Detect/prevent type variables from leaking across constraint systems.

lib/Sema/CSApply.cpp

@@ -6793,6 +6793,9 @@ Expr *TypeChecker::callWitness(Expr *base, DeclContext *dc,
 
   // Find the witness we need to use.
   auto type = base->getType();
+  assert(!type->hasTypeVariable() &&
+         "Building call to witness with unresolved base type!");
+
   if (auto metaType = type->getAs<AnyMetatypeType>())
     type = metaType->getInstanceType();
 

lib/Sema/CSGen.cpp

@@ -3101,6 +3101,11 @@ bool swift::isExtensionApplied(DeclContext &DC, Type BaseTy,
   bool Failed = false;
   SmallVector<Type, 3> TypeScratch;
 
+  // Don't allow type variables from an existing constraint system to
+  // leak into the new constraint system created below.
+  assert(!BaseTy->hasTypeVariable() &&
+         "Unexpected type variable in base type!");
+
   // Prepare type substitution map.
   TypeSubstitutionMap Substitutions = BaseTy->getMemberSubstitutions(ED);
   auto resolveType = [&](Type Ty) {
@@ -3144,6 +3149,9 @@ bool swift::isExtensionApplied(DeclContext &DC, Type BaseTy,
 static bool canSatisfy(Type T1, Type T2, DeclContext &DC, ConstraintKind Kind,
                        bool ReplaceArchetypeWithVariables,
                        bool AllowFreeVariables) {
+  assert(!T1->hasTypeVariable() && !T2->hasTypeVariable() &&
+         "Unexpected type variable in constraint satisfaction testing");
+
   std::unique_ptr<TypeChecker> CreatedTC;
   // If the current ast context has no type checker, create one for it.
   auto *TC = static_cast<TypeChecker*>(DC.getASTContext().getLazyResolver());

lib/Sema/TypeCheckProtocol.cpp

@@ -5445,7 +5445,7 @@ bool TypeChecker::isProtocolExtensionUsable(DeclContext *dc, Type type,
 
   // If the type still has parameters, the constrained extension is considered
   // unusable.
-  if (type->hasTypeParameter())
+  if (type->hasTypeParameter() || type->hasTypeVariable())
     return false;
 
   // Set up a constraint system where we open the generic parameters of the