Preprocessor: fix `undef` and redefinition (#204)

* Preprocessor: fix `undef` and redefinition

The logic for `undef` directives was failing to suppress macro expansion when reading the name to un-define, and so it wasn't actually working at all. We didn't notice this because we didn't have a test case, and users hadn't tried it.

The logic for `define` had a similar bug, which meant that any attempt to define an already-defined macro would fail with a cryptic error, rather than raising the intended warning.

Test cases have been added for both issues, along with the fixes.

* fixup: add expected output for tests added

4 files changed, 55 insertions, 0 deletions

diff --git a/tests/preprocessor/define-redefine.slang b/tests/preprocessor/define-redefine.slang
new file mode 100644
index 000000000..42b7a4dbe
--- /dev/null
+++ b/tests/preprocessor/define-redefine.slang
@@ -0,0 +1,10 @@
+//TEST:SIMPLE:
+
+// Confirm that we handle redefinition
+// of an existing define (with a diagnostic)
+
+#define FOO 1.0f
+
+float foo() { return FOO + 2.0; }
+
+#define FOO 2.0f
diff --git a/tests/preprocessor/define-redefine.slang.expected b/tests/preprocessor/define-redefine.slang.expected
new file mode 100644
index 000000000..2542c05d8
--- /dev/null
+++ b/tests/preprocessor/define-redefine.slang.expected
@@ -0,0 +1,7 @@
+result code = 0
+standard error = {
+tests/preprocessor/define-redefine.slang(10): warning 15400: redefinition of macro 'FOO'
+tests/preprocessor/define-redefine.slang(6): note: see previous definition of 'FOO'
+}
+standard output = {
+}
diff --git a/tests/preprocessor/undef.slang b/tests/preprocessor/undef.slang
new file mode 100644
index 000000000..42ce05be7
--- /dev/null
+++ b/tests/preprocessor/undef.slang
@@ -0,0 +1,32 @@
+//TEST:SIMPLE:
+// #undef support
+
+// warning: undef of something not defined
+#undef FOO
+
+#define BAR 1.0f
+
+float foo() { return BAR + 2.0; }
+
+#ifdef BAR
+// okay
+#else
+#error not okay
+#endif
+
+#undef BAR
+
+typedef float BAR;
+
+BAR bar() { return 2.0; }
+
+#if !defined(BAR)
+// okay
+#else
+#error not okay
+#endif
+
+
+#define FOO
+
+#undef FOO
diff --git a/tests/preprocessor/undef.slang.expected b/tests/preprocessor/undef.slang.expected
new file mode 100644
index 000000000..2f7026487
--- /dev/null
+++ b/tests/preprocessor/undef.slang.expected
@@ -0,0 +1,6 @@
+result code = 0
+standard error = {
+tests/preprocessor/undef.slang(5): warning 15401: macro 'FOO' is not defined
+}
+standard output = {
+}