From b04771e78f9bf9acb5ef0cfcae6d9fa6c44ebfcd Mon Sep 17 00:00:00 2001
From: Johan Engelen <jbc.engelen@gmail.com>
Date: Tue, 12 Jul 2016 11:14:47 +0200
Subject: [PATCH] Clarify spec for "+=" and friends

See https://forum.dlang.org/post/mxvdieydnicvdxbcklrd@forum.dlang.org

(submitted through the website and I cannot make changes to this PR. Because I am cleaning up after people, if you don't like it, please submit your own PR.)
---
 spec/expression.dd | 30 +++++++++++++++++++++++++++++-
 1 file changed, 29 insertions(+), 1 deletion(-)

diff --git a/spec/expression.dd b/spec/expression.dd
index 8f6c62d8cd..c0ccaa9cb0 100644
--- a/spec/expression.dd
+++ b/spec/expression.dd
@@ -96,10 +96,38 @@ $(H4 Assignment Operator Expressions)
     $(P except that:)
 
     $(UL
-        $(LI operand $(D a) is only evaluated once)
+        $(LI operand $(D a) is only evaluated once, see below.)
         $(LI overloading $(I op) uses a different function than overloading $(I op)= does)
         $(LI the left operand of $(D >>>=) does not undergo integral promotions before shifting)
     )
+    
+    $(P The expression evaluates according to
+
+        --------------
+        void doOpAssignment(ref int x, int y)
+        {
+            x = x op y;
+        }
+        doOpAssignment(a, b);
+        --------------
+
+        thus the following unittest passes
+
+        --------------
+        int sum;
+        int add7tosum_return1() { sum += 7; return 1; }
+        unittest
+        {
+            sum = 0;
+            sum += add7tosum_return1();
+            assert(sum == 8); // "eight" !
+        
+            sum = 0;
+            sum = cast(typeof(sum))(sum + add7tosum_return1());
+            assert(sum == 1); // "one" !
+        }
+        --------------
+    )
 
 $(H3 Conditional Expressions)