[CSSimplify] Adjust isBindable to reject binding if it's directly to a type variable

Although inference doesn't allow direct bindings to
type variables, they can still get through via `matchTypes`
when type is a partially resolved pack expansion that simplifies
down to a type variable.

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -4358,8 +4358,13 @@ static bool isBindable(TypeVariableType *typeVar, Type type) {
   // what the \c type is, because contextual type is just a hint
   // in this situation and type variable would be bound to its
   // opened type instead.
+  //
+  // Note that although inference doesn't allow direct bindings to
+  // type variables, they can still get through via `matchTypes`
+  // when type is a partially resolved pack expansion that simplifies
+  // down to a type variable.
   return typeVar->getImpl().isPackExpansion() ||
-         !type->is<DependentMemberType>();
+         !(type->is<TypeVariableType>() || type->is<DependentMemberType>());
 }
 
 ConstraintSystem::TypeMatchResult

validation-test/Sema/type_checker_crashers_fixed/issue-71701.swift

@@ -0,0 +1,10 @@
+// RUN: not %target-swift-frontend %s -typecheck
+
+// https://github.com/apple/swift/issues/71701
+
+struct S<each T> {
+  init(_ values: repeat each T) {}
+}
+
+_ = S(nil)
+_ = S.init(nil)