checker: allow noreturn in if expr (#12462)

vlib/v/checker/checker.v

@@ -6870,6 +6870,10 @@ pub fn (mut c Checker) if_expr(mut node ast.IfExpr) ast.Type {
 						&& c.table.get_type_symbol(former_expected_type).kind == .sum_type {
 						continue
 					}
+					if is_noreturn_callexpr(last_expr.expr) {
+						continue
+					}
+
 					c.error('mismatched types `${c.table.type_to_str(node.typ)}` and `${c.table.type_to_str(last_expr.typ)}`',
 						node.pos)
 				}

vlib/v/gen/c/cgen.v

@@ -5159,6 +5159,9 @@ fn (mut g Gen) need_tmp_var_in_if(node ast.IfExpr) bool {
 			if branch.stmts.len == 1 {
 				if branch.stmts[0] is ast.ExprStmt {
 					stmt := branch.stmts[0] as ast.ExprStmt
+					if is_noreturn_callexpr(stmt.expr) {
+						return true
+					}
 					if stmt.expr is ast.CallExpr {
 						if stmt.expr.is_method {
 							left_sym := g.table.get_type_symbol(stmt.expr.receiver_type)

vlib/v/tests/if_expression_test.v

@@ -234,3 +234,32 @@ fn test_if_expr_with_or_block() {
 	a := if arr.len == 0 || arr[0] == '-' { 123 } else { return_optional() or { -1 } }
 	assert a == 1
 }
+
+type Num = f32 | f64 | i64 | int
+
+[noreturn]
+fn assert_false_noreturn() {
+	assert false
+	exit(1)
+}
+
+fn test_noreturn() {
+	n := Num(int(0))
+	_ := if n is int {
+		n
+	} else if n is f32 {
+		int(n)
+	} else {
+		exit(1)
+	}
+
+	_ := if 1 == 0 {
+		0
+	} else if 1 == 1 {
+		1
+	} else if 1 == 2 {
+		panic('err')
+	} else {
+		assert_false_noreturn()
+	}
+}