Require parentheses around tuple in list comprehension filtering.

This affects only the tuple that appears after 'if' inside a list
comprehension. Since a truth value is expected, it's very unlikely to break
existing code. It's technically a breaking change, but it's not worth
introducing a flag for this.

RELNOTES:
  Parentheses around the tuple are now mandatory in [a for b in c if 1, 2]
PiperOrigin-RevId: 159218397

src/main/java/com/google/devtools/build/lib/syntax/Parser.java

diff --git a/src/main/java/com/google/devtools/build/lib/syntax/Parser.java b/src/main/java/com/google/devtools/build/lib/syntax/Parser.java
index 763b233..ed23432 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/Parser.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/Parser.java
@@ -817,7 +817,9 @@
         comprehensionBuilder.addFor(loopVar, listExpression);
       } else if (token.kind == TokenKind.IF) {
         nextToken();
-        comprehensionBuilder.addIf(parseExpression());
+        // [x for x in li if 1, 2]  # parse error
+        // [x for x in li if (1, 2)]  # ok
+        comprehensionBuilder.addIf(parseNonTupleExpression(0));
       } else if (token.kind == closingBracket) {
         nextToken();
         return comprehensionBuilder.build();