Resolve paths before comparing input/output destination

lib/output.js

@@ -1,5 +1,6 @@
 'use strict';
 
+const path = require('path');
 const is = require('./is');
 const sharp = require('./sharp');
 
@@ -59,7 +60,7 @@ function toFile (fileOut, callback) {
   let err;
   if (!is.string(fileOut)) {
     err = new Error('Missing output file path');
-  } else if (this.options.input.file === fileOut) {
+  } else if (is.string(this.options.input.file) && path.resolve(this.options.input.file) === path.resolve(fileOut)) {
     err = new Error('Cannot use same file for input and output');
   } else if (this.options.formatOut === 'input' && fileOut.toLowerCase().endsWith('.gif') && !this.constructor.format.magick.output.file) {
     err = errMagickSave;

test/unit/io.js

@@ -1,6 +1,7 @@
 'use strict';
 
 const fs = require('fs');
+const path = require('path');
 const assert = require('assert');
 const rimraf = require('rimraf');
 
@@ -316,6 +317,48 @@ describe('Input/output', function () {
     });
   });
 
+  it('Fail when output File is input File (relative output, absolute input)', function (done) {
+    const relativePath = path.relative(process.cwd(), fixtures.inputJpg);
+    sharp(fixtures.inputJpg).toFile(relativePath, function (err) {
+      assert(err instanceof Error);
+      assert.strictEqual('Cannot use same file for input and output', err.message);
+      done();
+    });
+  });
+
+  it('Fail when output File is input File via Promise (relative output, absolute input)', function (done) {
+    const relativePath = path.relative(process.cwd(), fixtures.inputJpg);
+    sharp(fixtures.inputJpg).toFile(relativePath).then(function (data) {
+      assert(false);
+      done();
+    }).catch(function (err) {
+      assert(err instanceof Error);
+      assert.strictEqual('Cannot use same file for input and output', err.message);
+      done();
+    });
+  });
+
+  it('Fail when output File is input File (relative input, absolute output)', function (done) {
+    const relativePath = path.relative(process.cwd(), fixtures.inputJpg);
+    sharp(relativePath).toFile(fixtures.inputJpg, function (err) {
+      assert(err instanceof Error);
+      assert.strictEqual('Cannot use same file for input and output', err.message);
+      done();
+    });
+  });
+
+  it('Fail when output File is input File via Promise (relative input, absolute output)', function (done) {
+    const relativePath = path.relative(process.cwd(), fixtures.inputJpg);
+    sharp(relativePath).toFile(fixtures.inputJpg).then(function (data) {
+      assert(false);
+      done();
+    }).catch(function (err) {
+      assert(err instanceof Error);
+      assert.strictEqual('Cannot use same file for input and output', err.message);
+      done();
+    });
+  });
+
   it('Fail when output File is empty', function (done) {
     sharp(fixtures.inputJpg).toFile('', function (err) {
       assert(err instanceof Error);