GenEx: Parse comma after colon tokens specially

Otherwise the comma is treated as plain text by ParseContent.

$<STREQUAL:,> should be valid and true.
$<STREQUAL:,something> should be valid and false.

$<STREQUAL:,,> should be non-valid as it is 3 parameters.
$<STREQUAL:something,,> should be non-valid as it is 3 parameters.

Additionally, this allows reporting the correct error for other
expressions. For example $<TARGET_PROPERTY:,> should be invalid
because it has an empty target and empty property. It shouldn't
attempt to read the property ',' on the 'implicit this' target.

1 files changed, 7 insertions, 1 deletions

diff --git a/Source/cmGeneratorExpressionParser.cxx b/Source/cmGeneratorExpressionParser.cxx
index 344e9f8..d95e1cc 100644
--- a/Source/cmGeneratorExpressionParser.cxx
+++ b/Source/cmGeneratorExpressionParser.cxx
@@ -118,10 +118,16 @@ void cmGeneratorExpressionParser::ParseGeneratorExpression(
     colonToken = this->it;
     parameters.resize(parameters.size() + 1);
     ++this->it;
+    while (this->it->TokenType == cmGeneratorExpressionToken::CommaSeparator)
+      {
+      commaTokens.push_back(this->it);
+      parameters.resize(parameters.size() + 1);
+      ++this->it;
+      }
     while(this->it->TokenType != cmGeneratorExpressionToken::EndExpression)
       {
       this->ParseContent(*(parameters.end() - 1));
-      if (this->it->TokenType == cmGeneratorExpressionToken::CommaSeparator)
+      while (this->it->TokenType == cmGeneratorExpressionToken::CommaSeparator)
         {
         commaTokens.push_back(this->it);
         parameters.resize(parameters.size() + 1);