Start work on #3950: add skeletal IterationType (to merge to 3.0)

src/main/java/com/fasterxml/jackson/databind/type/IterationType.java

@@ -0,0 +1,29 @@
+package com.fasterxml.jackson.databind.type;
+
+import com.fasterxml.jackson.databind.JavaType;
+
+/**
+ * Specialized {@link SimpleType} for types that are allow iteration
+ * over Collection(-like) types: this includes types like
+ * {@link java.util.Iterator}.
+ * Referenced type is accessible using {@link #getContentType()}.
+ *
+ * @since 2.16
+ */
+public abstract class IterationType extends SimpleType
+{
+    private static final long serialVersionUID = 1L;
+
+    protected final JavaType _iteratedType;
+
+    /**
+     * Constructor used when upgrading into this type (via {@link #upgradeFrom},
+     * the usual way for {@link IterationType}s to come into existence.
+     * Sets up what is considered the "base" reference type
+     */
+    protected IterationType(TypeBase base, JavaType iteratedType)
+    {
+        super(base);
+        _iteratedType = iteratedType;
+    }
+}