improve `unsafe` on undefined (#1548)

`unsafe` turns undefined keyword into a variable of the same name if found, but that interferes with other related optimisations.

Keep track of such transformations to ensure zero information loss in the process.

2 files changed, 37 insertions, 12 deletions

diff --git a/test/compress/issue-1443.js b/test/compress/issue-1443.js
index a2565872..304a71ac 100644
--- a/test/compress/issue-1443.js
+++ b/test/compress/issue-1443.js
@@ -2,6 +2,7 @@
 
 unsafe_undefined: {
     options = {
+        conditionals: true,
         if_return: true,
         unsafe: true
     }
@@ -19,12 +20,7 @@ unsafe_undefined: {
     expect: {
         function f(n) {
             return function() {
-                if (a)
-                    return b;
-                if (c)
-                    return d;
-                else
-                    return n;
+                return a ? b : c ? d : n;
             };
         }
     }
@@ -32,6 +28,7 @@ unsafe_undefined: {
 
 keep_fnames: {
     options = {
+        conditionals: true,
         if_return: true,
         unsafe: true
     }
@@ -57,12 +54,7 @@ keep_fnames: {
                 function n(n) {
                     return n * n;
                 }
-                if (a)
-                    return b;
-                if (c)
-                    return d;
-                else
-                    return r;
+                return a ? b : c ? d : r;
             };
         }
     }
diff --git a/test/compress/sequences.js b/test/compress/sequences.js
index 7bb274cb..41cfc726 100644
--- a/test/compress/sequences.js
+++ b/test/compress/sequences.js
@@ -251,3 +251,36 @@ iife: {
         function d() {}(), function e() {}(), function f() {}(), function g() {}();
     }
 }
+
+unsafe_undefined: {
+    options = {
+        conditionals: true,
+        if_return: true,
+        sequences: true,
+        side_effects: true,
+        unsafe: true,
+    }
+    input: {
+        function f(undefined) {
+            if (a)
+                return b;
+            if (c)
+                return d;
+        }
+        function g(undefined) {
+            if (a)
+                return b;
+            if (c)
+                return d;
+            e();
+        }
+    }
+    expect: {
+        function f(undefined) {
+            return a ? b : c ? d : undefined;
+        }
+        function g(undefined) {
+            return a ? b : c ? d : void e();
+        }
+    }
+}