From 5dafc456d29ae0fd76896993a3cd6ce8b2b86d3c Mon Sep 17 00:00:00 2001
From: "Iskander (Alex) Sharipov" <quasilyte@gmail.com>
Date: Sat, 2 Feb 2019 00:02:13 +0300
Subject: [PATCH] checkers: fix equalFold false positive (#788)

The following code makes sense and is correct:

	strings.ToLower(x) == x

Previously, equalFold would trigger on that.
Added a check that upper/lower casing call
operand is not the same as LHS/RHS.

Signed-off-by: Iskander Sharipov <quasilyte@gmail.com>
---
 checkers/equalFold_checker.go                 |  9 +++++++--
 checkers/testdata/equalFold/negative_tests.go | 17 +++++++++++++++++
 2 files changed, 24 insertions(+), 2 deletions(-)

diff --git a/checkers/equalFold_checker.go b/checkers/equalFold_checker.go
index 3f4fb914b..265b2f79b 100644
--- a/checkers/equalFold_checker.go
+++ b/checkers/equalFold_checker.go
@@ -7,6 +7,7 @@ import (
 	"github.com/go-lintpack/lintpack"
 	"github.com/go-lintpack/lintpack/astwalk"
 	"github.com/go-toolsmith/astcast"
+	"github.com/go-toolsmith/astequal"
 )
 
 func init() {
@@ -57,7 +58,9 @@ func (c *equalFoldChecker) checkBytes(expr *ast.CallExpr) {
 	if !ok1 && !ok2 {
 		return
 	}
-	c.warnBytes(expr, x, y)
+	if !astequal.Expr(x, y) {
+		c.warnBytes(expr, x, y)
+	}
 }
 
 func (c *equalFoldChecker) checkStrings(expr *ast.BinaryExpr) {
@@ -70,7 +73,9 @@ func (c *equalFoldChecker) checkStrings(expr *ast.BinaryExpr) {
 	if !ok1 && !ok2 {
 		return
 	}
-	c.warnStrings(expr, x, y)
+	if !astequal.Expr(x, y) {
+		c.warnStrings(expr, x, y)
+	}
 }
 
 func (c *equalFoldChecker) warnStrings(cause ast.Node, x, y ast.Expr) {
diff --git a/checkers/testdata/equalFold/negative_tests.go b/checkers/testdata/equalFold/negative_tests.go
index 35d041a50..1f6981b1a 100644
--- a/checkers/testdata/equalFold/negative_tests.go
+++ b/checkers/testdata/equalFold/negative_tests.go
@@ -5,6 +5,23 @@ import (
 	"strings"
 )
 
+func changeCaseOfSameExpr(x string, b []byte) {
+	_ = strings.ToLower(x) == x
+	_ = x == strings.ToLower(x)
+	_ = strings.ToLower(x) != x
+	_ = x != strings.ToLower(x)
+
+	_ = strings.ToUpper(x) == x
+	_ = x == strings.ToUpper(x)
+	_ = strings.ToUpper(x) != x
+	_ = x != strings.ToUpper(x)
+
+	_ = bytes.Equal(bytes.ToLower(b), b)
+	_ = bytes.Equal(b, bytes.ToLower(b))
+	_ = bytes.Equal(bytes.ToUpper(b), b)
+	_ = bytes.Equal(b, bytes.ToUpper(b))
+}
+
 func stringsEqualFold(x, y string) {
 	_ = strings.EqualFold(x, y)
 	_ = strings.EqualFold(x, concat(y, "123"))