RequirementMachine: Fix ConcreteContraction::substTypeParameter() for non-nominal base type

We can handle a non-nominal base type here as long as the DependentMemberType
we're substituting this into is resolved.

Author: slavapestov

lib/AST/RequirementMachine/ConcreteContraction.cpp

@@ -231,35 +231,44 @@ Optional<Type> ConcreteContraction::substTypeParameter(
   if (!substBaseType)
     return None;
 
-  auto *decl = (*substBaseType)->getAnyNominal();
-  if (decl == nullptr) {
-    if (Debug) {
-      llvm::dbgs() << "@@@ Not a nominal type: " << *substBaseType << "\n";
-    }
-
-    return None;
-  }
-
-  auto *module = decl->getParentModule();
-
   // A resolved DependentMemberType stores an associated type declaration.
   //
   // Handle this by looking up the corresponding type witness in the base
   // type's conformance to the associated type's protocol.
   if (auto *assocType = memberType->getAssocType()) {
-    auto conformance = module->lookupConformance(
-        *substBaseType, assocType->getProtocol());
+    auto *proto = assocType->getProtocol();
+    auto *module = proto->getParentModule();
+
+    auto conformance = ((*substBaseType)->isTypeParameter()
+                        ? ProtocolConformanceRef(proto)
+                        : module->lookupConformance(*substBaseType, proto));
 
     // The base type doesn't conform, in which case the requirement remains
     // unsubstituted.
-    if (!conformance)
+    if (!conformance) {
+      if (Debug) {
+        llvm::dbgs() << "@@@ " << substBaseType << " does not conform to "
+                     << proto->getName() << "\n";
+      }
       return None;
+    }
 
     return assocType->getDeclaredInterfaceType()
                     ->castTo<DependentMemberType>()
                     ->substBaseType(module, *substBaseType);
   }
 
+  auto *decl = (*substBaseType)->getAnyNominal();
+  if (decl == nullptr) {
+    if (Debug) {
+      llvm::dbgs() << "@@@ Not a nominal type: " << *substBaseType << "\n";
+    }
+
+    return None;
+  }
+
+  auto *module = decl->getParentModule();
+
   // An unresolved DependentMemberType stores an identifier. Handle this
   // by performing a name lookup into the base type.
   auto *typeDecl = lookupConcreteNestedType(module, decl,

test/Generics/concrete_contraction_substitute_non_nominal_base.swift

@@ -0,0 +1,18 @@
+// RUN: %target-swift-frontend -typecheck %s -debug-generic-signatures -requirement-machine-inferred-signatures=on 2>&1 | %FileCheck %s
+
+protocol P {
+  associatedtype T : Q
+}
+
+protocol Q {
+  associatedtype U
+}
+
+struct S<T : Q> : P {}
+
+// Make sure concrete contraction can transform X.T.U => S<Y>.T.U => Y.U
+struct G<X : P, Y : Q> where X.T.U == Int {}
+
+// CHECK-LABEL: ExtensionDecl line={{.*}} base=G
+// CHECK-NEXT: Generic signature: <X, Y where X == S<Y>, Y : Q, Y.[Q]U == Int>
+extension G where X == S<Y> {}