introduce "strict" to `pure_getters` (#1795)

4 files changed, 59 insertions, 16 deletions

diff --git a/README.md b/README.md
index d57a15ce..d15f114c 100644
--- a/README.md
+++ b/README.md
@@ -411,6 +411,8 @@ to set `true`; it's effectively a shortcut for `foo=true`).
 - `pure_getters` -- the default is `false`.  If you pass `true` for
   this, UglifyJS will assume that object property access
   (e.g. `foo.bar` or `foo["bar"]`) doesn't have any side effects.
+  Specify `"strict"` to treat `foo.bar` as side-effect-free only when
+  `foo` is certain to not throw, i.e. not `null` or `undefined`.
 
 - `pure_funcs` -- default `null`.  You can pass an array of names and
   UglifyJS will assume that those functions do not produce side
diff --git a/lib/compress.js b/lib/compress.js
index 8df6e58f..c199d13f 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -71,7 +71,7 @@ function Compressor(options, false_by_default) {
         negate_iife   : !false_by_default,
         passes        : 1,
         properties    : !false_by_default,
-        pure_getters  : false,
+        pure_getters  : !false_by_default && "strict",
         pure_funcs    : null,
         reduce_vars   : !false_by_default,
         screw_ie8     : true,
@@ -1165,7 +1165,13 @@ merge(Compressor.prototype, {
     // may_eq_null()
     // returns true if this node may evaluate to null or undefined
     (function(def) {
-        def(AST_Node, return_true);
+        function is_strict(compressor) {
+            return /strict/.test(compressor.option("pure_getters"));
+        }
+
+        def(AST_Node, function(compressor) {
+            return !is_strict(compressor);
+        });
         def(AST_Null, return_true);
         def(AST_Undefined, return_true);
         def(AST_Constant, return_false);
@@ -1198,12 +1204,9 @@ merge(Compressor.prototype, {
         def(AST_Seq, function(compressor) {
             return this.cdr.may_eq_null(compressor);
         });
-        def(AST_PropAccess, function(compressor) {
-            return !compressor.option("unsafe");
-        });
         def(AST_SymbolRef, function(compressor) {
             if (this.is_undefined) return true;
-            if (compressor.option("unsafe")) return false;
+            if (!is_strict(compressor)) return false;
             var fixed = this.fixed_value();
             return !fixed || fixed.may_eq_null(compressor);
         });
diff --git a/test/compress/pure_getters.js b/test/compress/pure_getters.js
index 338f8639..c2dcb95b 100644
--- a/test/compress/pure_getters.js
+++ b/test/compress/pure_getters.js
@@ -1,10 +1,9 @@
-side_effects: {
+strict: {
     options = {
-        pure_getters: true,
+        pure_getters: "strict",
         reduce_vars: false,
         side_effects: true,
         toplevel: true,
-        unsafe: false,
     }
     input: {
         var a, b = null, c = {};
@@ -28,13 +27,12 @@ side_effects: {
     }
 }
 
-side_effects_reduce_vars: {
+strict_reduce_vars: {
     options = {
-        pure_getters: true,
+        pure_getters: "strict",
         reduce_vars: true,
         side_effects: true,
         toplevel: true,
-        unsafe: false,
     }
     input: {
         var a, b = null, c = {};
@@ -57,13 +55,38 @@ side_effects_reduce_vars: {
     }
 }
 
-side_effects_unsafe: {
+unsafe: {
     options = {
         pure_getters: true,
         reduce_vars: false,
         side_effects: true,
         toplevel: true,
-        unsafe: true,
+    }
+    input: {
+        var a, b = null, c = {};
+        a.prop;
+        b.prop;
+        c.prop;
+        d.prop;
+        null.prop;
+        (void 0).prop;
+        undefined.prop;
+    }
+    expect: {
+        var a, b = null, c = {};
+        d;
+        null.prop;
+        (void 0).prop;
+        (void 0).prop;
+    }
+}
+
+unsafe_reduce_vars: {
+    options = {
+        pure_getters: true,
+        reduce_vars: true,
+        side_effects: true,
+        toplevel: true,
     }
     input: {
         var a, b = null, c = {};
diff --git a/test/compress/reduce_vars.js b/test/compress/reduce_vars.js
index 842d8de4..b6f711ad 100644
--- a/test/compress/reduce_vars.js
+++ b/test/compress/reduce_vars.js
@@ -1919,7 +1919,7 @@ side_effects_assign: {
 
 pure_getters_1: {
     options = {
-        pure_getters: true,
+        pure_getters: "strict",
         reduce_vars: true,
         side_effects: true,
         toplevel: true,
@@ -1941,7 +1941,7 @@ pure_getters_1: {
 
 pure_getters_2: {
     options = {
-        pure_getters: true,
+        pure_getters: "strict",
         reduce_vars: true,
         toplevel: true,
         unused: true,
@@ -1956,6 +1956,21 @@ pure_getters_2: {
     }
 }
 
+pure_getters_3: {
+    options = {
+        pure_getters: true,
+        reduce_vars: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        var a;
+        var a = a && a.b;
+    }
+    expect: {
+    }
+}
+
 catch_var: {
     options = {
         booleans: true,