cgen: fix typeof union sum type (#6794)

vlib/v/gen/cgen.v

@@ -381,6 +381,22 @@ pub fn (mut g Gen) write_typeof_functions() {
 			g.writeln('}')
 		}
 	}
+	for typ in g.table.types {
+		if typ.kind == .union_sum_type {
+			sum_info := typ.info as table.UnionSumType
+			tidx := g.table.find_type_idx(typ.name)
+			g.writeln('char * v_typeof_unionsumtype_${tidx}(int sidx) { /* $typ.name */ ')
+			g.writeln('	switch(sidx) {')
+			g.writeln('		case $tidx: return "${util.strip_main_name(typ.name)}";')
+			for v in sum_info.variants {
+				subtype := g.table.get_type_symbol(v)
+				g.writeln('		case $v: return "${util.strip_main_name(subtype.name)}";')
+			}
+			g.writeln('		default: return "unknown ${util.strip_main_name(typ.name)}";')
+			g.writeln('	}')
+			g.writeln('}')
+		}
+	}
 	g.writeln('// << typeof() support for sum types')
 	g.writeln('')
 }
@@ -2566,6 +2582,13 @@ fn (mut g Gen) typeof_expr(node ast.TypeOf) {
 		g.write('tos3( /* $sym.name */ v_typeof_sumtype_${sum_type_idx}( (')
 		g.expr(node.expr)
 		g.write(').typ ))')
+	} else if sym.kind == .union_sum_type {
+		// When encountering a .sum_type, typeof() should be done at runtime,
+		// because the subtype of the expression may change:
+		sum_type_idx := node.expr_type.idx()
+		g.write('tos3( /* $sym.name */ v_typeof_unionsumtype_${sum_type_idx}( (')
+		g.expr(node.expr)
+		g.write(').typ ))')
 	} else if sym.kind == .array_fixed {
 		fixed_info := sym.info as table.ArrayFixed
 		typ_name := g.table.get_type_name(fixed_info.elem_type)

vlib/v/tests/union_sum_type_test.v

@@ -24,7 +24,7 @@ fn handle(e Expr) string {
 	assert is_literal
 	assert !(e !is IntegerLiteral)
 	if e is IntegerLiteral {
-		println('int')
+		assert typeof(e.val) == 'string'
 	}
 	match union e {
 		IntegerLiteral {
@@ -294,7 +294,7 @@ fn test_nested_if_is() {
 	mut b := Outer(InnerStruct{Inner(0)})
 	if b is InnerStruct {
 		if b.x is int {
-			println(b.x)
+			assert b.x == 0
 		}
 	}
 }
@@ -303,20 +303,30 @@ fn test_casted_sum_type_selector_reassign() {
 	mut b := InnerStruct{Inner(0)}
 	if b.x is int {
 		assert typeof(b.x) == 'int'
+		// this check works only if x is castet
+		assert b.x == 0
 		b.x = 'test'
+		// this check works only if x is castet
+		assert b.x[0] == `t`
 		assert typeof(b.x) == 'string'
 	}
-	assert typeof(b.x) == 'Inner'
+	// this check works only if x is not castet
+	assert b.x is string
 }
 
 fn test_casted_sum_type_ident_reassign() {
 	mut x := Inner(0)
 	if x is int {
+		// this check works only if x is castet
+		assert x == 0
 		assert typeof(x) == 'int'
 		x = 'test'
+		// this check works only if x is castet
+		assert x[0] == `t`
 		assert typeof(x) == 'string'
 	}
-	assert typeof(x) == 'Inner'
+	// this check works only if x is not castet
+	assert x is string
 }
 
 __type Expr2 = int | string
@@ -427,11 +437,17 @@ fn test_match_multi_branch() {
 	mut y := ''
 	match union f {
 		CallExpr2, CTempVarExpr {
-			assert typeof(f) == 'Expr4'
+			// this check works only if f is not castet
+			assert f is CTempVarExpr
 		}
 	}
 }
 
+fn test_typeof() {
+    x := Expr4(CTempVarExpr{})
+	assert typeof(x) == 'CTempVarExpr'
+}
+
 struct Outer2 {
 	e Expr4
 }