[CSSimplify] Add special handling if specialized type comes from TypeExpr

Add a special case for `TypeExpr` due to i.e. context specialization,
in such cases there is nothing for the solver to "open" so we need
to form opened type map manually.

Resolves: rdar://111059036

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -13544,6 +13544,38 @@ ConstraintSystem::simplifyExplicitGenericArgumentsConstraint(
       auto *genericParam = typeVar->getImpl().getGenericParameter();
       openedTypes.push_back({genericParam, typeVar});
     }
+  } else if (locator.directlyAt<TypeExpr>()) {
+    auto *BGT = type1->getAs<BoundGenericType>();
+    if (!BGT)
+      return SolutionKind::Error;
+
+    decl = BGT->getDecl();
+
+    auto genericParams = BGT->getDecl()->getInnermostGenericParamTypes();
+    if (genericParams.size() != BGT->getGenericArgs().size())
+      return SolutionKind::Error;
+
+    for (unsigned i = 0, n = genericParams.size(); i != n; ++i) {
+      auto argType = BGT->getGenericArgs()[i];
+      if (auto *typeVar = argType->getAs<TypeVariableType>()) {
+        openedTypes.push_back({genericParams[i], typeVar});
+      } else {
+        // If we have a concrete substitution then we need to create
+        // a new type variable to be able to add it to the list as-if
+        // it is opened generic parameter type.
+        auto *GP = genericParams[i];
+
+        unsigned options = TVO_CanBindToNoEscape;
+        if (GP->isParameterPack())
+          options |= TVO_CanBindToPack;
+
+        auto *argVar = createTypeVariable(
+            getConstraintLocator(locator, LocatorPathElt::GenericArgument(i)),
+            options);
+        addConstraint(ConstraintKind::Bind, argVar, argType, locator);
+        openedTypes.push_back({GP, argVar});
+      }
+    }
   } else {
     // If the overload hasn't been resolved, we can't simplify this constraint.
     auto overloadLocator = getCalleeLocator(getConstraintLocator(locator));

test/decl/ext/specialize.swift

@@ -74,3 +74,15 @@ func testNestedExtensions() {
 
   Tree<Int>.Branch<String>.Nest<Void>.Egg.twite()
 }
+
+// rdar://111059036 - failed to produce a diagnostic in specialized extension
+struct Test {
+  struct Key<Value> {}
+}
+
+class State {
+}
+
+extension Test.Key<State> {
+  static let state = Self<State>()
+}