cgen: fix a[0]+='str'

vlib/builtin/array_test.v

@@ -871,3 +871,9 @@ fn test_mutli_array_index() {
 	b[0][0] = 1
 	assert '$b' == '[[1, 0, 0], [0, 0, 0]]'
 }
+
+fn test_plus_assign_string() {
+	mut a := ['']
+	a[0] += 'abc'
+	assert a == ['abc']
+}

vlib/v/gen/cgen.v

@@ -1144,9 +1144,17 @@ fn (mut g Gen) gen_assign_stmt(assign_stmt ast.AssignStmt) {
 			}
 			mut str_add := false
 			if var_type == table.string_type_idx && assign_stmt.op == .plus_assign {
-				// str += str2 => `str = string_add(str, str2)`
-				g.expr(left)
-				g.write(' = /*f*/string_add(')
+				if left is ast.IndexExpr {
+					// a[0] += str => `array_set(&a, 0, &(string[]) {string_add(...))})`
+					g.expr(left)
+					g.write('string_add(')
+				} else {
+					// str += str2 => `str = string_add(str, str2)`
+					g.expr(left)
+					g.write(' = /*f*/string_add(')
+				}
+				g.is_assign_lhs = false
+				g.is_assign_rhs = true
 				str_add = true
 			}
 			if right_sym.kind == .function && is_decl {
@@ -1209,11 +1217,12 @@ fn (mut g Gen) gen_assign_stmt(assign_stmt ast.AssignStmt) {
 			if unwrap_optional {
 				g.write('.data')
 			}
+			if str_add {
+				g.write(')')
+			}
 			if g.is_array_set {
 				g.write(' })')
 				g.is_array_set = false
-			} else if str_add {
-				g.write(')')
 			}
 		}
 		g.right_is_opt = false
@@ -2196,7 +2205,8 @@ fn (mut g Gen) index_expr(node ast.IndexExpr) {
 					g.write(', &')
 				}
 				// `x[0] *= y`
-				if g.assign_op != .assign && g.assign_op in token.assign_tokens {
+				if g.assign_op != .assign && g.assign_op in token.assign_tokens &&
+						info.elem_type != table.string_type {
 					// TODO move this
 					g.write('*($elem_type_str*)array_get(')
 					if left_is_ptr {