Warn when setting used on the command line more than once

See emscripten-core#19938

ChangeLog.md

@@ -20,6 +20,9 @@ See docs/process.md for more on how version tagging works.
 
 3.1.56 (in development)
 -----------------------
+- emscripten will now generate an `unused-command-line-argument` warning if
+  a `-s` setting is specified more than once on the command line with
+  conflicting values.  In this case the first setting is ignored. (#21464)
 
 3.1.55 - 03/01/24
 -----------------

emcc.py

@@ -684,6 +684,9 @@ def phase_parse_arguments(state):
   for s in settings_changes:
     key, value = s.split('=', 1)
     key, value = normalize_boolean_setting(key, value)
+    old_value = user_settings.get(key)
+    if old_value and old_value != value:
+      diagnostics.warning('unused-command-line-argument', f'-s{key} specified multiple times. Ignoring previous value (`{old_value}`)')
     user_settings[key] = value
 
   # STRICT is used when applying settings so it needs to be applied first before

test/common.py

@@ -868,7 +868,7 @@ def setUp(self):
     # For historical reasons emcc compiles and links as C++ by default.
     # However we want to run our tests in a more strict manner.  We can
     # remove this if the issue above is ever fixed.
-    self.set_setting('NO_DEFAULT_TO_CXX')
+    self.set_setting('DEFAULT_TO_CXX', 0)
     self.ldflags = []
     # Increate stack trace limit to maximise usefulness of test failure reports
     self.node_args = ['--stack-trace-limit=50']

test/test_core.py

@@ -9153,8 +9153,9 @@ def test_pthread_create_embind_stack_check(self):
     # embind should work with stack overflow checks (see #12356)
     self.set_setting('STACK_OVERFLOW_CHECK', 2)
     self.set_setting('EXIT_RUNTIME')
+    self.set_setting('DEFAULT_TO_CXX')
     self.emcc_args += ['-lembind']
-    self.do_run_in_out_file_test('core/pthread/create.c', emcc_args=['-sDEFAULT_TO_CXX'])
+    self.do_run_in_out_file_test('core/pthread/create.c')
 
   @node_pthreads
   def test_pthread_exceptions(self):

test/test_other.py

@@ -7811,6 +7811,10 @@ def test_dash_s_bad_json_types(self):
     err = self.expect_fail([EMCC, test_file('hello_world.c'), '-sEXPORTED_FUNCTIONS=[{"a":1}]'])
     self.assertContained("list members in settings must be strings (not $<class 'dict'>)", err)
 
+  def test_dash_s_repeated(self):
+    err = self.expect_fail([EMCC, '-Werror', test_file('hello_world.c'), '-sEXPORTED_FUNCTIONS=foo', '-sEXPORTED_FUNCTIONS=bar'])
+    self.assertContained('emcc: error: -sEXPORTED_FUNCTIONS specified multiple times. Ignoring previous value (`foo`) [-Wunused-command-line-argument]', err)
+
   def test_zeroinit(self):
     create_file('src.c', r'''
 #include <stdio.h>