[Diagnostics] Diagnose use of member with mutating getter in key path

Author: xedin

lib/Sema/CSDiagnostics.cpp

@@ -2470,3 +2470,8 @@ bool InvalidStaticMemberRefInKeyPath::diagnoseAsError() {
   emitDiagnostic(getLoc(), diag::expr_keypath_static_member, getName());
   return true;
 }
+
+bool InvalidMemberWithMutatingGetterInKeyPath::diagnoseAsError() {
+  emitDiagnostic(getLoc(), diag::expr_keypath_mutating_getter, getName());
+  return true;
+}

lib/Sema/CSDiagnostics.h

@@ -1070,6 +1070,34 @@ class InvalidStaticMemberRefInKeyPath final : public InvalidMemberRefInKeyPath {
   bool diagnoseAsError() override;
 };
 
+/// Diagnose an attempt to reference a member which has a mutating getter as a
+/// key path component e.g.
+///
+/// ```swift
+/// struct S {
+///   var foo: Int {
+///     mutating get { return 42 }
+///   }
+///
+///   subscript(_: Int) -> Bool {
+///     mutating get { return false }
+///   }
+/// }
+///
+/// _ = \S.foo
+/// _ = \S.[42]
+/// ```
+class InvalidMemberWithMutatingGetterInKeyPath final
+    : public InvalidMemberRefInKeyPath {
+public:
+  InvalidMemberWithMutatingGetterInKeyPath(Expr *root, ConstraintSystem &cs,
+                                           ValueDecl *member,
+                                           ConstraintLocator *locator)
+      : InvalidMemberRefInKeyPath(root, cs, member, locator) {}
+
+  bool diagnoseAsError() override;
+};
+
 } // end namespace constraints
 } // end namespace swift
 

lib/Sema/CSFix.cpp

@@ -432,7 +432,9 @@ bool AllowInvalidRefInKeyPath::diagnose(Expr *root, bool asNote) const {
   }
 
   case RefKind::MutatingGetter: {
-    return false;
+    InvalidMemberWithMutatingGetterInKeyPath failure(
+        root, getConstraintSystem(), Member, getLocator());
+    return failure.diagnose(asNote);
   }
   }
 }

test/expr/unary/keypath/keypath.swift

@@ -732,7 +732,7 @@ func test_keypath_with_static_members(_ p: P_With_Static_Members) {
     static var baz: Int = 42
   }
 
-func foo(_ s: S) {
+  func foo(_ s: S) {
     let _ = \S.Type.foo
     // expected-error@-1 {{key path cannot refer to static member 'foo'}}
     let _ = s[keyPath: \.foo]
@@ -748,6 +748,34 @@ func foo(_ s: S) {
   }
 }
 
+func test_keypath_with_mutating_getter() {
+  struct S {
+    var foo: Int {
+      mutating get { return 42 }
+    }
+
+    subscript(_: Int) -> [Int] {
+      mutating get { return [] }
+    }
+  }
+
+  _ = \S.foo
+  // expected-error@-1 {{key path cannot refer to 'foo', which has a mutating getter}}
+  let _: KeyPath<S, Int> = \.foo
+  // expected-error@-1 {{key path cannot refer to 'foo', which has a mutating getter}}
+  _ = \S.[0]
+  // expected-error@-1 {{key path cannot refer to 'subscript(_:)', which has a mutating getter}}
+  _ = \S.[0].count
+  // expected-error@-1 {{key path cannot refer to 'subscript(_:)', which has a mutating getter}}
+
+  func test_via_subscript(_ s: S) {
+    _ = s[keyPath: \.foo]
+    // expected-error@-1 {{key path cannot refer to 'foo', which has a mutating getter}}
+    _ = s[keyPath: \.[0].count]
+    // expected-error@-1 {{key path cannot refer to 'subscript(_:)', which has a mutating getter}}
+  }
+}
+
 func testSyntaxErrors() { // expected-note{{}}
   _ = \.  ; // expected-error{{expected member name following '.'}}
   _ = \.a ;