fix missing corner case in #2855 (#2868)

lib/compress.js

@@ -1044,13 +1044,15 @@ merge(Compressor.prototype, {
                 // but are otherwise not safe to scan into or beyond them.
                 var sym;
                 if (node instanceof AST_Call
+                    || node instanceof AST_Exit
+                        && (side_effects || lhs instanceof AST_PropAccess || may_modify(lhs))
                     || node instanceof AST_PropAccess
                         && (side_effects || node.expression.may_throw_on_access(compressor))
                     || node instanceof AST_SymbolRef
                         && !(parent instanceof AST_Assign && parent.operator == "=" && parent.left === node)
-                        && (lvalues[node.name] || may_modify(node))
+                        && (lvalues[node.name] || side_effects && may_modify(node))
                     || node instanceof AST_VarDef && node.value
-                        && (node.name.name in lvalues || may_modify(node.name))
+                        && (node.name.name in lvalues || side_effects && may_modify(node.name))
                     || (sym = is_lhs(node.left, node))
                         && (sym instanceof AST_PropAccess || sym.name in lvalues)
                     || may_throw
@@ -1356,7 +1358,6 @@ merge(Compressor.prototype, {
             }
 
             function may_modify(sym) {
-                if (!side_effects) return false;
                 var def = sym.definition();
                 if (def.orig.length == 1 && def.orig[0] instanceof AST_SymbolDefun) return false;
                 if (def.scope !== scope) return true;

test/compress/collapse_vars.js

@@ -4191,6 +4191,31 @@ return_3: {
     expect_stdout: "0"
 }
 
+return_4: {
+    options = {
+        collapse_vars: true,
+    }
+    input: {
+        var a = "FAIL";
+        (function(b) {
+            a = "PASS";
+            return;
+            b(a);
+        })();
+        console.log(a);
+    }
+    expect: {
+        var a = "FAIL";
+        (function(b) {
+            a = "PASS";
+            return;
+            b(a);
+        })();
+        console.log(a);
+    }
+    expect_stdout: "PASS"
+}
+
 issue_2858: {
     options = {
         collapse_vars: true,