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.

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);

Tests/GeneratorExpression/CMakeLists.txt

@@ -37,6 +37,8 @@ add_custom_target(check ALL
     -Dtest_strequal_angle_r=$<STREQUAL:$<ANGLE-R>,$<ANGLE-R>>
     -Dtest_strequal_comma=$<STREQUAL:$<COMMA>,$<COMMA>>
     -Dtest_strequal_angle_r_comma=$<STREQUAL:$<ANGLE-R>,$<COMMA>>
+    -Dtest_strequal_both_empty=$<STREQUAL:,>
+    -Dtest_strequal_one_empty=$<STREQUAL:something,>
     -Dtest_angle_r=$<ANGLE-R>
     -Dtest_comma=$<COMMA>
     -P ${CMAKE_CURRENT_SOURCE_DIR}/check.cmake

Tests/GeneratorExpression/check.cmake

@@ -38,5 +38,7 @@ check(test_strequal_no_yes "0")
 check(test_strequal_angle_r "1")
 check(test_strequal_comma "1")
 check(test_strequal_angle_r_comma "0")
+check(test_strequal_both_empty "1")
+check(test_strequal_one_empty "0")
 check(test_angle_r ">")
 check(test_comma ",")

Tests/RunCMake/GeneratorExpression/BadNOT-stderr.txt

@@ -20,7 +20,7 @@ CMake Error at BadNOT.cmake:1 \(add_custom_target\):
 
     \$<NOT:,>
 
-  \$<NOT> parameter must resolve to exactly one '0' or '1' value.
+  \$<NOT> expression requires exactly one parameter.
 Call Stack \(most recent call first\):
   CMakeLists.txt:3 \(include\)
 +

Tests/RunCMake/GeneratorExpression/BadStrEqual-result.txt

@@ -0,0 +1 @@
+1

Tests/RunCMake/GeneratorExpression/BadStrEqual-stderr.txt

@@ -0,0 +1,38 @@
+CMake Error at BadStrEqual.cmake:1 \(add_custom_target\):
+  Error evaluating generator expression:
+
+    \$<STREQUAL>
+
+  \$<STREQUAL> expression requires 2 comma separated parameters, but got 0
+  instead.
+Call Stack \(most recent call first\):
+  CMakeLists.txt:3 \(include\)
++CMake Error at BadStrEqual.cmake:1 \(add_custom_target\):
+  Error evaluating generator expression:
+
+    \$<STREQUAL:>
+
+  \$<STREQUAL> expression requires 2 comma separated parameters, but got 1
+  instead.
+Call Stack \(most recent call first\):
+  CMakeLists.txt:3 \(include\)
++
+CMake Error at BadStrEqual.cmake:1 \(add_custom_target\):
+  Error evaluating generator expression:
+
+    \$<STREQUAL:,,>
+
+  \$<STREQUAL> expression requires 2 comma separated parameters, but got 3
+  instead.
+Call Stack \(most recent call first\):
+  CMakeLists.txt:3 \(include\)
++
+CMake Error at BadStrEqual.cmake:1 \(add_custom_target\):
+  Error evaluating generator expression:
+
+    \$<STREQUAL:something,,>
+
+  \$<STREQUAL> expression requires 2 comma separated parameters, but got 3
+  instead.
+Call Stack \(most recent call first\):
+  CMakeLists.txt:3 \(include\)$

Tests/RunCMake/GeneratorExpression/BadStrEqual.cmake

@@ -0,0 +1,6 @@
+add_custom_target(check ALL COMMAND check
+  $<STREQUAL>
+  $<STREQUAL:>
+  $<STREQUAL:,,>
+  $<STREQUAL:something,,>
+  VERBATIM)

Tests/RunCMake/GeneratorExpression/RunCMakeTest.cmake

@@ -4,3 +4,4 @@ run_cmake(BadCONFIG)
 run_cmake(BadOR)
 run_cmake(BadAND)
 run_cmake(BadNOT)
+run_cmake(BadStrEqual)