checker: check for empty branches in match expressions (#10593)

vlib/v/ast/ast.v

@@ -773,7 +773,8 @@ pub:
 	stmts         []Stmt      // right side
 	pos           token.Position
 	is_else       bool
-	post_comments []Comment // comments below ´... }´
+	post_comments []Comment      // comments below ´... }´
+	branch_pos    token.Position // for checker errors about invalid branches
 pub mut:
 	exprs []Expr // left side
 	scope &Scope

vlib/v/checker/checker.v

@@ -5497,19 +5497,27 @@ pub fn (mut c Checker) match_expr(mut node ast.MatchExpr) ast.Type {
 	node.is_sum_type = cond_type_sym.kind in [.interface_, .sum_type]
 	c.match_exprs(mut node, cond_type_sym)
 	c.expected_type = cond_type
+	mut first_iteration := true
 	mut ret_type := ast.void_type
 	mut nbranches_with_return := 0
 	mut nbranches_without_return := 0
 	for branch in node.branches {
 		c.stmts(branch.stmts)
-		if node.is_expr && branch.stmts.len > 0 {
-			// ignore last statement - workaround
-			// currently the last statement in a match branch does not have an
-			// expected value set, so e.g. IfExpr.is_expr is not set.
-			// probably any mismatch will be caught by not producing a value instead
-			for st in branch.stmts[0..branch.stmts.len - 1] {
-				// must not contain C statements
-				st.check_c_expr() or { c.error('`match` expression branch has $err.msg', st.pos) }
+		if node.is_expr {
+			if branch.stmts.len > 0 {
+				// ignore last statement - workaround
+				// currently the last statement in a match branch does not have an
+				// expected value set, so e.g. IfExpr.is_expr is not set.
+				// probably any mismatch will be caught by not producing a value instead
+				for st in branch.stmts[0..branch.stmts.len - 1] {
+					// must not contain C statements
+					st.check_c_expr() or {
+						c.error('`match` expression branch has $err.msg', st.pos)
+					}
+				}
+			} else if ret_type != ast.void_type {
+				c.error('`match` expression requires an expression as the last statement of every branch',
+					branch.branch_pos)
 			}
 		}
 		// If the last statement is an expression, return its type
@@ -5521,7 +5529,7 @@ pub fn (mut c Checker) match_expr(mut node ast.MatchExpr) ast.Type {
 						c.expected_type = node.expected_type
 					}
 					expr_type := c.expr(stmt.expr)
-					if ret_type == ast.void_type {
+					if first_iteration {
 						if node.is_expr && !node.expected_type.has_flag(.optional)
 							&& c.table.get_type_symbol(node.expected_type).kind == .sum_type {
 							ret_type = node.expected_type
@@ -5540,15 +5548,14 @@ pub fn (mut c Checker) match_expr(mut node ast.MatchExpr) ast.Type {
 					}
 				}
 				else {
-					// TODO: ask alex about this
-					// typ := c.expr(stmt.expr)
-					// type_sym := c.table.get_type_symbol(typ)
-					// p.warn('match expr ret $type_sym.name')
-					// node.typ = typ
-					// return typ
+					if node.is_expr && ret_type != ast.void_type {
+						c.error('`match` expression requires an expression as the last statement of every branch',
+							stmt.pos)
+					}
 				}
 			}
 		}
+		first_iteration = false
 		if has_return := c.has_return(branch.stmts) {
 			if has_return {
 				nbranches_with_return++

vlib/v/checker/tests/match_expr_empty_branch.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/match_expr_empty_branch.vv:3:2: error: `match` expression requires an expression as the last statement of every branch
+    1 | _ := match true {
+    2 |     true { 0 }
+    3 |     false {}
+      |     ~~~~~~~~
+    4 | }

vlib/v/checker/tests/match_expr_empty_branch.vv

@@ -0,0 +1,4 @@
+_ := match true {
+	true { 0 }
+	false {}
+}

vlib/v/checker/tests/match_expr_non_void_stmt_last.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/match_expr_non_void_stmt_last.vv:9:4: error: `match` expression requires an expression as the last statement of every branch
+    7 |         }
+    8 |         []int {
+    9 |             return arr.str()
+      |             ~~~~~~~~~~~~~~~~
+   10 |         }
+   11 |     }

vlib/v/checker/tests/match_expr_non_void_stmt_last.vv

@@ -0,0 +1,17 @@
+type Arr = []int | []string
+
+fn (arr Arr) str() string {
+	return match arr {
+		[]string {
+			arr.join(' ')
+		}
+		[]int {
+			return arr.str()
+		}
+	}
+}
+
+fn main() {
+	println(Arr([0, 0]))
+	println(Arr(['0', '0']))
+}

vlib/v/parser/if_match.v

@@ -237,12 +237,14 @@ fn (mut p Parser) match_expr() ast.MatchExpr {
 		p.close_scope()
 		p.inside_match_body = false
 		pos := branch_first_pos.extend_with_last_line(branch_last_pos, p.prev_tok.line_nr)
+		branch_pos := branch_first_pos.extend_with_last_line(p.tok.position(), p.tok.line_nr)
 		post_comments := p.eat_comments({})
 		branches << ast.MatchBranch{
 			exprs: exprs
 			ecmnts: ecmnts
 			stmts: stmts
 			pos: pos
+			branch_pos: branch_pos
 			is_else: is_else
 			post_comments: post_comments
 			scope: branch_scope