compiler: Initialize named returns to default values

Make them behave as locals. We must initialize them at the start
because the default value could also be used inside the function body.

Signed-off-by: Evgenii Stratonikov <fyfyrchik@runbox.com>

pkg/compiler/codegen.go

@@ -522,6 +522,19 @@ func (c *codegen) convertFuncDecl(file ast.Node, decl *ast.FuncDecl, pkg *types.
 		}
 	}
 
+	// Emit defaults for named returns.
+	if decl.Type.Results.NumFields() != 0 {
+		for _, arg := range decl.Type.Results.List {
+			for _, id := range arg.Names {
+				if id.Name != "_" {
+					i := c.scope.newLocal(id.Name)
+					c.emitDefault(c.typeOf(arg.Type))
+					c.emitStoreByIndex(varLocal, i)
+				}
+			}
+		}
+	}
+
 	ast.Walk(c, decl.Body)
 
 	// If we have reached the end of the function without encountering `return` statement,
@@ -763,10 +776,14 @@ func (c *codegen) Visit(node ast.Node) ast.Visitor {
 				for i := len(results.List) - 1; i >= 0; i-- {
 					names := results.List[i].Names
 					for j := len(names) - 1; j >= 0; j-- {
+						if names[j].Name == "_" {
+							c.emitDefault(c.typeOf(results.List[i].Type))
+						} else {
 							c.emitLoadVar("", names[j].Name)
 						}
 					}
 				}
+			}
 		} else {
 			// first result should be on top of the stack
 			for i := len(n.Results) - 1; i >= 0; i-- {

pkg/compiler/return_test.go

@@ -120,6 +120,19 @@ func TestNamedReturn(t *testing.T) {
 	t.Run("AnotherVariable", runCase("b, c", big.NewInt(5)))
 }
 
+func TestNamedReturnDefault(t *testing.T) {
+	src := `package foo
+	func Main() int {
+		a, b, c := f()
+		return a + b + c
+	}
+	func f() (_ int, b int, c int) {
+		b += 1
+		return
+	}`
+	eval(t, src, big.NewInt(1))
+}
+
 func TestTypeAssertReturn(t *testing.T) {
 	src := `
 		package main