checker: make implementing an interface with another interface an error (#8398)

vlib/v/checker/checker.v

@@ -1870,6 +1870,11 @@ fn (mut c Checker) type_implements(typ table.Type, inter_typ table.Type, pos tok
 	typ_sym := c.table.get_type_symbol(typ)
 	mut inter_sym := c.table.get_type_symbol(inter_typ)
 	styp := c.table.type_to_str(typ)
+	same_base_type := typ.idx() == inter_typ.idx()
+	if typ_sym.kind == .interface_ && inter_sym.kind == .interface_ && !same_base_type {
+		c.error('cannot implement interface `$inter_sym.name` with a different interface `$styp`',
+			pos)
+	}
 	imethods := if inter_sym.kind == .interface_ {
 		(inter_sym.info as table.Interface).methods
 	} else {
@@ -3566,6 +3571,8 @@ pub fn (mut c Checker) cast_expr(mut node ast.CastExpr) table.Type {
 			type_name := c.table.type_to_str(node.expr_type)
 			c.error('cannot cast `$type_name` to struct', node.pos)
 		}
+	} else if to_type_sym.kind == .interface_ {
+		c.type_implements(node.expr_type, node.typ, node.pos)
 	} else if node.typ == table.bool_type {
 		c.error('cannot cast to bool - use e.g. `some_int != 0` instead', node.pos)
 	} else if node.expr_type == table.none_type {
@@ -3575,8 +3582,6 @@ pub fn (mut c Checker) cast_expr(mut node ast.CastExpr) table.Type {
 		if (node.typ.is_ptr() || to_type_sym.kind !in [.sum_type, .interface_]) && !c.is_builtin_mod {
 			type_name := c.table.type_to_str(node.typ)
 			c.error('cannot cast struct to `$type_name`', node.pos)
-		} else if to_type_sym.kind == .interface_ {
-			c.type_implements(node.expr_type, node.typ, node.pos)
 		}
 	} else if node.expr_type.has_flag(.optional) || node.expr_type.has_flag(.variadic) {
 		// variadic case can happen when arrays are converted into variadic

vlib/v/checker/tests/interface_implementing_interface.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/interface_implementing_interface.vv:15:10: error: cannot implement interface `Thing` with a different interface `Animal`
+   13 | dog := Dog{}
+   14 | animal := Animal(dog)
+   15 | thing := Thing(animal)
+      |          ~~~~~~~~~~~~~
+   16 | println(thing)

vlib/v/checker/tests/interface_implementing_interface.vv

@@ -0,0 +1,16 @@
+interface Thing {
+    kind string
+}
+
+interface Animal {
+    kind string
+}
+
+struct Dog {
+    kind string = 'labrador'
+}
+
+dog := Dog{}
+animal := Animal(dog)
+thing := Thing(animal)
+println(thing)

vlib/v/checker/tests/struct_type_cast_err.out

@@ -60,11 +60,4 @@ vlib/v/checker/tests/struct_type_cast_err.vv:13:10: error: cannot cast struct to
    13 |     _ := int(foo)
       |          ~~~~~~~~
    14 |     _ = &I1(foo)
-   15 | }
+   15 | }
-vlib/v/checker/tests/struct_type_cast_err.vv:14:10: error: cannot cast struct to `&I1`
-   12 |     _ := i64(foo)
-   13 |     _ := int(foo)
-   14 |     _ = &I1(foo)
-      |          ~~~~~~~
-   15 | }
-   16 |