K2: add more tests/fix augmented assignment case around KT-59748

compiler/testData/diagnostics/testsWithStdLib/listAssignmentInWhen.fir.kt

@@ -10,3 +10,20 @@ fun bar(list: MutableList<Any?>, condition: Boolean): Unit = <!RETURN_TYPE_MISMA
     condition -> list.set(0, "")
     else -> Unit
 }<!>
+
+fun plusFoo(list: MutableList<String>, condition: Boolean): Unit = when {
+    condition -> list[0] += ""
+    else -> Unit
+}
+
+var x: String = ""
+
+fun assign(condition: Boolean): Unit = when {
+    condition -> x = ""
+    else -> Unit
+}
+
+fun plugAssign(condition: Boolean): Unit = when {
+    condition -> x += ""
+    else -> Unit
+}

compiler/testData/diagnostics/testsWithStdLib/listAssignmentInWhen.fir.txt

@@ -25,3 +25,48 @@ FILE: listAssignmentInWhen.fir.kt
         }
 
     }
+    public final fun plusFoo(list: R|kotlin/collections/MutableList<kotlin/String>|, condition: R|kotlin/Boolean|): R|kotlin/Unit| {
+        ^plusFoo when () {
+            R|<local>/condition| ->  {
+                 {
+                     {
+                        lval <array>: R|kotlin/collections/MutableList<kotlin/String>| = R|<local>/list|
+                        lval <index_0>: R|kotlin/Int| = Int(0)
+                        R|<local>/<array>|.R|SubstitutionOverride<kotlin/collections/MutableList.set: R|kotlin/String|>|(R|<local>/<index_0>|, R|<local>/<array>|.R|SubstitutionOverride<kotlin/collections/MutableList.get: R|kotlin/String|>|(R|<local>/<index_0>|).R|kotlin/String.plus|(String()))
+                    }
+
+                    Unit
+                }
+
+            }
+            else ->  {
+                Q|kotlin/Unit|
+            }
+        }
+
+    }
+    public final var x: R|kotlin/String| = String()
+        public get(): R|kotlin/String|
+        public set(value: R|kotlin/String|): R|kotlin/Unit|
+    public final fun assign(condition: R|kotlin/Boolean|): R|kotlin/Unit| {
+        ^assign when () {
+            R|<local>/condition| ->  {
+                R|/x| = String()
+            }
+            else ->  {
+                Q|kotlin/Unit|
+            }
+        }
+
+    }
+    public final fun plugAssign(condition: R|kotlin/Boolean|): R|kotlin/Unit| {
+        ^plugAssign when () {
+            R|<local>/condition| ->  {
+                R|/x| = R|/x|.R|kotlin/String.plus|(String())
+            }
+            else ->  {
+                Q|kotlin/Unit|
+            }
+        }
+
+    }

compiler/testData/diagnostics/testsWithStdLib/listAssignmentInWhen.kt

@@ -10,3 +10,20 @@ fun bar(list: MutableList<Any?>, condition: Boolean): Unit = <!TYPE_MISMATCH!>wh
     condition -> list.set(0, "")
     else -> Unit
 }<!>
+
+fun plusFoo(list: MutableList<String>, condition: Boolean): Unit = when {
+    condition -> list[0] += ""
+    else -> Unit
+}
+
+var x: String = ""
+
+fun assign(condition: Boolean): Unit = when {
+    condition -> x = ""
+    else -> Unit
+}
+
+fun plugAssign(condition: Boolean): Unit = when {
+    condition -> x += ""
+    else -> Unit
+}