fix corner case in unused (#3747)

fixes #3746

lib/compress.js

@@ -3916,14 +3916,18 @@ merge(Compressor.prototype, {
             } else if (node instanceof AST_Unary) {
                 if (node.write_only) sym = node.expression;
             }
-            if (!/strict/.test(compressor.option("pure_getters"))) return sym instanceof AST_SymbolRef && sym;
-            while (sym instanceof AST_PropAccess && !sym.expression.may_throw_on_access(compressor)) {
-                if (sym instanceof AST_Sub) props.unshift(sym.property);
-                sym = sym.expression;
+            if (/strict/.test(compressor.option("pure_getters"))) {
+                while (sym instanceof AST_PropAccess && !sym.expression.may_throw_on_access(compressor)) {
+                    if (sym instanceof AST_Sub) props.unshift(sym.property);
+                    sym = sym.expression;
+                }
             }
-            return sym instanceof AST_SymbolRef && all(sym.definition().orig, function(sym) {
+            if (!(sym instanceof AST_SymbolRef)) return;
+            if (compressor.exposed(sym.definition())) return;
+            if (!all(sym.definition().orig, function(sym) {
                 return !(sym instanceof AST_SymbolLambda);
-            }) && sym;
+            })) return;
+            return sym;
         };
         var in_use = [];
         var in_use_ids = Object.create(null); // avoid expensive linear scans of in_use

test/compress/drop-unused.js

@@ -2413,3 +2413,34 @@ issue_3673: {
     }
     expect_stdout: "PASS"
 }
+
+issue_3746: {
+    options = {
+        keep_fargs: "strict",
+        side_effects: true,
+        unused: true,
+    }
+    input: {
+        try {
+            A;
+        } catch (e) {
+            var e;
+        }
+        (function f(a) {
+            e = a;
+        })();
+        console.log("PASS");
+    }
+    expect: {
+        try {
+            A;
+        } catch (e) {
+            var e;
+        }
+        (function(a) {
+            e = a;
+        })();
+        console.log("PASS");
+    }
+    expect_stdout: "PASS"
+}