diff --git a/docs/contracts/using-for.rst b/docs/contracts/using-for.rst
index 782519644..ad78dd40f 100644
--- a/docs/contracts/using-for.rst
+++ b/docs/contracts/using-for.rst
@@ -45,7 +45,7 @@ Note that private library functions can only be specified when ``using for`` is
 
 If you define an operator (e.g. ``using {f as +} for T``), then the type (``T``) must be a
 :ref:`user-defined value type <user-defined-value-types>`.
-The definition of an operator must be a function with the types of all parameters and
+The definition of an operator must be a ``pure`` function with the types of all parameters and
 the return value matching ``T``, except for comparison operators, where the return value must
 be of type ``bool``.
 
diff --git a/libsolidity/analysis/TypeChecker.cpp b/libsolidity/analysis/TypeChecker.cpp
index c43fff136..f90d72e7c 100644
--- a/libsolidity/analysis/TypeChecker.cpp
+++ b/libsolidity/analysis/TypeChecker.cpp
@@ -4001,6 +4001,13 @@ void TypeChecker::endVisit(UsingForDirective const& _usingFor)
 			}
 			solAssert(usingForType->typeDefinition());
 
+			if (functionType->stateMutability() != StateMutability::Pure)
+				m_errorReporter.typeError(
+					7775_error,
+					path->location(),
+					"Only pure functions can be used to define operators."
+				);
+
 			bool identicalFirstTwoParameters = (parameterCount < 2 || *parameterTypes.at(0) == *parameterTypes.at(1));
 			bool isUnaryOnlyOperator = (!TokenTraits::isBinaryOp(*operator_) && TokenTraits::isUnaryOp(*operator_));
 			bool isBinaryOnlyOperator =
diff --git a/libsolidity/ast/AST.h b/libsolidity/ast/AST.h
index c2688728f..9db0873fc 100644
--- a/libsolidity/ast/AST.h
+++ b/libsolidity/ast/AST.h
@@ -653,7 +653,7 @@ private:
  * all functions, and this is checked at the point of the using statement. For versions 1 and
  * 2, this check is only done when a function is called.
  *
- * For version 4, T has to be user-defined value type.
+ * For version 4, T has to be user-defined value type and the function must be pure.
  * All parameters and return value of all the functions have to be of type T.
  * This version can be combined with version 3 - a single directive may attach functions to the
  * type and define operators on it at the same time.
diff --git a/test/libsolidity/semanticTests/operators/userDefined/calling_operator_that_makes_external_call.sol b/test/libsolidity/syntaxTests/operators/userDefined/calling_operator_that_makes_external_call.sol
similarity index 81%
rename from test/libsolidity/semanticTests/operators/userDefined/calling_operator_that_makes_external_call.sol
rename to test/libsolidity/syntaxTests/operators/userDefined/calling_operator_that_makes_external_call.sol
index dedf88b56..99f4f11c3 100644
--- a/test/libsolidity/semanticTests/operators/userDefined/calling_operator_that_makes_external_call.sol
+++ b/test/libsolidity/syntaxTests/operators/userDefined/calling_operator_that_makes_external_call.sol
@@ -21,5 +21,4 @@ contract C {
     }
 }
 // ----
-// test() -> 3
-// gas legacy: 119695
+// TypeError 7775: (27-30): Only pure functions can be used to define operators.
diff --git a/test/libsolidity/syntaxTests/operators/userDefined/implementing_operator_with_non_pure_function.sol b/test/libsolidity/syntaxTests/operators/userDefined/implementing_operator_with_non_pure_function.sol
new file mode 100644
index 000000000..5b29a0941
--- /dev/null
+++ b/test/libsolidity/syntaxTests/operators/userDefined/implementing_operator_with_non_pure_function.sol
@@ -0,0 +1,12 @@
+using {add as +, sub as -, mul as *} for A;
+
+function add(A, A) view returns (A) {}
+function sub(A, A) returns (A) {}
+function mul(A, A) payable returns (A) {}
+
+type A is address payable;
+// ----
+// TypeError 7775: (7-10): Only pure functions can be used to define operators.
+// TypeError 7775: (17-20): Only pure functions can be used to define operators.
+// TypeError 7775: (27-30): Only pure functions can be used to define operators.
+// TypeError 9559: (118-159): Free functions cannot be payable.
diff --git a/test/libsolidity/syntaxTests/viewPureChecker/user_operator_with_view_modifier.sol b/test/libsolidity/syntaxTests/viewPureChecker/user_operator_with_view_modifier.sol
deleted file mode 100644
index dbe7796f6..000000000
--- a/test/libsolidity/syntaxTests/viewPureChecker/user_operator_with_view_modifier.sol
+++ /dev/null
@@ -1,17 +0,0 @@
-type Int is uint8;
-
-using {
-    add as +
-} for Int;
-
-
-function f_view() view {}
-
-function add(Int, Int) view returns (Int) {
-    f_view();
-    return Int.wrap(0);
-}
-
-function f() view {
-    Int.wrap(0) + Int.wrap(0);
-}
diff --git a/test/libsolidity/syntaxTests/viewPureChecker/user_operator_with_view_modifier_can_be_restricted.sol b/test/libsolidity/syntaxTests/viewPureChecker/user_operator_with_view_modifier_can_be_restricted.sol
deleted file mode 100644
index 95063d44c..000000000
--- a/test/libsolidity/syntaxTests/viewPureChecker/user_operator_with_view_modifier_can_be_restricted.sol
+++ /dev/null
@@ -1,28 +0,0 @@
-type Int is uint8;
-
-using {
-    add as +
-} for Int;
-
-
-function f_view() view {}
-
-function add(Int, Int) view returns (Int) {
-    f_view();
-    return Int.wrap(0);
-}
-
-function f() view {
-    Int.wrap(0) + Int.wrap(0);
-}
-
-function g() {
-    Int.wrap(0) + Int.wrap(0);
-}
-
-function h() pure {
-    Int.wrap(0) + Int.wrap(0);
-}
-// ----
-// Warning 2018: (220-267): Function state mutability can be restricted to view
-// TypeError 2527: (293-318): Function declared as pure, but this expression (potentially) reads from the environment or state and thus requires "view".