Warn when undefined identifier used in preprocessor conditional (#642)

This can mask an error when the user either typos a macro name when writing a conditional, or (as was the case for the user who pointed out this issue) they mistakenly assume that a `#define` in an `import`ed file has been made visible to them.

This change just adds the warning in the obvious place, with a test code to ensure it triggers.

1 files changed, 6 insertions, 0 deletions

diff --git a/tests/diagnostics/undefined-in-preprocessor-conditional.slang.expected b/tests/diagnostics/undefined-in-preprocessor-conditional.slang.expected
new file mode 100644
index 000000000..ac8be68a2
--- /dev/null
+++ b/tests/diagnostics/undefined-in-preprocessor-conditional.slang.expected
@@ -0,0 +1,6 @@
+result code = 0
+standard error = {
+tests/diagnostics/undefined-in-preprocessor-conditional.slang(8): warning 15205: undefined idenfier 'BART' in preprocessor expression will evaluate to zero
+}
+standard output = {
+}