Forbid private library functions to be attached outside their declaration scope

Changelog.md

@@ -30,6 +30,7 @@ Bugfixes:
  * SMTChecker: Fix internal error on chain assignments using static fully specified state variables.
  * SMTChecker: Fix internal error when using user defined types as mapping indices or struct members.
  * SMTChecker: Fix internal error when deleting struct member of function type.
+ * TypeChecker: Fix bug where private library functions could be attached with ``using for`` outside of their declaration scope.
 
 
 ### 0.8.17 (2022-09-08)

libsolidity/analysis/TypeChecker.cpp

@@ -3809,6 +3809,10 @@ void TypeChecker::endVisit(UsingForDirective const& _usingFor)
 			m_errorReporter.fatalTypeError(
 				4731_error,
 				path->location(),
+				SecondarySourceLocation().append(
+						"Function defined here:",
+						functionDefinition.location()
+				),
 				fmt::format(
 					"The function \"{}\" does not have any parameters, and therefore cannot be attached to the type \"{}\".",
 					joinHumanReadable(path->path(), "."),
@@ -3816,6 +3820,27 @@ void TypeChecker::endVisit(UsingForDirective const& _usingFor)
 				)
 			);
 
+		if (
+			functionDefinition.visibility() == Visibility::Private &&
+			functionDefinition.scope() != m_currentContract
+		)
+		{
+			solAssert(functionDefinition.libraryFunction());
+			m_errorReporter.typeError(
+				6772_error,
+				path->location(),
+				SecondarySourceLocation().append(
+						"Function defined here:",
+						functionDefinition.location()
+				),
+				fmt::format(
+					"Function \"{}\" is private and therefore cannot be attached"
+					" to a type outside of the library where it is defined.",
+					joinHumanReadable(path->path(), ".")
+				)
+			);
+		}
+
 		FunctionType const* functionType = dynamic_cast<FunctionType const&>(*functionDefinition.type()).withBoundFirstArgument();
 		solAssert(functionType && functionType->selfType(), "");
 		BoolResult result = normalizedType->isImplicitlyConvertibleTo(

test/libsolidity/semanticTests/using/private_library_function.sol

@@ -0,0 +1,10 @@
+library L {
+    using {L.privateFunction} for uint;
+    function privateFunction(uint x) private pure returns (uint) { return x + 1; }
+    function f() public pure returns (uint) {
+        uint x = 1;
+        return x.privateFunction();
+    }
+}
+// ----
+// f() -> 2

test/libsolidity/syntaxTests/using/private_library_function_inside_scope.sol

@@ -0,0 +1,5 @@
+library L {
+    using {L.privateFunction} for uint;
+    function privateFunction(uint) private pure {}
+}
+// ----

test/libsolidity/syntaxTests/using/private_library_function_outside_scope.sol

@@ -0,0 +1,12 @@
+library L {
+    function privateFunction(uint) private pure {}
+}
+
+using {L.privateFunction} for uint;
+
+contract C {
+    using {L.privateFunction} for uint;
+}
+// ----
+// TypeError 6772: (73-90): Function "L.privateFunction" is private and therefore cannot be attached to a type outside of the library where it is defined.
+// TypeError 6772: (127-144): Function "L.privateFunction" is private and therefore cannot be attached to a type outside of the library where it is defined.