FIR resolve: add extra test & comments to multiple receiver resolve

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/calls/CallResolver.kt

@@ -146,8 +146,8 @@ abstract class SessionBasedTowerLevel(val session: FirSession) : TowerScopeLevel
 // This is more like "dispatch receiver-based tower level"
 // Here we always have an explicit or implicit dispatch receiver, and can access members of its scope
 // (which is separated from currently accessible scope, see below)
-// So: dispatch receiver = given explicit or implicit receiver
+// So: dispatch receiver = given explicit or implicit receiver (always present)
-// So: extension receiver = NONE
+// So: extension receiver = either none, if dispatch receiver = explicit receiver, or given explicit receiver, otherwise
 class MemberScopeTowerLevel(
     session: FirSession,
     val dispatchReceiver: ReceiverValue

compiler/fir/resolve/testData/resolve/expresssions/access.kt

@@ -10,15 +10,21 @@ class Foo {
 class Bar {
     val x = ""
 
+    // NB: unused
     fun Foo.abc() = x
 
     fun bar(): Bar = this
 
+    // NB: unused
     operator fun String.plus(bar: Bar): String {
         return ""
     }
 
+    // NB! abc() here is resolved to member Foo.abc(), and not to extension member of Bar
     fun Foo.check() = abc() + bar()
+
+    // NB! + here is resolved to member String.plus (not to extension member above)
+    fun Foo.check2() = "" + bar()
 }
 
 fun Foo.ext() = x

compiler/fir/resolve/testData/resolve/expresssions/access.txt

@@ -40,6 +40,10 @@ FILE: access.kt
             ^check R|/Foo.abc|().<Inapplicable(INAPPLICABLE): [kotlin/Int.plus, kotlin/Int.plus, kotlin/Int.plus, kotlin/Int.plus, kotlin/Int.plus, kotlin/Int.plus]>#(R|/bar|())
         }
 
+        public final fun R|Foo|.check2(): R|kotlin/String| {
+            ^check2 String().R|kotlin/String.plus|(R|/bar|())
+        }
+
     }
     public final fun R|Foo|.ext(): R|kotlin/Int| {
         ^ext R|/Foo.x|

compiler/fir/resolve/testData/resolve/expresssions/memberExtension.kt

@@ -0,0 +1,15 @@
+class Foo {
+    fun bar(arg: Bar) {
+        arg.foo()
+    }
+}
+
+fun Bar.foo() {}
+
+class Bar {
+    fun Foo.foo() {}
+
+    fun bar(arg: Foo) {
+        arg.foo()
+    }
+}

compiler/fir/resolve/testData/resolve/expresssions/memberExtension.txt

@@ -0,0 +1,26 @@
+FILE: memberExtension.kt
+    public final class Foo : R|kotlin/Any| {
+        public constructor(): R|Foo| {
+            super<R|kotlin/Any|>()
+        }
+
+        public final fun bar(arg: R|Bar|): R|kotlin/Unit| {
+            R|<local>/arg|.R|/foo|()
+        }
+
+    }
+    public final fun R|Bar|.foo(): R|kotlin/Unit| {
+    }
+    public final class Bar : R|kotlin/Any| {
+        public constructor(): R|Bar| {
+            super<R|kotlin/Any|>()
+        }
+
+        public final fun R|Foo|.foo(): R|kotlin/Unit| {
+        }
+
+        public final fun bar(arg: R|Foo|): R|kotlin/Unit| {
+            R|<local>/arg|.R|/Bar.foo|()
+        }
+
+    }

compiler/fir/resolve/tests/org/jetbrains/kotlin/fir/FirResolveTestCaseGenerated.java

@@ -222,6 +222,11 @@ public class FirResolveTestCaseGenerated extends AbstractFirResolveTestCase {
             runTest("compiler/fir/resolve/testData/resolve/expresssions/localImplicitBodies.kt");
         }
 
+        @TestMetadata("memberExtension.kt")
+        public void testMemberExtension() throws Exception {
+            runTest("compiler/fir/resolve/testData/resolve/expresssions/memberExtension.kt");
+        }
+
         @TestMetadata("objectVsProperty.kt")
         public void testObjectVsProperty() throws Exception {
             runTest("compiler/fir/resolve/testData/resolve/expresssions/objectVsProperty.kt");