Fix issue where closure implicit self check was being run on expr that wasn't member of closure

Author: calda

lib/Sema/MiscDiagnostics.cpp

@@ -2004,6 +2004,30 @@ static void diagnoseImplicitSelfUseInClosure(const Expr *E,
       diag.fixItInsertAfter(closureExpr->getLoc(), " [self] in" + trailing);
     }
   };
+  
+  // A walker that verifies whether or not `exprToMatch`
+  // is a descendent of the AST node being walked
+  class ExprIsDescendantWalker : public BaseDiagnosticWalker {
+    Expr *exprToMatch;
+  public:
+    bool exprIsDescendant = false;
+    
+    explicit ExprIsDescendantWalker(Expr *exprToMatch)
+      : exprToMatch(exprToMatch) { }
+    
+    bool shouldWalkIntoSeparatelyCheckedClosure(ClosureExpr *expr) override {
+      return true;
+    }
+    
+    PreWalkResult<Expr *> walkToExprPre(Expr *E) override {
+      if (E == exprToMatch) {
+        exprIsDescendant = true;
+        return Action::Stop();
+      }
+      
+      return Action::Continue(E);
+    }
+  };
 
   auto &ctx = DC->getASTContext();
   AbstractClosureExpr *ACE = nullptr;
@@ -2015,6 +2039,18 @@ static void diagnoseImplicitSelfUseInClosure(const Expr *E,
       DC = DC->getParent();
     }
   }
+  
+  // Verify E is actually a descendant of ACE before we use it.
+  // Otherwise we'd be diagnosing E within the context of a closure
+  // that it isn't actually a part of.
+  if (ACE) {
+    auto isDescendantWalker = ExprIsDescendantWalker(const_cast<Expr *>(E));
+    ACE->walk(isDescendantWalker);
+    if (!isDescendantWalker.exprIsDescendant) {
+      ACE = nullptr;
+    }
+  }
+  
   const_cast<Expr *>(E)->walk(DiagnoseWalker(ctx, ACE));
 }