cgen: interface fix + add more interface tests (#7776)

vlib/v/gen/cgen.v

@@ -4196,6 +4196,10 @@ fn (mut g Gen) return_statement(node ast.Return) {
 			if !g.fn_decl.return_type.is_ptr() && node.types[0].is_ptr() {
 				// Automatic Dereference for optional
 				g.write('*')
+				// Fixes returning a mutable receiver with interface as return type
+				if node.exprs[0] is ast.Ident && !g.is_amp {
+					g.write('&')
+				}
 			}
 			for i, expr in node.exprs {
 				g.expr(expr)

vlib/v/gen/fn.v

@@ -788,7 +788,9 @@ fn (mut g Gen) call_args(node ast.CallExpr) {
 				exp_sym := g.table.get_type_symbol(expected_types[i])
 				// exp_styp := g.typ(expected_types[arg_no]) // g.table.get_type_symbol(expected_types[arg_no])
 				// styp := g.typ(arg.typ) // g.table.get_type_symbol(arg.typ)
-				if exp_sym.kind == .interface_ {
+				// NB: the second check avoids casting the interface into itself
+				// aka avoid 'I__Speaker_to_Interface_Speaker' thing for example
+				if exp_sym.kind == .interface_ && expected_types[i] != arg.typ {
 					g.interface_call(arg.typ, expected_types[i])
 					is_interface = true
 				}

vlib/v/tests/interface_test.v

@@ -188,6 +188,52 @@ fn test_register() {
 interface Speaker2 {
 	name() string
 	speak()
+	return_speaker() Speaker2
+	return_speaker2() ?Speaker2
+}
+
+struct Boss {
+mut:
+	name string
+}
+
+fn (b Boss) name() string {
+	return b.name
+}
+
+fn (b Boss) speak() {
+	println("i'm $b.name")
+}
+
+fn (b &Boss) return_speaker() Speaker2 {
+	return b
+}
+
+fn (mut b Boss) return_speaker2() ?Speaker2 {
+	if b.name == 'richard' {
+		return none
+	}
+	b.name = 'boss'
+	return b
+}
+
+fn return_speaker2(sp Speaker2) Speaker2 {
+	s := sp.return_speaker()
+	s2 := sp.return_speaker2() or {
+		return sp
+	}
+	s.speak()
+	s2.speak()
+	return s2
+}
+
+fn test_interface_returning_interface() {
+	mut b := Boss{'bob'}
+	assert b.name == 'bob'
+	s2 := return_speaker2(b)
+	if s2 is Boss {
+		assert s2.name == 'boss'
+	}
 }
 
 struct Foo {