[CSSimplify] Teach specialization constraint to look through non-generic typealiases

If type alias declaration doesn't add new generic parameters
refer to its underlying type to find them.

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -13595,15 +13595,36 @@ ConstraintSystem::simplifyExplicitGenericArgumentsConstraint(
     }
 
     decl = overloadChoice.getDecl();
+
     auto openedOverloadTypes = getOpenedTypes(overloadLocator);
     openedTypes.append(openedOverloadTypes.begin(), openedOverloadTypes.end());
   }
 
-  auto genericContext = decl->getAsGenericContext();
-  if (!genericContext)
+  std::function<GenericParamList *(ValueDecl *)> getGenericParams =
+      [&](ValueDecl *decl) -> GenericParamList * {
+    auto genericContext = decl->getAsGenericContext();
+    if (!genericContext)
+      return nullptr;
+
+    auto genericParams = genericContext->getGenericParams();
+    if (!genericParams || genericParams->size() == 0) {
+      // If declaration is a non-generic typealias, let's point
+      // to the underlying generic declaration.
+      if (auto *TA = dyn_cast<TypeAliasDecl>(decl)) {
+        if (TA->isGeneric())
+          return nullptr;
+        if (auto underlying = TA->getUnderlyingType()->getAnyNominal())
+          return getGenericParams(underlying);
+      }
+    }
+
+    return genericParams;
+  };
+
+  if (!decl->getAsGenericContext())
     return SolutionKind::Error;
 
-  auto genericParams = genericContext->getGenericParams();
+  auto genericParams = getGenericParams(decl);
   if (!genericParams || genericParams->size() == 0) {
     // FIXME: Record an error here that we're ignoring the parameters.
     return SolutionKind::Solved;