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diff --git a/tiger-compiler/src/type/type.hh b/tiger-compiler/src/type/type.hh
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+/**
+ ** \file type/type.hh
+ ** \brief The class Type.
+ */
+#pragma once
+
+#include <misc/xalloc.hh>
+#include <type/fwd.hh>
+
+namespace type
+{
+  /// Abstract a type.
+  class Type
+  {
+    /** \name Ctor & dtor.
+     ** \{ */
+  public:
+    /// Destroys a Type.
+    virtual ~Type() = default;
+    /** \} */
+
+    /// \name Visitors entry point.
+    /// \{ */
+    /// Accept a const visitor \a v.
+    virtual void accept(ConstVisitor& v) const = 0;
+    /// Accept a non-const visitor \a v.
+    virtual void accept(Visitor& v) = 0;
+    /// \}
+
+    /** \name Accessors.
+     ** \{ */
+    /// Return the actual type held by THIS.
+    virtual const Type& actual() const;
+    /** \} */
+
+    /** \brief Whether two types are "compatible".
+     **
+     ** I.e., whether "a = b", "a <> b", "a := b" are correctly typed
+     ** with a of type \a this, and b of type \a other).
+     **
+     ** By default two types are compatible (in the sense of "=" and "<>",
+     ** not w.r.t. an order) only when they are equal.
+     **
+     ** In the case of assignment, "rec := nil" is valid, but "nil := rec"
+     ** is not, which suggest that we should have a non commutative
+     ** assignment specific compatibility check.  But since "nil := ..."
+     ** is incorrect syntactically, that is not needed.
+     */
+    virtual bool compatible_with(const Type& other) const;
+  };
+
+  /** \brief Compare two Type s.
+   **
+   ** Return true if \a a and \a b are equivalent Tiger Types.  E.g.,
+   ** if \a a and \a b are different but point to the same type, then
+   ** return true. */
+  bool operator==(const Type& lhs, const Type& rhs);
+  /// !(a == b).
+  bool operator!=(const Type& lhs, const Type& rhs);
+
+  /// Hide actual types?  (i.e., print only the surface type?)
+  extern const misc::xalloc<bool> hide_actual_types;
+
+} // namespace type
+
+#include <type/type.hxx>