ABI encoding functions are pure and should be usable in constants.

libsolidity/ast/Types.cpp

@@ -2865,7 +2865,11 @@ bool FunctionType::isPure() const
 		m_kind == Kind::RIPEMD160 ||
 		m_kind == Kind::AddMod ||
 		m_kind == Kind::MulMod ||
-		m_kind == Kind::ObjectCreation;
+		m_kind == Kind::ObjectCreation ||
+		m_kind == Kind::ABIEncode ||
+		m_kind == Kind::ABIEncodePacked ||
+		m_kind == Kind::ABIEncodeWithSelector ||
+		m_kind == Kind::ABIEncodeWithSignature;
 }
 
 TypePointers FunctionType::parseElementaryTypeVector(strings const& _types)

libsolidity/ast/Types.h

@@ -1046,8 +1046,8 @@ public:
 		return *m_declaration;
 	}
 	bool hasDeclaration() const { return !!m_declaration; }
-	/// @returns true if the result of this function only depends on its arguments
-	/// and it does not modify the state.
+	/// @returns true if the result of this function only depends on its arguments,
+	/// does not modify the state and is a compile-time constant.
 	/// Currently, this will only return true for internal functions like keccak and ecrecover.
 	bool isPure() const;
 	bool isPayable() const { return m_stateMutability == StateMutability::Payable; }