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.

2 files changed, 2 insertions, 0 deletions

diff --git a/source/slang/diagnostic-defs.h b/source/slang/diagnostic-defs.h
index 83e82fc81..02edb6521 100644
--- a/source/slang/diagnostic-defs.h
+++ b/source/slang/diagnostic-defs.h
@@ -114,6 +114,7 @@ DIAGNOSTIC(15201, Error, syntaxErrorInPreprocessorExpression, "syntax error in p
 DIAGNOSTIC(15202, Error, divideByZeroInPreprocessorExpression, "division by zero in preprocessor expression");
 DIAGNOSTIC(15203, Error, expectedTokenInDefinedExpression, "expected '$0' in 'defined' expression");
 DIAGNOSTIC(15204, Warning, directiveExpectsExpression, "'$0' directive requires an expression");
+DIAGNOSTIC(15205, Warning, undefinedIdentifierInPreprocessorExpression, "undefined idenfier '$0' in preprocessor expression will evaluate to zero")
 
 DIAGNOSTIC(-1, Note, seeOpeningToken, "see opening '$0'")
 
diff --git a/source/slang/preprocessor.cpp b/source/slang/preprocessor.cpp
index 71c6fd4bb..d652d07bb 100644
--- a/source/slang/preprocessor.cpp
+++ b/source/slang/preprocessor.cpp
@@ -1213,6 +1213,7 @@ static PreprocessorExpressionValue ParseAndEvaluateUnaryExpression(PreprocessorD
             // An identifier here means it was not defined as a macro (or
             // it is defined, but as a function-like macro. These should
             // just evaluate to zero (possibly with a warning)
+            GetSink(context)->diagnose(token.loc, Diagnostics::undefinedIdentifierInPreprocessorExpression, token.getName());
             return 0;
         }