From 59367c96d1b674a76c6e898c99dd8af4b2801d58 Mon Sep 17 00:00:00 2001
From: Evgenii Stratonikov <evgeniy@nspcc.ru>
Date: Sun, 23 Aug 2020 13:12:10 +0300
Subject: [PATCH] compiler: allow to use `+=` on strings

---
 pkg/compiler/assign_test.go  | 10 ++++++++++
 pkg/compiler/codegen.go      | 12 +++++-------
 pkg/compiler/codegen_test.go | 13 +++++++++----
 3 files changed, 24 insertions(+), 11 deletions(-)

diff --git a/pkg/compiler/assign_test.go b/pkg/compiler/assign_test.go
index f521d3a4a..4c7007a0f 100644
--- a/pkg/compiler/assign_test.go
+++ b/pkg/compiler/assign_test.go
@@ -108,6 +108,16 @@ var assignTestCases = []testCase{
 		`,
 		big.NewInt(15),
 	},
+	{
+		"add assign for string",
+		`package foo
+		func Main() string {
+			s := "Hello, "
+			s += "world!"
+			return s
+		}`,
+		[]byte("Hello, world!"),
+	},
 	{
 		"decl assign",
 		`
diff --git a/pkg/compiler/codegen.go b/pkg/compiler/codegen.go
index 089660638..24669b72d 100644
--- a/pkg/compiler/codegen.go
+++ b/pkg/compiler/codegen.go
@@ -1440,7 +1440,11 @@ func (c *codegen) emitToken(tok token.Token, typ types.Type) {
 
 func convertToken(tok token.Token, typ types.Type) (opcode.Opcode, error) {
 	switch tok {
-	case token.ADD_ASSIGN:
+	case token.ADD_ASSIGN, token.ADD:
+		// VM has separate opcodes for number and string concatenation
+		if isString(typ) {
+			return opcode.CAT, nil
+		}
 		return opcode.ADD, nil
 	case token.SUB_ASSIGN:
 		return opcode.SUB, nil
@@ -1450,12 +1454,6 @@ func convertToken(tok token.Token, typ types.Type) (opcode.Opcode, error) {
 		return opcode.DIV, nil
 	case token.REM_ASSIGN:
 		return opcode.MOD, nil
-	case token.ADD:
-		// VM has separate opcodes for number and string concatenation
-		if isString(typ) {
-			return opcode.CAT, nil
-		}
-		return opcode.ADD, nil
 	case token.SUB:
 		return opcode.SUB, nil
 	case token.MUL:
diff --git a/pkg/compiler/codegen_test.go b/pkg/compiler/codegen_test.go
index 9245edef9..3de3dea7f 100644
--- a/pkg/compiler/codegen_test.go
+++ b/pkg/compiler/codegen_test.go
@@ -18,12 +18,12 @@ func TestConvertToken(t *testing.T) {
 	}
 
 	testCases := []testCase{
-		{"ADD (string)",
+		{"ADD (number)",
 			token.ADD,
 			opcode.ADD,
 			types.Typ[types.Int],
 		},
-		{"ADD (number)",
+		{"ADD (string)",
 			token.ADD,
 			opcode.CAT,
 			types.Typ[types.String],
@@ -48,10 +48,15 @@ func TestConvertToken(t *testing.T) {
 			opcode.MOD,
 			nil,
 		},
-		{"ADD_ASSIGN",
+		{"ADD_ASSIGN (number)",
 			token.ADD_ASSIGN,
 			opcode.ADD,
-			nil,
+			types.Typ[types.Int],
+		},
+		{"ADD_ASSIGN (string)",
+			token.ADD_ASSIGN,
+			opcode.CAT,
+			types.Typ[types.String],
 		},
 		{"SUB_ASSIGN",
 			token.SUB_ASSIGN,