account for exceptions in AST_Assign.left (#2892)

fixes #2891

lib/compress.js

@@ -2758,8 +2758,13 @@ merge(Compressor.prototype, {
             return any(this.elements, compressor);
         });
         def(AST_Assign, function(compressor){
-            return this.operator != "=" && this.left.may_throw(compressor)
+            if (this.right.may_throw(compressor)) return true;
-                || this.right.may_throw(compressor);
+            if (!compressor.has_directive("use strict")
+                && this.operator == "="
+                && this.left instanceof AST_SymbolRef) {
+                return false;
+            }
+            return this.left.may_throw(compressor);
         });
         def(AST_Binary, function(compressor){
             return this.left.may_throw(compressor)

test/compress/collapse_vars.js

@@ -4461,3 +4461,59 @@ issue_2878: {
     }
     expect_stdout: "1"
 }
+
+issue_2891_1: {
+    options = {
+        collapse_vars: true,
+    }
+    input: {
+        var a = "PASS", b;
+        try {
+            b = c.p = 0;
+            a = "FAIL";
+            b();
+        } catch (e) {
+        }
+        console.log(a);
+    }
+    expect: {
+        var a = "PASS", b;
+        try {
+            b = c.p = 0;
+            a = "FAIL";
+            b();
+        } catch (e) {
+        }
+        console.log(a);
+    }
+    expect_stdout: "PASS"
+}
+
+issue_2891_2: {
+    options = {
+        collapse_vars: true,
+    }
+    input: {
+        "use strict";
+        var a = "PASS", b;
+        try {
+            b = c = 0;
+            a = "FAIL";
+            b();
+        } catch (e) {
+        }
+        console.log(a);
+    }
+    expect: {
+        "use strict";
+        var a = "PASS", b;
+        try {
+            b = c = 0;
+            a = "FAIL";
+            b();
+        } catch (e) {
+        }
+        console.log(a);
+    }
+    expect_stdout: true
+}