Fix ICE when printing an error message related to mappings

libsolidity/ast/Types.cpp

@@ -2559,6 +2559,11 @@ string UserDefinedValueType::toString(bool /* _short */) const
 	return *definition().annotation().canonicalName;
 }
 
+string UserDefinedValueType::canonicalName() const
+{
+	return *definition().annotation().canonicalName;
+}
+
 vector<tuple<string, Type const*>> UserDefinedValueType::makeStackItems() const
 {
 	return underlyingType().stackItems();

libsolidity/ast/Types.h

@@ -1141,7 +1141,7 @@ public:
 	}
 
 	std::string toString(bool _short) const override;
-	std::string canonicalName() const override { solAssert(false, ""); }
+	std::string canonicalName() const override;
 	std::string signatureInExternalFunction(bool) const override { solAssert(false, ""); }
 
 protected: