[CSDiagnostics] Support static "Type." fix-it

Check whether we found a static property, and if
so, suggest inserting "Type." instead of "self.".

Resolves SR-11788.

Author: hamishknight

include/swift/AST/DiagnosticsSema.def

@@ -3424,9 +3424,9 @@ ERROR(assignment_bang_has_immutable_subcomponent,none,
 NOTE(change_to_mutating,none,
      "mark %select{method|accessor}0 'mutating' to make 'self' mutable",
      (bool))
-NOTE(masked_instance_variable,none,
-     "add explicit 'self.' to refer to mutable property of %0",
-     (Type))
+NOTE(masked_mutable_property,none,
+     "add explicit '%0' to refer to mutable %1 of %2",
+     (StringRef, DescriptiveDeclKind, Type))
 
 ERROR(assignment_let_property_delegating_init,none,
       "'let' property %0 may not be initialized directly; use "

lib/Sema/CSDiagnostics.cpp

@@ -1616,8 +1616,8 @@ bool AssignmentFailure::diagnoseAsError() {
       emitDiagnostic(Loc, DeclDiagnostic, message)
           .highlight(immutableExpr->getSourceRange());
 
-      // If there is a masked instance variable of the same type, emit a
-      // note to fixit prepend a 'self.'.
+      // If there is a masked property of the same type, emit a
+      // note to fixit prepend a 'self.' or 'Type.'.
       if (auto typeContext = DC->getInnermostTypeContext()) {
         SmallVector<ValueDecl *, 2> results;
         DC->lookupQualified(typeContext->getSelfNominalTypeDecl(),
@@ -1648,9 +1648,20 @@ bool AssignmentFailure::diagnoseAsError() {
 
         if (foundProperty != results.end()) {
           auto startLoc = immutableExpr->getStartLoc();
-          emitDiagnostic(startLoc, diag::masked_instance_variable,
-                         typeContext->getSelfTypeInContext())
-              .fixItInsert(startLoc, "self.");
+          auto *property = *foundProperty;
+          auto selfTy = typeContext->getSelfTypeInContext();
+
+          // If we found an instance property, suggest inserting "self.",
+          // otherwise suggest "Type." for a static property.
+          std::string fixItText;
+          if (property->isInstanceMember()) {
+            fixItText = "self.";
+          } else {
+            fixItText = selfTy->getString() + ".";
+          }
+          emitDiagnostic(startLoc, diag::masked_mutable_property,
+                         fixItText, property->getDescriptiveKind(), selfTy)
+              .fixItInsert(startLoc, fixItText);
         }
       }
 

test/Sema/immutability.swift

@@ -704,6 +704,7 @@ extension JustAProtocol {
 
 struct S {
   var x = 0
+  static var y = 0
 
   struct Nested {
     func foo() {
@@ -724,5 +725,19 @@ struct S {
 
     x = 1 // expected-error {{cannot assign to value: 'x' is a 'let' constant}}
     // expected-note@-1 {{add explicit 'self.' to refer to mutable property of 'S'}} {{5-5=self.}}
+
+    // SR-11788: Insert "Type." for a static property.
+    let y = 0 // expected-note {{change 'let' to 'var' to make it mutable}}
+    y += 1 // expected-error {{left side of mutating operator isn't mutable: 'y' is a 'let' constant}}
+    // expected-note@-1 {{add explicit 'S.' to refer to mutable static property of 'S'}} {{5-5=S.}}
+  }
+}
+
+struct S2<T> {
+  static var y: Int { get { 0 } set {} }
+  func foo() {
+    let y = 0 // expected-note {{change 'let' to 'var' to make it mutable}}
+    y += 1 // expected-error {{left side of mutating operator isn't mutable: 'y' is a 'let' constant}}
+    // expected-note@-1 {{add explicit 'S2<T>.' to refer to mutable static property of 'S2<T>'}} {{5-5=S2<T>.}}
   }
 }