Genex: Add `IF` generator expression

This allows a single condition to be used to choose between two
alternatives.  Without this the condition must be duplicated with
one surrounded by `NOT`.

Closes: #15585

6 files changed, 31 insertions, 0 deletions

diff --git a/Tests/GeneratorExpression/CMakeLists.txt b/Tests/GeneratorExpression/CMakeLists.txt
index f0d6abf..f920188 100644
--- a/Tests/GeneratorExpression/CMakeLists.txt
+++ b/Tests/GeneratorExpression/CMakeLists.txt
@@ -246,6 +246,10 @@ add_custom_target(check-part4 ALL
     # CMake as command-line argument
     -Dtest_shell_path=${path_prefix}$<SHELL_PATH:${test_shell_path}>
     -Dpath_prefix=${path_prefix}
+    -Dif_1=$<IF:1,a,b>
+    -Dif_2=$<IF:0,a,b>
+    -Dif_3=$<IF:$<EQUAL:10,30>,a,b>
+    -Dif_4=$<IF:$<EQUAL:30,30>,a,b>
     -DWIN32=${WIN32}
     -DCMAKE_GENERATOR=${CMAKE_GENERATOR}
     -P ${CMAKE_CURRENT_SOURCE_DIR}/check-part4.cmake
diff --git a/Tests/GeneratorExpression/check-part4.cmake b/Tests/GeneratorExpression/check-part4.cmake
index 9e516d5..f5d14dd 100644
--- a/Tests/GeneratorExpression/check-part4.cmake
+++ b/Tests/GeneratorExpression/check-part4.cmake
@@ -13,3 +13,8 @@ if(WIN32)
 else()
   check(test_shell_path [[/shell/path]])
 endif()
+
+check(if_1 "a")
+check(if_2 "b")
+check(if_3 "b")
+check(if_4 "a")
diff --git a/Tests/RunCMake/GeneratorExpression/BadIF-result.txt b/Tests/RunCMake/GeneratorExpression/BadIF-result.txt
new file mode 100644
index 0000000..d00491f
--- /dev/null
+++ b/Tests/RunCMake/GeneratorExpression/BadIF-result.txt
@@ -0,0 +1 @@
+1
diff --git a/Tests/RunCMake/GeneratorExpression/BadIF-stderr.txt b/Tests/RunCMake/GeneratorExpression/BadIF-stderr.txt
new file mode 100644
index 0000000..7c7506c
--- /dev/null
+++ b/Tests/RunCMake/GeneratorExpression/BadIF-stderr.txt
@@ -0,0 +1,15 @@
+CMake Error at BadIF.cmake:2 \(add_custom_target\):
+  Error evaluating generator expression:
+
+    \$<IF:asdf,a,b>
+
+  First parameter to \$<IF> must resolve to exactly one '0' or '1' value.
+Call Stack \(most recent call first\):
+  CMakeLists.txt:3 \(include\)
++
+CMake Error at BadIF.cmake:2 \(add_custom_target\):
+  Error evaluating generator expression:
+
+    \$<IF:asdf,a>
+
+  \$<IF> expression requires 3 comma separated parameters, but got 2 instead.
diff --git a/Tests/RunCMake/GeneratorExpression/BadIF.cmake b/Tests/RunCMake/GeneratorExpression/BadIF.cmake
new file mode 100644
index 0000000..583f68d
--- /dev/null
+++ b/Tests/RunCMake/GeneratorExpression/BadIF.cmake
@@ -0,0 +1,5 @@
+
+add_custom_target(check ALL COMMAND check
+    $<IF:asdf,a,b>
+    $<IF:asdf,a>
+    )
diff --git a/Tests/RunCMake/GeneratorExpression/RunCMakeTest.cmake b/Tests/RunCMake/GeneratorExpression/RunCMakeTest.cmake
index f5584d5..084b5c3 100644
--- a/Tests/RunCMake/GeneratorExpression/RunCMakeTest.cmake
+++ b/Tests/RunCMake/GeneratorExpression/RunCMakeTest.cmake
@@ -1,5 +1,6 @@
 include(RunCMake)
 
+run_cmake(BadIF)
 run_cmake(BadCONFIG)
 run_cmake(BadOR)
 run_cmake(BadAND)