checker: fix missing struct field type checking for type mismatch (re…

…f vs non-ref in `mt sync.Mutex = sync.new_mutex()`) (fix #18088) (#21949)

vlib/v/checker/struct.v

@@ -111,9 +111,20 @@ fn (mut c Checker) struct_decl(mut node ast.StructDecl) {
 			if field.has_default_expr {
 				c.expected_type = field.typ
 				field.default_expr_typ = c.expr(mut field.default_expr)
-				if field.typ.is_ptr() && !field.default_expr_typ.is_ptr()
-					&& field.default_expr is ast.StructInit {
-					c.error('reference field must be initialized with reference', field.default_expr.pos())
+				if field.typ.is_ptr() != field.default_expr_typ.is_ptr() {
+					match field.default_expr {
+						ast.CallExpr {
+							err_desc := if field.typ.is_ptr() { 'is' } else { 'is not' }
+							val_desc := if field.default_expr_typ.is_ptr() { 'is' } else { 'is not' }
+							c.error('field ${err_desc} reference but default value ${val_desc} reference',
+								field.default_expr.pos())
+						}
+						ast.StructInit {
+							c.error('reference field must be initialized with reference',
+								field.default_expr.pos())
+						}
+						else {}
+					}
 				}
 
 				// disallow map `mut a = b`

vlib/v/checker/tests/struct_field_mismatch_ref_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/struct_field_mismatch_ref_err.vv:6:26: error: field is not reference but default value is reference
+    4 | struct SafeCounter {
+    5 | mut:
+    6 |     mt sync.Mutex = sync.new_mutex()
+      |                          ~~~~~~~~~~~
+    7 | pub mut:
+    8 |     value map[string]int

vlib/v/checker/tests/struct_field_mismatch_ref_err.vv

@@ -0,0 +1,40 @@
+import sync
+import time
+
+struct SafeCounter {
+mut:
+    mt sync.Mutex = sync.new_mutex()
+pub mut:
+    value map[string]int
+}
+
+fn (mut c SafeCounter) inc(key string) {
+    c.mt.@lock()
+    defer {
+        c.mt.unlock()
+    }
+    c.value[key]++
+}
+
+fn (mut c SafeCounter) value(key string) int {
+    c.mt.@lock()
+    defer {
+        c.mt.unlock()
+    }
+    return c.value[key]
+}
+
+fn main() {
+    mut counter := &SafeCounter{}
+
+    for i := 0; i < 5; i++ {
+        spawn fn [mut counter] () {
+            for j := 0; j < 100; j++ {
+                counter.inc('key')
+            }
+        }()
+    }
+
+    time.sleep(1 * time.second)
+    println(counter.value('key'))
+}