Don't forget side effects in prefixes of inlined function calls

compiler/src/dotty/tools/dotc/typer/Inliner.scala

@@ -546,7 +546,10 @@ class Inliner(call: tpd.Tree, rhsToInline: tpd.Tree)(using Context) {
     for ((level, selfSym) <- sortedProxies) {
       lazy val rhsClsSym = selfSym.info.widenDealias.classSymbol
       val rhs = selfSym.info.dealias match
-        case info: TermRef if info.isStable =>
+        case info: TermRef
+        if info.isStable && (lastSelf.exists || isPureExpr(inlineCallPrefix)) =>
+          // If this is the first proxy, optimize to `ref(info)` only if call prefix is pure.
+          // Otherwise we might forget side effects. See run/i12829.scala.
           ref(info)
         case info =>
           val rhsClsSym = info.widenDealias.classSymbol

tests/run/i12829.check

@@ -0,0 +1,2 @@
+abc
+abc

tests/run/i12829.scala

@@ -0,0 +1,19 @@
+object Test:
+
+  class Foo:
+    var buf = ""
+
+    def add(v: String): Unit = buf += v
+
+    inline def += (inline v: String): this.type = {add(v); this }
+
+
+  def main(sa: Array[String]): Unit =
+
+    var fooVar = new Foo
+    fooVar += "a" += "b" += "c"
+    println(fooVar.buf)              // Prints: abc
+
+    val fooVal = new Foo
+    fooVal += "a" += "b" += "c"
+    println(fooVal.buf)              // Printed: c     // Expected abc