fix corner cases in `templates` (#4610)

3 files changed, 63 insertions, 4 deletions

diff --git a/README.md b/README.md
index 9a0be306..9f69ce00 100644
--- a/README.md
+++ b/README.md
@@ -769,6 +769,9 @@ to be `false` and all symbol names will be omitted.
 
 - `switches` (default: `true`) -- de-duplicate and remove unreachable `switch` branches
 
+- `templates` (default: `true`) -- compact template literals by embedding expressions
+  and/or converting to string literals, e.g. `` `foo ${42}` → "foo 42"``
+
 - `top_retain` (default: `null`) -- prevent specific toplevel functions and
   variables from `unused` removal (can be array, comma-separated, RegExp or
   function. Implies `toplevel`)
diff --git a/lib/compress.js b/lib/compress.js
index 7293f9df..dbd3dd35 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -4416,8 +4416,8 @@ merge(Compressor.prototype, {
             }
             var exprs = eval_all(this.expressions, compressor, ignore_side_effects, cached, depth);
             if (!exprs) return this;
-            var ret = decode(this.strings[0]);
             var malformed = false;
+            var ret = decode(this.strings[0]);
             for (var i = 0; i < exprs.length; i++) {
                 ret += exprs[i] + decode(this.strings[i + 1]);
             }
@@ -4426,7 +4426,7 @@ merge(Compressor.prototype, {
             return this;
 
             function decode(str) {
-                return str.replace(/\\(u[0-9a-fA-F]{4}|u\{[0-9a-fA-F]+\}|x[0-9a-fA-F]{2}|[0-9]+|[\s\S])/g, function(match, seq) {
+                return str.replace(/\\(u\{[^}]*\}?|u[\s\S]{0,4}|x[\s\S]{0,2}|[0-9]+|[\s\S])/g, function(match, seq) {
                     var s = decode_escape_sequence(seq);
                     if (typeof s != "string") malformed = true;
                     return s;
@@ -9952,8 +9952,18 @@ merge(Compressor.prototype, {
                     return "\\" + (s == "\r" ? "r" : s);
                 });
                 if (ev.length > node.print_to_string().length + 3) continue;
+                var combined = strs[i] + ev + strs[i + 1];
+                if (typeof make_node(AST_Template, self, {
+                    expressions: [],
+                    strings: [ combined ],
+                    tag: self.tag,
+                }).evaluate(compressor) != typeof make_node(AST_Template, self, {
+                    expressions: [ node ],
+                    strings: strs.slice(i, i + 2),
+                    tag: self.tag,
+                }).evaluate(compressor)) continue;
                 exprs.splice(i, 1);
-                strs.splice(i, 2, strs[i] + ev + strs[i + 1]);
+                strs.splice(i, 2, combined);
             }
         }
         return try_evaluate(compressor, self);
diff --git a/test/compress/templates.js b/test/compress/templates.js
index 3c7957f8..0be46df0 100644
--- a/test/compress/templates.js
+++ b/test/compress/templates.js
@@ -134,7 +134,7 @@ partial_evaluate: {
     node_version: ">=4"
 }
 
-malformed_evaluate: {
+malformed_evaluate_1: {
     options = {
         evaluate: true,
         templates: true,
@@ -149,6 +149,52 @@ malformed_evaluate: {
     node_version: ">=4"
 }
 
+malformed_evaluate_2: {
+    options = {
+        evaluate: true,
+        templates: true,
+    }
+    input: {
+        console.log(`\u0${0}b${5}`);
+    }
+    expect: {
+        console.log(`\u0${0}b5`);
+    }
+    expect_stdout: true
+    node_version: ">=4"
+}
+
+malformed_evaluate_3: {
+    options = {
+        evaluate: true,
+        templates: true,
+    }
+    input: {
+        console.log(`\u${0}b${5}`);
+    }
+    expect: {
+        console.log(`\u0b5`);
+    }
+    expect_stdout: true
+    node_version: ">=4"
+}
+
+malformed_evaluate_4: {
+    options = {
+        evaluate: true,
+        templates: true,
+        unsafe: true,
+    }
+    input: {
+        console.log(String.raw`\u0${0}b${5}`);
+    }
+    expect: {
+        console.log("\\u00b5");
+    }
+    expect_stdout: "\\u00b5"
+    node_version: ">=8"
+}
+
 unsafe_evaluate: {
     options = {
         evaluate: true,