aws_resource_missing_tags: fix provider reference without alias

When a resource uses `provider = aws` without an alias, the provider
alias should be treated as "default" to match provider-level default_tags.

Fixes #1002

Author: bendrucker

rules/aws_resource_missing_tags.go

@@ -172,7 +172,10 @@ func (r *AwsResourceMissingTagsRule) Check(runner tflint.Runner) error {
 					logger.Error("error decoding provider: %w", diagnostics)
 					return diagnostics
 				}
-				providerAlias = provider.Alias
+				if provider.Alias != "" {
+					providerAlias = provider.Alias
+				}
+				// else: keep providerAlias as "default" when no alias is specified
 			}
 
 			// If the resource has a tags attribute

rules/aws_resource_missing_tags_test.go

@@ -525,6 +525,30 @@ rule "aws_resource_missing_tags" {
 				},
 			},
 		},
+		{
+			Name: "Provider reference without alias (issue #1002)",
+			Content: `
+provider "aws" {
+  region = "eu-west-1"
+  default_tags {
+    tags = {
+      "layer": "app"
+      "type": "service"
+    }
+  }
+}
+
+resource "aws_iam_user" "bedrock" {
+  provider = aws
+  name = "test"
+}`,
+			Config: `
+rule "aws_resource_missing_tags" {
+  enabled = true
+  tags = ["layer", "type"]
+}`,
+			Expected: helper.Issues{},
+		},
 	}
 
 	rule := NewAwsResourceMissingTagsRule()