checker: show available sumtype variants in match branches on typos

vlib/v/checker/match.v

@@ -236,7 +236,11 @@ fn (mut c Checker) match_exprs(mut node ast.MatchExpr, cond_type_sym ast.TypeSym
 				if expr_type !in cond_type_sym.info.variants {
 					expr_str := c.table.type_to_str(expr_type)
 					expect_str := c.table.type_to_str(node.cond_type)
-					c.error('`$expect_str` has no variant `$expr_str`', expr.pos())
+					sumtype_variant_names := cond_type_sym.info.variants.map(c.table.type_to_str_using_aliases(it,
+						{}))
+					suggestion := util.new_suggestion(expr_str, sumtype_variant_names)
+					c.error(suggestion.say('`$expect_str` has no variant `$expr_str`'),
+						expr.pos())
 				}
 			} else if cond_type_sym.info is ast.Alias && expr_type_sym.info is ast.Struct {
 				expr_str := c.table.type_to_str(expr_type)

vlib/v/checker/tests/amod/amod.v

@@ -0,0 +1,5 @@
+module amod
+
+pub struct Xyz {}
+
+pub struct Bcg {}

vlib/v/checker/tests/match_invalid_type.out

@@ -1,4 +1,5 @@
-vlib/v/checker/tests/match_invalid_type.vv:5:3: error: `IoS` has no variant `byte`
+vlib/v/checker/tests/match_invalid_type.vv:5:3: error: `IoS` has no variant `byte`.
+2 possibilities: `int`, `string`.
     3 | fn sum() {
     4 |     match IoS(1) {
     5 |         byte {

vlib/v/checker/tests/sumtype_has_no_variant_suggestion.out

@@ -0,0 +1,8 @@
+vlib/v/checker/tests/sumtype_has_no_variant_suggestion.vv:14:5: error: `Abc` has no variant `amod.NonExisting`.
+5 possibilities: `amod.Bcg`, `amod.Xyz`, `AnotherStruct`, `Struct1`, `ThirdStruct`.
+   12 |     a := Abc(Struct1{})
+   13 |     match a {
+   14 |         x.NonExisting { println('----') }
+      |           ~~~~~~~~~~~
+   15 |         else {}
+   16 |     }

vlib/v/checker/tests/sumtype_has_no_variant_suggestion.vv

@@ -0,0 +1,17 @@
+import v.checker.tests.amod as x
+
+struct Struct1 {}
+
+struct AnotherStruct {}
+
+struct ThirdStruct {}
+
+type Abc = AnotherStruct | Struct1 | ThirdStruct | x.Bcg | x.Xyz
+
+fn main() {
+	a := Abc(Struct1{})
+	match a {
+		x.NonExisting { println('----') }
+		else {}
+	}
+}