checker: handle invalid smartcast with if/match none ident or selector (fix #12317) (#13762)

vlib/v/checker/checker.v

@@ -91,9 +91,10 @@ pub mut:
 	inside_fn_arg             bool // `a`, `b` in `a.f(b)`
 	inside_ct_attr            bool // true inside `[if expr]`
 	inside_comptime_for_field bool
-	skip_flags                bool // should `#flag` and `#include` be skipped
-	fn_level                  int  // 0 for the top level, 1 for `fn abc() {}`, 2 for a nested fn, etc
-	smartcast_mut_pos         token.Pos
+	skip_flags                bool      // should `#flag` and `#include` be skipped
+	fn_level                  int       // 0 for the top level, 1 for `fn abc() {}`, 2 for a nested fn, etc
+	smartcast_mut_pos         token.Pos // match mut foo, if mut foo is Foo
+	smartcast_cond_pos        token.Pos // match cond
 	ct_cond_stack             []ast.Expr
 mut:
 	stmt_level int // the nesting level inside each stmts list;
@@ -1769,6 +1770,10 @@ pub fn (mut c Checker) selector_expr(mut node ast.SelectorExpr) ast.Type {
 			c.note('smartcasting requires either an immutable value, or an explicit mut keyword before the value',
 				c.smartcast_mut_pos)
 		}
+		if c.smartcast_cond_pos != token.Pos{} {
+			c.note('smartcast can only be used on the ident or selector, e.g. match foo, match foo.bar',
+				c.smartcast_cond_pos)
+		}
 		c.error(unknown_field_msg, node.pos)
 	}
 	return ast.void_type
@@ -3357,7 +3362,9 @@ fn (mut c Checker) smartcast(expr_ ast.Expr, cur_type ast.Type, to_type_ ast.Typ
 				c.smartcast_mut_pos = expr.pos
 			}
 		}
-		else {}
+		else {
+			c.smartcast_cond_pos = expr.pos()
+		}
 	}
 }
 

vlib/v/checker/if.v

@@ -152,9 +152,8 @@ pub fn (mut c Checker) if_expr(mut node ast.IfExpr) ast.Type {
 			} else {
 				c.stmts(branch.stmts)
 			}
-			if c.smartcast_mut_pos != token.Pos{} {
-				c.smartcast_mut_pos = token.Pos{}
-			}
+			c.smartcast_mut_pos = token.Pos{}
+			c.smartcast_cond_pos = token.Pos{}
 		}
 		if expr_required {
 			if branch.stmts.len > 0 && branch.stmts[branch.stmts.len - 1] is ast.ExprStmt {

vlib/v/checker/match.v

@@ -43,9 +43,8 @@ pub fn (mut c Checker) match_expr(mut node ast.MatchExpr) ast.Type {
 		} else {
 			c.stmts(branch.stmts)
 		}
-		if c.smartcast_mut_pos != token.Pos{} {
-			c.smartcast_mut_pos = token.Pos{}
-		}
+		c.smartcast_mut_pos = token.Pos{}
+		c.smartcast_cond_pos = token.Pos{}
 		if node.is_expr {
 			if branch.stmts.len > 0 {
 				// ignore last statement - workaround

vlib/v/checker/tests/incorrect_smartcast2_err.out

@@ -0,0 +1,14 @@
+vlib/v/checker/tests/incorrect_smartcast2_err.vv:24:9: notice: smartcast can only be used on the ident or selector, e.g. match foo, match foo.bar
+   22 |
+   23 | fn doesntwork(v []Either<int, int>) {
+   24 |     match v[0] {
+      |            ~~~
+   25 |         Left<int> {
+   26 |             println(v[0].error)
+vlib/v/checker/tests/incorrect_smartcast2_err.vv:26:17: error: field `error` does not exist or have the same type in all sumtype variants
+   24 |     match v[0] {
+   25 |         Left<int> {
+   26 |             println(v[0].error)
+      |                          ~~~~~
+   27 |         }
+   28 |         else {}

vlib/v/checker/tests/incorrect_smartcast2_err.vv

@@ -0,0 +1,32 @@
+struct Left<E> {
+	error E
+}
+
+struct Right<T> {
+	inner T
+}
+
+type Either<T, E> =
+	Left<E> |
+	Right<T>
+
+fn works(v []Either<int, int>) {
+	first := v[0]
+	match first {
+		Left<int> {
+			println(first.error)
+		}
+		else {}
+	}
+}
+
+fn doesntwork(v []Either<int, int>) {
+	match v[0] {
+		Left<int> {
+			println(v[0].error)
+		}
+		else {}
+	}
+}
+
+fn main() {}