compilers: corretify deduplication direction

so: when building compile args, meson is deduplicating flags. When a
compiler argument is appended, a later appearance of a dedup'ed is going
to remove a earlier one. If the argument is prepended, the element
*before* the new one is going to be removed. And that is where the
problem reported in https://github.com/mesonbuild/meson/pull/7119 is
coming in. In the revision linked there, the order of replacement in the
prepend case was revesered.

With this patch, we restore this behaviour again.

mesonbuild/compilers/compilers.py

@@ -481,10 +481,10 @@ class CompilerArgs(collections.abc.MutableSequence):
         post_flush_set = set()
 
         #The two lists are here walked from the front to the back, in order to not need removals for deduplication
-        for a in reversed(self.pre):
+        for a in self.pre:
             dedup = self._can_dedup(a)
             if a not in pre_flush_set:
-                pre_flush.appendleft(a)
+                pre_flush.append(a)
                 if dedup == 2:
                     pre_flush_set.add(a)
         for a in reversed(self.post):

run_unittests.py

@@ -350,6 +350,25 @@ class InternalTests(unittest.TestCase):
                          stat.S_IRWXU | stat.S_ISUID |
                          stat.S_IRGRP | stat.S_IXGRP)
 
+    def test_compiler_args_class_none_flush(self):
+        cargsfunc = mesonbuild.compilers.CompilerArgs
+        cc = mesonbuild.compilers.CCompiler([], 'fake', False, MachineChoice.HOST, mock.Mock())
+        a = cargsfunc(cc, ['-I.'])
+        #first we are checking if the tree construction deduplicates the correct -I argument
+        a += ['-I..']
+        a += ['-I./tests/']
+        a += ['-I./tests2/']
+        #think this here as assertion, we cannot apply it, otherwise the CompilerArgs would already flush the changes:
+        # assertEqual(a, ['-I.', '-I./tests2/', '-I./tests/', '-I..', '-I.'])
+        a += ['-I.']
+        a += ['-I.', '-I./tests/']
+        self.assertEqual(a, ['-I.', '-I./tests/', '-I./tests2/', '-I..'])
+
+        #then we are checking that when CompilerArgs already have a build container list, that the deduplication is taking the correct one
+        a += ['-I.', '-I./tests2/']
+        self.assertEqual(a, ['-I.', '-I./tests2/', '-I./tests/', '-I..'])
+
+
     def test_compiler_args_class(self):
         cargsfunc = mesonbuild.compilers.CompilerArgs
         cc = mesonbuild.compilers.CCompiler([], 'fake', False, MachineChoice.HOST, mock.Mock())