Fix the edge case re: free var comparison

spec/ameba/rule/lint/literals_comparison_spec.cr

@@ -41,6 +41,13 @@ module Ameba::Rule::Lint
         CRYSTAL
     end
 
+    pending "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
         expect_issue subject, <<-CRYSTAL
@@ -48,6 +55,12 @@ module Ameba::Rule::Lint
            # ^^^^^^^^^^^^^^ 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

@@ -59,6 +59,17 @@ module Ameba::Rule::Lint
       return unless obj.class.in?(LITERAL_TYPES) &&
                     arg.class.in?(LITERAL_TYPES)
 
+      # Edge-case: `{{ T == Nil }}`
+      #
+      # Current implementation works for any single name path comparisons:
+      # `String == Nil`, regardless of the free variable being present.
+      #
+      # Ideally we should only check whether either of the sides
+      # is a free var
+      if obj.is_a?(Crystal::Path) && arg.is_a?(Crystal::Path)
+        return if obj.single_name? && arg.single_name? && (obj != arg)
+      end
+
       is_dynamic = obj.class.in?(DYNAMIC_LITERAL_TYPES) ||
                    arg.class.in?(DYNAMIC_LITERAL_TYPES)