Change error diagnosed for when self isn't available in scope

include/swift/AST/DiagnosticsSema.def

@@ -905,6 +905,8 @@ ERROR(cannot_find_in_scope,none,
 ERROR(cannot_find_in_scope_corrected,none,
       "cannot %select{find|find operator}1 %0 in scope; did you mean '%2'?",
       (DeclNameRef, bool, StringRef))
+ERROR(cannot_find_self_in_scope,none,
+      "cannot find 'self' in scope; did you mean to use it in a type or extension context?", ())
 NOTE(confusable_character,none,
       "%select{identifier|operator}0 '%1' contains possibly confused characters; "
       "did you mean to use '%2'?",

lib/Sema/PreCheckExpr.cpp

@@ -586,10 +586,19 @@ Expr *TypeChecker::resolveDeclRefExpr(UnresolvedDeclRefExpr *UDRE,
     }
 
     auto emitBasicError = [&] {
-      Context.Diags
-          .diagnose(Loc, diag::cannot_find_in_scope, Name,
-                    Name.isOperator())
-          .highlight(UDRE->getSourceRange());
+
+      if (Name.isSimpleName(Context.Id_self)) {
+        // `self` gets diagnosed with a different error when it can't be found.
+        Context.Diags
+            .diagnose(Loc, diag::cannot_find_self_in_scope)
+            .highlight(UDRE->getSourceRange());
+      } else {
+        Context.Diags
+            .diagnose(Loc, diag::cannot_find_in_scope, Name,
+                      Name.isOperator())
+            .highlight(UDRE->getSourceRange());
+      }
+
       if (!Context.LangOpts.DisableExperimentalClangImporterDiagnostics) {
         Context.getClangModuleLoader()->diagnoseTopLevelValue(
             Name.getFullName());

test/Sema/self_cannot_be_found.swift

@@ -0,0 +1,21 @@
+// RUN: %target-typecheck-verify-swift
+
+let _ = self // expected-error {{cannot find 'self' in scope; did you mean to use it in a type or extension context?}} 
+let s = self.b // // expected-error {{cannot find 'self' in scope; did you mean to use it in a type or extension context?}} 
+let w = self.method() // expected-error {{cannot find 'self' in scope; did you mean to use it in a type or extension context?}} 
+
+struct aStruct {
+    let b = "a"
+
+    func testFunc() {
+        print(self.b)
+    }
+
+    var c: String {
+        self.b
+    }
+}
+
+let _ = aStruct.self
+let initialized = aStruct().self
+_ = initialized.b