RequirementMachine: Concrete contraction needs to substitute the parent type of a subject type sometimes

If you have generic parameters <T, U> and requirements of the form:

- T : P
- T == ConcreteType<U>
- T.[P]U : SomeClass
- T.[P]U : SomeProto

And furthermore SomeClass does not conform to SomeProto, we can't leave
`T.[P]U : SomeClass` unsubstituted; we still have to replace `T` with
`ConcreteType<U>` to transform the latter two requirements into:

- U : SomeClass
- U : SomeProto

"Concrete contraction" is easily the hackiest part of the Requirement
Machine; I need to come up with a more principled solution for the
problem that it solves sooner or later.

Fixes rdar://problem/94150249.

lib/AST/RequirementMachine/ConcreteContraction.cpp

@@ -227,8 +227,8 @@ public:
 Optional<Type> ConcreteContraction::substTypeParameterRec(
     Type type, Position position) const {
 
-  // If the requirement is of the form 'T == C' or 'T : C', don't
-  // substitute T, since then we end up with 'C == C' or 'C : C',
+  // If we have a superclass (T : C) or same-type requirement (T == C),
+  // don't substitute T, since then we end up with 'C == C' or 'C : C',
   // losing the requirement.
   if (position == Position::BaseType ||
       position == Position::ConformanceRequirement) {
@@ -399,9 +399,10 @@ ConcreteContraction::substRequirement(const Requirement &req) const {
         !module->lookupConformance(substFirstType, proto,
                                    allowMissing, allowUnavailable)) {
       // Handle the case of <T where T : P, T : C> where C is a class and
-      // C does not conform to P by leaving the conformance requirement
-      // unsubstituted.
-      return req;
+      // C does not conform to P and only substitute the parent type of T
+      // by pretending we have a same-type requirement here.
+      substFirstType = substTypeParameter(
+          firstType, Position::SameTypeRequirement);
     }
 
     // Otherwise, replace the generic parameter in the conformance
@@ -418,9 +419,11 @@ ConcreteContraction::substRequirement(const Requirement &req) const {
     if (!substFirstType->isTypeParameter() &&
         !substFirstType->satisfiesClassConstraint() &&
         req.getLayoutConstraint()->isClass()) {
-      // If the concrete type doesn't satisfy the layout constraint,
-      // leave it unsubstituted so that we produce a better diagnostic.
-      return req;
+      // If the concrete type doesn't satisfy the layout constraint, produce
+      // a better diagnostic and only substitute the parent type by pretending
+      // we have a same-type requirement here.
+      substFirstType = substTypeParameter(
+          firstType, Position::SameTypeRequirement);
     }
 
     return Requirement(req.getKind(),

test/Generics/rdar94150249.swift

@@ -12,14 +12,16 @@ class G<Value> : P2 {}
 
 protocol P3 {}
 
+class C {}
+
 extension P1 where Value: P2 {
   typealias Element = Value.Value
 
   // Make sure we can resolve 'Element' to 'V' on the left hand side of 'Element: P3'.
 
   // CHECK-LABEL: .P1 extension.set()@
-  // CHECK-NEXT: Generic signature: <Self, V where Self : P1, V : P3, Self.[P1]Value == G<V>>
-  func set<V>() where Element: P3, Value == G<V> {
+  // CHECK-NEXT: Generic signature: <Self, V where Self : P1, V : C, V : P3, Self.[P1]Value == G<V>>
+  func set<V>() where Element: P3 & C, Value == G<V> {
     takeP3(V.self)
   }
 }