[CSGen] Abstract checking for a nil literal in visitSubscriptExpr into a

general isValidBaseOfMemberRef function.

Author: hborla

lib/Sema/CSGen.cpp

@@ -930,6 +930,16 @@ namespace {
       = { nullptr, nullptr };
     unsigned currentEditorPlaceholderVariable = 0;
 
+    /// Returns false and emits the specified diagnostic if the member reference
+    /// base is a nil literal. Returns true otherwise.
+    bool isValidBaseOfMemberRef(Expr *base, Diag<> diagnostic) {
+      if (auto nilLiteral = dyn_cast<NilLiteralExpr>(base)) {
+        CS.getASTContext().Diags.diagnose(nilLiteral->getLoc(), diagnostic);
+        return false;
+      }
+      return true;
+    }
+
     /// Add constraints for a reference to a named member of the given
     /// base type, and return the type of such a reference.
     Type addMemberRefConstraints(Expr *expr, Expr *base, DeclNameRef name,
@@ -1785,21 +1795,18 @@ namespace {
     }
 
     Type visitSubscriptExpr(SubscriptExpr *expr) {
-      auto &ctx = CS.getASTContext();
       ValueDecl *decl = nullptr;
       if (expr->hasDecl()) {
         decl = expr->getDecl().getDecl();
         if (decl->isInvalid())
           return Type();
       }
 
-      if (auto nilLiteral = dyn_cast<NilLiteralExpr>(expr->getBase())) {
-        ctx.Diags.diagnose(nilLiteral->getLoc(),
-                           diag::cannot_subscript_nil_literal);
+      auto *base = expr->getBase();
+      if (!isValidBaseOfMemberRef(base, diag::cannot_subscript_nil_literal))
         return nullptr;
-      }
 
-      return addSubscriptConstraints(expr, CS.getType(expr->getBase()),
+      return addSubscriptConstraints(expr, CS.getType(base),
                                      expr->getIndex(),
                                      decl, expr->getArgumentLabels(),
                                      expr->hasTrailingClosure());