checker: check smartcasting requires either an immutable value, or an explicit mut keyword before the value (#16654)

vlib/v/checker/fn.v

@@ -1925,6 +1925,16 @@ fn (mut c Checker) method_call(mut node ast.CallExpr) ast.Type {
 			return info.func.return_type
 		}
 	}
+	if left_sym.kind in [.struct_, .aggregate, .interface_, .sum_type] {
+		if c.smartcast_mut_pos != token.Pos{} {
+			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)
+		}
+	}
 	if left_type != ast.void_type {
 		suggestion := util.new_suggestion(method_name, left_sym.methods.map(it.name))
 		c.error(suggestion.say(unknown_method_msg), node.pos)

vlib/v/checker/tests/incorrect_smartcast3_err.out

@@ -0,0 +1,28 @@
+vlib/v/checker/tests/incorrect_smartcast3_err.vv:25:5: notice: smartcasting requires either an immutable value, or an explicit mut keyword before the value
+   23 |
+   24 |     mut r := la.regex
+   25 |     if r is RE {
+      |        ^
+   26 |         println(r)
+   27 |         println(r.matches_string(item))
+vlib/v/checker/tests/incorrect_smartcast3_err.vv:30:8: notice: smartcasting requires either an immutable value, or an explicit mut keyword before the value
+   28 |     }
+   29 |
+   30 |     match r {
+      |           ^
+   31 |         RE { r.matches_string(item) }
+   32 |         else {}
+vlib/v/checker/tests/incorrect_smartcast3_err.vv:27:13: error: unknown method or field: `OurRegex.matches_string`
+   25 |     if r is RE {
+   26 |         println(r)
+   27 |         println(r.matches_string(item))
+      |                   ~~~~~~~~~~~~~~~~~~~~
+   28 |     }
+   29 |
+vlib/v/checker/tests/incorrect_smartcast3_err.vv:31:10: error: unknown method or field: `OurRegex.matches_string`
+   29 |
+   30 |     match r {
+   31 |         RE { r.matches_string(item) }
+      |                ~~~~~~~~~~~~~~~~~~~~
+   32 |         else {}
+   33 |     }

vlib/v/checker/tests/incorrect_smartcast3_err.vv

@@ -0,0 +1,34 @@
+module main
+
+import regex { RE, regex_opt }
+
+pub struct EmptyRegex {
+}
+
+type OurRegex = EmptyRegex | RE
+
+pub struct ListArgs {
+pub mut:
+	regex OurRegex
+}
+
+fn main() {
+	query := r'(c(pa)+z ?)+'
+	mut re := regex_opt(query) or { panic(err) }
+
+	item := 'sss'
+	mut la := ListArgs{
+		regex: re
+	}
+
+	mut r := la.regex
+	if r is RE {
+		println(r)
+		println(r.matches_string(item))
+	}
+
+	match r {
+		RE { r.matches_string(item) }
+		else {}
+	}
+}