parser: disallow multi else branches in match (#18526)

vlib/v/parser/if_match.v

@@ -213,6 +213,7 @@ fn (mut p Parser) is_only_array_type() bool {
 
 fn (mut p Parser) match_expr() ast.MatchExpr {
 	match_first_pos := p.tok.pos()
+	mut else_count := 0
 	p.inside_match = true
 	p.check(.key_match)
 	mut is_sum_type := false
@@ -233,6 +234,7 @@ fn (mut p Parser) match_expr() ast.MatchExpr {
 		mut is_else := false
 		if p.tok.kind == .key_else {
 			is_else = true
+			else_count += 1
 			p.next()
 		} else if (p.tok.kind == .name && !(p.tok.lit == 'C' && p.peek_tok.kind == .dot)
 			&& (((ast.builtin_type_names_matcher.matches(p.tok.lit) || p.tok.lit[0].is_capital())
@@ -328,6 +330,9 @@ fn (mut p Parser) match_expr() ast.MatchExpr {
 		if is_else && branches.len == 1 {
 			p.error_with_pos('`match` must have at least one non `else` branch', pos)
 		}
+		if else_count > 1 {
+			p.error_with_pos('`match` can have only one `else` branch', pos)
+		}
 		if p.tok.kind == .rcbr || (is_else && no_lcbr) {
 			break
 		}

vlib/v/parser/tests/match_multi_else_branch_err.out

@@ -0,0 +1,7 @@
+vlib/v/parser/tests/match_multi_else_branch_err.vv:5:3: error: `match` can have only one `else` branch
+    3 |         true { 1 }
+    4 |         else { 3 }
+    5 |         else { 5 }
+      |         ~~~~
+    6 |     }
+    7 | }

vlib/v/parser/tests/match_multi_else_branch_err.vv

@@ -0,0 +1,7 @@
+fn main() {
+	_ := match true {
+		true { 1 }
+		else { 3 }
+		else { 5 }
+	}
+}