C++: Reduce FPs by excluding all commas in loop heads

This leads to a 50% reduction of alerts in MRVA 1000.

Author: d10c

cpp/ql/src/Best Practices/Likely Errors/CommaBeforeMisleadingIndentation.ql

@@ -20,6 +20,12 @@ Expr normalizeExpr(Expr e) {
     else result = e
 }
 
+predicate isInLoopHead(CommaExpr ce) {
+  ce.getParent*() = [any(Loop l).getCondition(), any(ForStmt f).getUpdate()]
+  or
+  ce.getEnclosingStmt() = any(ForStmt f).getInitialization()
+}
+
 from CommaExpr ce, Expr left, Expr right, Location leftLoc, Location rightLoc
 where
   ce.fromSource() and
@@ -28,6 +34,7 @@ where
   right = normalizeExpr(ce.getRightOperand()) and
   leftLoc = left.getLocation() and
   rightLoc = right.getLocation() and
+  not isInLoopHead(ce) and // HACK to reduce FPs in loop heads; assumption: unlikely to be misread due to '(', ')' delimiters
   leftLoc.getEndLine() < rightLoc.getStartLine() and
   leftLoc.getStartColumn() > rightLoc.getStartColumn()
-select right, "The indentation level after the comma can be misleading (for some tab sizes)."
+select right, "The indentation after the comma may be misleading (for some tab sizes)."

cpp/ql/test/query-tests/Best Practices/Likely Errors/CommaBeforeMisleadingIndentation/test.cpp

@@ -86,6 +86,15 @@ int test(int i, int j, int (*foo)(int), int (*bar)(int, int))
         i = j = i + j;
     }
 
+	for (i = 0,         // GOOD? Currently ignoring loop heads.
+	    j = 1;
+		i + j < 10;
+		i++, j++);
+
+	for (i = 0,
+         j = 1; i < 10; i += 2, // GOOD? Currently ignoring loop heads.
+        j++) {}
+
     // Mixed tabs and spaces (ugly case):
 
     for (i = 0,         // GOOD if tab >= 4 spaces else BAD -- can't exclude w/o source code text :/
@@ -98,17 +107,6 @@ int test(int i, int j, int (*foo)(int), int (*bar)(int, int))
 	    (void)i,	    // GOOD if tab >= 4 spaces else BAD -- can't exclude w/o source code text :/
 		(void)j;
 
-	// One char difference (common but borderline):
-
-	for (i = 0,         // GOOD? [FALSE POSITIVE] -- can't exclude w/o source code text :/
-	    j = 1;
-		i + j < 10;
-		i++, j++);
-
-	for (i = 0,
-         j = 1; i < 10; i += 2, // GOOD? [FALSE POSITIVE] -- can't exclude w/o source code text :/
-        j++) {}
-
 	// LHS ends on same line RHS begins on:
 
 	int k = (foo(