[CS] Re-introduce some assertions

These FIXMEs appear to now be outdated.

This commit also adds an additional assertion in
`bindTypeVariable`.

Author: hamishknight

lib/Sema/CSSimplify.cpp

@@ -2394,10 +2394,7 @@ ConstraintSystem::matchTypesBindTypeVar(
     TypeMatchOptions flags, ConstraintLocatorBuilder locator,
     llvm::function_ref<TypeMatchResult()> formUnsolvedResult) {
   assert(typeVar->is<TypeVariableType>() && "Expected a type variable!");
-  // FIXME: Due to some SE-0110 related code farther up we can end
-  // up with type variables wrapped in parens that will trip this
-  // assert. For now, maintain the existing behavior.
-  // assert(!type->is<TypeVariableType>() && "Expected a non-type variable!");
+  assert(!type->is<TypeVariableType>() && "Expected a non-type variable!");
 
   // Simplify the right-hand type and perform the "occurs" check.
   typeVar = getRepresentative(typeVar);
@@ -4104,14 +4101,8 @@ ConstraintSystem::matchTypes(Type type1, Type type2, ConstraintKind kind,
         return getTypeMatchSuccess();
       }
 
-      assert((type1->is<TypeVariableType>() || type2->is<TypeVariableType>()) &&
-             "Expected a type variable!");
-      // FIXME: Due to some SE-0110 related code farther up we can end
-      // up with type variables wrapped in parens that will trip this
-      // assert. For now, maintain the existing behavior.
-      // assert(
-      //     (!type1->is<TypeVariableType>() || !type2->is<TypeVariableType>())
-      //     && "Expected a non-type variable!");
+      assert((type1->is<TypeVariableType>() != type2->is<TypeVariableType>()) &&
+             "Expected a type variable and a non type variable!");
 
       auto *typeVar = typeVar1 ? typeVar1 : typeVar2;
       auto type = typeVar1 ? type2 : type1;

lib/Sema/ConstraintGraph.cpp

@@ -428,6 +428,9 @@ void ConstraintGraph::mergeNodes(TypeVariableType *typeVar1,
 }
 
 void ConstraintGraph::bindTypeVariable(TypeVariableType *typeVar, Type fixed) {
+  assert(!fixed->is<TypeVariableType>() &&
+         "Cannot bind to type variable; merge equivalence classes instead");
+
   // If there are no type variables in the fixed type, there's nothing to do.
   if (!fixed->hasTypeVariable())
     return;