Merge pull request #297 from crystal-ameba/Sija/fix-issue-295

Fix the edge case re: free var comparison

spec/ameba/rule/lint/literals_comparison_spec.cr

@@ -41,13 +41,26 @@ module Ameba::Rule::Lint
         CRYSTAL
     end
 
+    it "reports if there is a static path comparison evaluating to false" do
+      expect_issue subject, <<-CRYSTAL
+        String == Nil
+        # ^^^^^^^^^^^ error: Comparison always evaluates to false
+        CRYSTAL
+    end
+
     context "macro" do
-      it "reports in macro scope" do
+      pending "reports in macro scope" do
         expect_issue subject, <<-CRYSTAL
           {{ "foo" == "foo" }}
            # ^^^^^^^^^^^^^^ error: Comparison always evaluates to true
           CRYSTAL
       end
+
+      it "passes for free variables comparisons in macro scope" do
+        expect_no_issues subject, <<-CRYSTAL
+          {{ T == Nil }}
+          CRYSTAL
+      end
     end
 
     it "reports rule, pos and message" do

src/ameba/rule/lint/literals_comparison.cr

@@ -52,6 +52,22 @@ module Ameba::Rule::Lint
     LITERAL_TYPES =
       PRIMITIVE_LITERAL_TYPES + DYNAMIC_LITERAL_TYPES
 
+    # Edge-case: `{{ T == Nil }}`
+    #
+    # Current implementation just skips all macro contexts,
+    # regardless of the free variable being present.
+    #
+    # Ideally we should only check whether either of the sides
+    # is a free var
+    def test(source)
+      AST::NodeVisitor.new self, source, skip: [
+        Crystal::Macro,
+        Crystal::MacroExpression,
+        Crystal::MacroIf,
+        Crystal::MacroFor,
+      ]
+    end
+
     def test(source, node : Crystal::Call)
       return unless node.name.in?(OP_NAMES)
       return unless (obj = node.obj) && (arg = node.args.first?)