checker: verify use of blank identifier (#6412)

vlib/v/checker/checker.v

@@ -2718,6 +2718,9 @@ pub fn (mut c Checker) ident(mut ident ast.Ident) table.Type {
 		c.const_deps << name
 	}
 	if ident.kind == .blank_ident {
+		if ident.tok_kind !in [.assign, .decl_assign] {
+			c.error('undefined ident: `_` (may only be used in assignments)', ident.pos)
+		}
 		return table.void_type
 	}
 	// second use
@@ -2828,7 +2831,6 @@ pub fn (mut c Checker) ident(mut ident ast.Ident) table.Type {
 	if ident.language == .c {
 		return table.int_type
 	}
-	if ident.name != '_' {
 	if c.inside_sql {
 		if field := c.table.struct_find_field(c.cur_orm_ts, ident.name) {
 			return field.typ
@@ -2846,12 +2848,10 @@ pub fn (mut c Checker) ident(mut ident ast.Ident) table.Type {
 		ident.mod = saved_mod
 	}
 	if ident.tok_kind == .assign {
-			c.error('undefined ident: `$ident.name` (use `:=` to declare a variable)',
+		c.error('undefined ident: `$ident.name` (use `:=` to declare a variable)', ident.pos)
-				ident.pos)
 	} else {
 		c.error('undefined ident: `$ident.name`', ident.pos)
 	}
-	}
 	if c.table.known_type(ident.name) {
 		// e.g. `User`  in `json.decode(User, '...')`
 		return table.void_type

vlib/v/checker/tests/blank_ident_invalid_use.out

@@ -0,0 +1,5 @@
+vlib/v/checker/tests/blank_ident_invalid_use.vv:2:8: error: undefined ident: `_` (may only be used in assignments)
+    1 | fn main() {
+    2 |     _ := [_]
+      |           ^
+    3 | }

vlib/v/checker/tests/blank_ident_invalid_use.vv

@@ -0,0 +1,3 @@
+fn main() {
+	_ := [_]
+}