fix corner case in collapse_vars (#4199)

fixes #4197

lib/compress.js

@@ -1599,9 +1599,13 @@ merge(Compressor.prototype, {
                 }
                 if (node instanceof AST_SymbolRef) {
                     if (symbol_in_lvalues(node, parent)) {
-                        return !parent || parent.operator != "=" || parent.left !== node;
+                        return !(parent instanceof AST_Assign && parent.operator == "=" && parent.left === node);
                     }
-                    return side_effects && may_modify(node);
+                    if (side_effects && may_modify(node)) return true;
+                    var def = node.definition();
+                    return (in_try || def.scope.resolve() !== scope) && !all(def.orig, function(sym) {
+                        return !(sym instanceof AST_SymbolConst);
+                    });
                 }
                 if (node instanceof AST_This) return symbol_in_lvalues(node, parent);
                 if (node instanceof AST_VarDef) {

test/compress/const.js

@@ -776,3 +776,32 @@ issue_4195: {
     }
     expect_stdout: "undefined"
 }
+
+issue_4197: {
+    options = {
+        collapse_vars: true,
+    }
+    input: {
+        var a = 0;
+        try {
+            const b = function() {
+                a = 1;
+                b[1];
+            }();
+        } catch (e) {
+            console.log(a);
+        }
+    }
+    expect: {
+        var a = 0;
+        try {
+            const b = function() {
+                a = 1;
+                b[1];
+            }();
+        } catch (e) {
+            console.log(a);
+        }
+    }
+    expect_stdout: "1"
+}

test/ufuzz/index.js

@@ -1348,7 +1348,7 @@ for (var round = 1; round <= num_iterations; round++) {
                 ok = sandbox.same_stdout(sandbox.run_code(sort_globals(original_code)), sandbox.run_code(sort_globals(uglify_code)));
             }
             // ignore numerical imprecision caused by `unsafe_math`
-            if (!ok && typeof uglify_result == "string" && o.compress && o.compress.unsafe_math) {
+            if (!ok && o.compress && o.compress.unsafe_math && typeof original_result == "string" && typeof uglify_result == "string") {
                 ok = fuzzy_match(original_result, uglify_result);
                 if (!ok) {
                     var fuzzy_result = sandbox.run_code(original_code.replace(/( - 0\.1){3}/g, " - 0.3"), toplevel);