Preserve standalone -D arguments always.

mesonbuild/compilers.py

@@ -430,6 +430,17 @@ class CompilerArgs(list):
         to recursively search for symbols in the libraries. This is not needed
         with other linkers.
         '''
+
+        # A standalone argument must never be deduplicated because it is
+        # defined by what comes _after_ it. Thus dedupping this:
+        # -D FOO -D BAR
+        # would yield either
+        # -D FOO BAR
+        # or
+        # FOO -D BAR
+        # both of which are invalid.
+        if arg in cls.dedup2_prefixes:
+            return 0
         if arg in cls.dedup2_args or \
            arg.startswith(cls.dedup2_prefixes) or \
            arg.endswith(cls.dedup2_suffixes):

run_unittests.py

@@ -1225,9 +1225,10 @@ int main(int argc, char **argv) {
         testdir = os.path.join(self.unit_test_dir, '10 d dedup')
         self.init(testdir)
         cmd = self.get_compdb()[0]['command']
-        self.assertTrue('-D FOO -D BAR' in cmd or \
-                        '/D FOO /D BAR' in cmd)
-
+        self.assertTrue('-D FOO -D BAR' in cmd or
+                        '"-D" "FOO" "-D" "BAR"' in cmd or
+                        '/D FOO /D BAR' in cmd or
+                        '"/D" "FOO" "/D" "BAR"' in cmd)
 
 
 class FailureTests(BasePlatformTests):