cgen: fix multi-return match in closure (fix #26558) (#26566)

Author: kbkpbot

vlib/v/gen/c/cgen.v

@@ -5963,13 +5963,25 @@ fn (mut g Gen) concat_expr(node ast.ConcatExpr) {
 	} else if g.inside_or_block {
 		typ = g.or_expr_return_type.clear_option_and_result()
 	}
-	styp := g.styp(typ)
 	sym := g.table.sym(node.return_type)
 	is_multi := sym.kind == .multi_return
 	if !is_multi {
 		g.expr(node.vals[0])
 	} else {
-		types := (g.table.sym(typ).info as ast.MultiReturn).types
+		// When typ is also a multi-return type, use it to get the types array,
+		// as it may have more accurate type information (e.g., with option types).
+		// Otherwise, fall back to using node.return_type.
+		typ_sym := g.table.sym(typ)
+		types := if typ_sym.kind == .multi_return {
+			(typ_sym.info as ast.MultiReturn).types
+		} else {
+			(sym.info as ast.MultiReturn).types
+		}
+		styp := if typ_sym.kind == .multi_return {
+			g.styp(typ)
+		} else {
+			g.styp(node.return_type.clear_option_and_result())
+		}
 		g.write('(${styp}){')
 		for i, expr in node.vals {
 			g.write('.arg${i}=')

vlib/v/tests/match_multi_return_closure_heap_test.v

@@ -0,0 +1,36 @@
+// Regression test for https://github.com/vlang/v/issues/26558
+// Multi-return match assignment inside closure should work
+// when the closure is a field of a @[heap] struct and the
+// struct init is inside a return expression.
+
+@[heap]
+struct Cfg {
+	handler fn (string) = unsafe { nil }
+}
+
+fn call_it(cfg Cfg) {
+	if cfg.handler != unsafe { nil } {
+		cfg.handler('s')
+	}
+}
+
+fn make_cfg(ch string) Cfg {
+	return Cfg{
+		handler: fn [ch] (s string) {
+			x, y, z := match ch {
+				'h' { f32(1), f32(2), f32(3) }
+				's' { f32(4), f32(5), f32(6) }
+				else { f32(7), f32(8), f32(9) }
+			}
+			println('${x} ${y} ${z}')
+		}
+	}
+}
+
+fn test_multi_return_match_in_closure_heap_struct() {
+	// This test verifies that the compiler does not panic with:
+	// "as cast: cannot cast `v.ast.UnknownTypeInfo` to `v.ast.MultiReturn`"
+	call_it(make_cfg('h'))
+	call_it(make_cfg('s'))
+	call_it(make_cfg('x'))
+}