builder: error on implicit_function_declaration

Author: kbkpbot

vlib/v/builder/cc.v

@@ -511,6 +511,14 @@ fn (v &Builder) only_compile_args(ccoptions CcompilerOptions) []string {
 				all << '-Werror=implicit-function-declaration'
 			}
 		}
+	} $else {
+		// On non-Windows platforms, always add -Werror=implicit-function-declaration
+		// to catch implicit function declarations at compile time
+		if ccoptions.cc != .msvc {
+			if !v.pref.is_cstrict {
+				all << '-Werror=implicit-function-declaration'
+			}
+		}
 	}
 	all << ccoptions.pre_args
 	all << ccoptions.source_args

vlib/v/tests/implicit_function_declaration_test.v

@@ -0,0 +1,38 @@
+// Test for implicit C function declaration detection
+// This test verifies that V correctly catches implicit C function declarations
+
+module main
+
+import os
+
+// `test_imp_c_function.v` filename can't contains `implicit`
+const test_file = os.join_path(os.vtmp_dir(), 'test_imp_c_function.v')
+
+fn testsuite_begin() {
+	os.rm(test_file) or {}
+	// Test code that should fail due to implicit C function declaration
+	test_code := 'module main
+fn C.custom_undeclared_function()
+fn main() {
+        C.custom_undeclared_function()
+}
+'
+	// Write the test code to the temporary file
+	os.write_file(test_file, test_code) or {
+		eprintln('FAIL: Unable to write test file')
+		exit(1)
+	}
+}
+
+fn testsuite_end() {
+	os.rm(test_file) or { eprintln('Warning: Unable to delete temporary file') }
+}
+
+fn test_implicit_c_function_declaration() {
+	result := os.execute('v ${test_file}')
+	assert result.exit_code != 0
+	println(result.output)
+	assert result.output.contains('custom_undeclared_function')
+	// C4013 is for msvc
+	assert result.output.contains('implicit') || result.output.contains('C4013')
+}