From 82dad350a9662763cb5662669412ed4bcf7064f4 Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Thu, 26 Feb 2026 21:13:27 +0300
Subject: [PATCH] checker: fix confusing error message when test file gets an
 invalid name (fixes #15642)

---
 vlib/v/checker/checker.v                   | 20 ++++++++++++++++++--
 vlib/v/checker/tests/test_invalid_name.out |  6 ++++++
 vlib/v/checker/tests/test_invalid_name.vv  |  5 +++++
 3 files changed, 29 insertions(+), 2 deletions(-)
 create mode 100644 vlib/v/checker/tests/test_invalid_name.out
 create mode 100644 vlib/v/checker/tests/test_invalid_name.vv

diff --git a/vlib/v/checker/checker.v b/vlib/v/checker/checker.v
index 473faaad3..cd085c9ea 100644
--- a/vlib/v/checker/checker.v
+++ b/vlib/v/checker/checker.v
@@ -439,6 +439,8 @@ pub fn (mut c Checker) check_files(ast_files []&ast.File) {
 	mut has_main_mod_file := false
 	mut has_no_main_mod_file := false
 	mut has_main_fn := false
+	mut invalid_test_file_name := ''
+	mut invalid_test_file_pos := token.Pos{}
 	// Determine the project directory when using -line-info
 	mut project_dir := ''
 	if c.pref.is_vls && c.pref.line_info != '' {
@@ -472,6 +474,10 @@ pub fn (mut c Checker) check_files(ast_files []&ast.File) {
 					has_main_fn = true
 				}
 			}
+			if invalid_test_file_name == '' && is_likely_invalid_test_file_name(file) {
+				invalid_test_file_name = file.path_base
+				invalid_test_file_pos = file.mod.pos
+			}
 			c.timers.show('checker_check ${file.path}')
 		}
 		if has_main_mod_file && !has_main_fn && files_from_main_module.len > 0 {
@@ -569,13 +575,23 @@ pub fn (mut c Checker) check_files(ast_files []&ast.File) {
 		return
 	}
 	if !has_main_mod_file {
-		c.error('project must include a `main` module or be a shared library (compile with `v -shared`)',
-			token.Pos{})
+		if invalid_test_file_name != '' {
+			c.add_error_detail('Test files should have names ending with `_test.v`.')
+			c.error('invalid test file name `${invalid_test_file_name}`', invalid_test_file_pos)
+		} else {
+			c.error('project must include a `main` module or be a shared library (compile with `v -shared`)',
+				token.Pos{})
+		}
 	} else if !has_main_fn && !c.pref.is_o {
 		c.error('function `main` must be declared in the main module', token.Pos{})
 	}
 }
 
+fn is_likely_invalid_test_file_name(file &ast.File) bool {
+	return !file.is_test && (file.path_base.ends_with('.v') || file.path_base.ends_with('.vv'))
+		&& file.path_base.starts_with('test_')
+}
+
 fn (mut c Checker) stmts_has_main_fn(stmts []ast.Stmt) bool {
 	mut has_main_fn := false
 	for stmt in stmts {
diff --git a/vlib/v/checker/tests/test_invalid_name.out b/vlib/v/checker/tests/test_invalid_name.out
new file mode 100644
index 000000000..f05f7cddd
--- /dev/null
+++ b/vlib/v/checker/tests/test_invalid_name.out
@@ -0,0 +1,6 @@
+vlib/v/checker/tests/test_invalid_name.vv:1:1: error: invalid test file name `test_invalid_name.vv`
+    1 | module custom
+      | ~~~~~~~~~~~~~
+    2 | 
+    3 | fn test_result() {
+Details: Test files should have names ending with `_test.v`.
diff --git a/vlib/v/checker/tests/test_invalid_name.vv b/vlib/v/checker/tests/test_invalid_name.vv
new file mode 100644
index 000000000..36275664d
--- /dev/null
+++ b/vlib/v/checker/tests/test_invalid_name.vv
@@ -0,0 +1,5 @@
+module custom
+
+fn test_result() {
+	println('whatever')
+}
-- 
2.39.5