RequirementMachine: Enable concrete contraction for nested type parameters

Fixes rdar://problem/91594361.

Author: slavapestov

lib/AST/RequirementMachine/ConcreteContraction.cpp

@@ -518,9 +518,6 @@ bool ConcreteContraction::performConcreteContraction(
     assert(subjectType->isTypeParameter() &&
            "You forgot to call desugarRequirement()");
 
-    if (!subjectType->is<GenericTypeParamType>())
-      continue;
-
     auto kind = req.req.getKind();
     switch (kind) {
     case RequirementKind::SameType: {

test/Generics/rdar91594361.swift

@@ -0,0 +1,20 @@
+// RUN: %target-swift-frontend -typecheck %s -debug-generic-signatures -requirement-machine-inferred-signatures=on 2>&1 | %FileCheck %s
+
+protocol P1 {
+  associatedtype T
+}
+
+protocol P2 {
+  associatedtype T : P3
+}
+
+protocol P3 {}
+
+struct G<T : P3> : P2 {}
+
+extension P1 where T : P2 {
+  // CHECK-LABEL: .foo@
+  // CHECK-NEXT: <Self, X where Self : P1, X : P3, Self.[P1]T == G<X>>
+  func foo<X>(_: X) where T == G<X> {}
+}
+

test/Generics/requirement_inference.swift

@@ -1,5 +1,5 @@
-// RUN: %target-typecheck-verify-swift -requirement-machine-protocol-signatures=verify -requirement-machine-inferred-signatures=off
-// RUN: not %target-swift-frontend -typecheck %s -debug-generic-signatures -requirement-machine-protocol-signatures=verify -requirement-machine-inferred-signatures=on 2>&1 | %FileCheck %s
+// RUN: %target-typecheck-verify-swift -requirement-machine-protocol-signatures=on -requirement-machine-inferred-signatures=verify
+// RUN: not %target-swift-frontend -typecheck %s -debug-generic-signatures -requirement-machine-protocol-signatures=on -requirement-machine-inferred-signatures=on 2>&1 | %FileCheck %s
 
 protocol P1 { 
   func p1()

test/Generics/requirement_machine_diagnostics.swift

@@ -295,18 +295,18 @@ func sameTypeConflicts() {
     fatalError()
   }
 
-  // expected-error@+1{{no type for 'T.Bar.Foo' can satisfy both 'T.Bar.Foo == X' and 'T.Bar.Foo == Z'}}
   func fail4<T: Barrable>(_ t: T) -> (Y, Z)
     where
     T.Bar == Y,
     T.Bar.Foo == Z {
+    // expected-error@-1{{generic signature requires types 'Y.Foo' (aka 'X') and 'Z' to be the same}}
     fatalError()
   }
 
-  // expected-error@+1{{no type for 'T.Bar.Foo' can satisfy both 'T.Bar.Foo == X' and 'T.Bar.Foo == Z'}}
   func fail5<T: Barrable>(_ t: T) -> (Y, Z)
     where
     T.Bar.Foo == Z,
+    // expected-error@-1{{generic signature requires types 'Y.Foo' (aka 'X') and 'Z' to be the same}}
     T.Bar == Y {
     fatalError()
   }

test/attr/accessibility_where_clause.swift

@@ -1,4 +1,4 @@
-// RUN: %target-typecheck-verify-swift -requirement-machine-inferred-signatures=off
+// RUN: %target-typecheck-verify-swift -requirement-machine-inferred-signatures=on
 
 public class OuterClass {
   class InnerClass {}
@@ -17,9 +17,6 @@ public protocol PublicProto2 {
 // FIXME: Once we no longer do that, come up with another strategy
 // to make the above diagnose.
 
-// FIXME: Get this working with the Requirement Machine, or decide that it should
-// be unsupported: rdar://90469477
-
 extension PublicProto2 where Self.T : OuterClass, Self.U == Self.T.InnerClass {
   public func cannotBePublic() {}
   // expected-error@-1 {{cannot declare a public instance method in an extension with internal requirements}}