[CS2] Fix #4629: interpolations (whether in strings or CSX tags) with only comments (#4659)

* Fix #4629: interpolations (whether in strings or CSX tags) that contain nothing but comments should not be optimized out

* Template literals need an expression inside their interpolations, so if we only have a comment to put in there, toss in an empty string as well

lib/coffeescript/lexer.js

@@ -1089,7 +1089,7 @@
     // of `'NEOSTRING'`s are converted using `fn` and turned into strings using
     // `options` first.
     mergeInterpolationTokens(tokens, options, fn) {
-      var converted, firstEmptyStringIndex, firstIndex, i, j, lastToken, len, locationToken, lparen, plusToken, rparen, tag, token, tokensToPush, value;
+      var converted, firstEmptyStringIndex, firstIndex, i, j, k, lastToken, len, len1, locationToken, lparen, placeholderToken, plusToken, rparen, tag, token, tokensToPush, val, value;
       if (tokens.length > 1) {
         lparen = this.token('STRING_START', '(', 0, 0);
       }
@@ -1100,8 +1100,32 @@
         switch (tag) {
           case 'TOKENS':
             if (value.length === 2) {
-              // Optimize out empty interpolations (an empty pair of parentheses).
-              continue;
+              if (!(value[0].comments || value[1].comments)) {
+                // Optimize out empty interpolations (an empty pair of parentheses).
+                continue;
+              }
+              // There are comments (and nothing else) in this interpolation.
+              if (this.csxDepth === 0) {
+                // This is an interpolated string, not a CSX tag; and for whatever
+                // reason `` `a${/*test*/}b` `` is invalid JS. So compile to
+                // `` `a${/*test*/''}b` `` instead.
+                placeholderToken = this.makeToken('STRING', "''");
+              } else {
+                placeholderToken = this.makeToken('JS', '');
+              }
+              // Use the same location data as the first parenthesis.
+              placeholderToken[2] = value[0][2];
+              for (k = 0, len1 = value.length; k < len1; k++) {
+                val = value[k];
+                if (!val.comments) {
+                  continue;
+                }
+                if (placeholderToken.comments == null) {
+                  placeholderToken.comments = [];
+                }
+                placeholderToken.comments.push(...val.comments);
+              }
+              value.splice(1, 0, placeholderToken);
             }
             // Push all the tokens in the fake `'TOKENS'` token. These already have
             // sane location data.

src/lexer.coffee

@@ -139,7 +139,7 @@ exports.Lexer = class Lexer
       return id.length
     if id is 'do' and regExSuper = /^(\s*super)(?!\(\))/.exec @chunk[3...]
       @token 'SUPER', 'super'
-      @token 'CALL_START', '('      
+      @token 'CALL_START', '('
       @token 'CALL_END', ')'
       [input, sup] = regExSuper
       return sup.length + 3
@@ -820,8 +820,23 @@ exports.Lexer = class Lexer
       [tag, value] = token
       switch tag
         when 'TOKENS'
-          # Optimize out empty interpolations (an empty pair of parentheses).
-          continue if value.length is 2
+          if value.length is 2
+            # Optimize out empty interpolations (an empty pair of parentheses).
+            continue unless value[0].comments or value[1].comments
+            # There are comments (and nothing else) in this interpolation.
+            if @csxDepth is 0
+              # This is an interpolated string, not a CSX tag; and for whatever
+              # reason `` `a${/*test*/}b` `` is invalid JS. So compile to
+              # `` `a${/*test*/''}b` `` instead.
+              placeholderToken = @makeToken 'STRING', "''"
+            else
+              placeholderToken = @makeToken 'JS', ''
+            # Use the same location data as the first parenthesis.
+            placeholderToken[2] = value[0][2]
+            for val in value when val.comments
+              placeholderToken.comments ?= []
+              placeholderToken.comments.push val.comments...
+            value.splice 1, 0, placeholderToken
           # Push all the tokens in the fake `'TOKENS'` token. These already have
           # sane location data.
           locationToken = value[0]

test/comments.coffee

@@ -721,6 +721,26 @@ test "Empty lines between comments are preserved", ->
     3;
   }'''
 
+test "Block comment in an interpolated string", ->
+  eqJS '"a#{### Comment ###}b"', "`a${/* Comment */''}b`;"
+  eqJS '"a#{### 1 ###}b#{### 2 ###}c"', "`a${/* 1 */''}b${/* 2 */''}c`;"
+
+test "#4629: Block comment in CSX interpolation", ->
+  eqJS '<div>{### Comment ###}</div>', '<div>{/* Comment */}</div>;'
+  eqJS '''
+  <div>
+  {###
+    Multiline
+    Comment
+  ###}
+  </div>''', '''
+  <div>
+  {/*
+    Multiline
+    Comment
+  */}
+  </div>;'''
+
 test "Line comment in an interpolated string", ->
   eqJS '''
   "a#{# Comment