checker, cgen: fix check and cgen for match x { StructName {} } and match x { y {} }, where x is a mutable receiver (#17755)

vlib/v/checker/match.v

@@ -194,6 +194,10 @@ fn (mut c Checker) match_exprs(mut node ast.MatchExpr, cond_type_sym ast.TypeSym
 				// ensure that the sub expressions of the branch are actually checked, before anything else:
 				_ := c.expr(expr)
 			}
+			if expr is ast.TypeNode && cond_sym.kind == .struct_ {
+				c.error('struct instances cannot be matched by type name, they can only be matched to other instances of the same struct type',
+					branch.pos)
+			}
 			if mut expr is ast.RangeExpr {
 				// Allow for `match enum_value { 4..5 { } }`, even though usually int and enum values,
 				// are considered incompatible outside unsafe{}, and are not allowed to be compared directly

vlib/v/checker/tests/struct_match_same_type_node.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/struct_match_same_type_node.vv:10:3: error: struct instances cannot be matched by type name, they can only be matched to other instances of the same struct type
+    8 | fn (mut t Type) returns() Union {
+    9 |     match t {
+   10 |         Type {
+      |         ~~~~
+   11 |             t.something = 'else'
+   12 |         }

vlib/v/checker/tests/struct_match_same_type_node.vv

@@ -0,0 +1,25 @@
+struct Type {
+mut:
+	something string
+}
+
+type Union = Type
+
+fn (mut t Type) returns() Union {
+	match t {
+		Type {
+			t.something = 'else'
+		}
+		else {}
+	}
+
+	return t
+}
+
+fn main() {
+	mut t := &Type{
+		something: 'something'
+	}
+	t.returns()
+	println(t)
+}

vlib/v/gen/c/match.v

@@ -472,8 +472,10 @@ fn (mut g Gen) match_expr_classic(node ast.MatchExpr, is_expr bool, cond_var str
 						g.write(')')
 					}
 					.struct_ {
+						derefs_expr := '*'.repeat(g.get_expr_type(expr).nr_muls())
+						derefs_ctype := '*'.repeat(node.cond_type.nr_muls())
 						ptr_typ := g.equality_fn(node.cond_type)
-						g.write('${ptr_typ}_struct_eq(${cond_var}, ')
+						g.write('${ptr_typ}_struct_eq(${derefs_ctype}${cond_var}, ${derefs_expr}')
 						g.expr(expr)
 						g.write(')')
 					}

vlib/v/tests/match_struct_type_test.v

@@ -0,0 +1,69 @@
+struct Type {
+mut:
+	something string
+}
+
+fn (mut t Type) match_mut_receiver_against_zero_struct_value() {
+	zero := Type{}
+	match t {
+		zero {
+			t.something = 'else'
+		}
+		else {}
+	}
+}
+
+fn (t &Type) match_ref_receiver_against_zero_struct_value() bool {
+	zero := Type{}
+	match t {
+		zero {
+			return true
+		}
+		else {}
+	}
+	return false
+}
+
+//
+
+fn test_match_struct_type_mut_receiver() {
+	mut a := &Type{}
+	a.match_mut_receiver_against_zero_struct_value()
+	assert a.something == 'else'
+	mut b := &Type{
+		something: 'another'
+	}
+	b.match_mut_receiver_against_zero_struct_value()
+	assert b.something == 'another'
+}
+
+fn test_match_struct_type_mut_receiver_with_non_ref_receiver_in_caller() {
+	mut a := Type{}
+	a.match_mut_receiver_against_zero_struct_value()
+	assert a.something == 'else'
+	mut b := Type{
+		something: 'another'
+	}
+	b.match_mut_receiver_against_zero_struct_value()
+	assert b.something == 'another'
+}
+
+//
+
+fn test_match_struct_type_ref_receiver() {
+	a := &Type{}
+	assert a.match_ref_receiver_against_zero_struct_value()
+	b := &Type{
+		something: 'another'
+	}
+	assert !b.match_ref_receiver_against_zero_struct_value()
+}
+
+fn test_match_struct_type_ref_receiver_with_non_ref_receiver_in_caller() {
+	a := Type{}
+	assert a.match_ref_receiver_against_zero_struct_value()
+	b := Type{
+		something: 'another'
+	}
+	assert !b.match_ref_receiver_against_zero_struct_value()
+}