite system: {
+// CHECK-NEXT: - [P].[P] => [P] [permanent]
 // CHECK-NEXT: - [P].[superclass: Base] => [P]
 // CHECK-NEXT: - [P].[layout: _NativeClass] => [P]
 // CHECK-NEXT: - τ_0_0.[superclass: Derived] => τ_0_0