FIR: Support proper loading of Map.merge

compiler/fir/jvm/src/org/jetbrains/kotlin/fir/scopes/jvm/DescriptorUtils.kt

@@ -116,6 +116,10 @@ private fun StringBuilder.appendConeType(coneType: ConeKotlinType) {
                 appendClassLikeType(representative.coneTypeUnsafe())
             }
         }
+        is ConeDefinitelyNotNullType -> {
+            appendConeType(coneType.original)
+            return
+        }
         is ConeFlexibleType -> {
             appendConeType(coneType.lowerBound)
             return

compiler/fir/resolve/testData/resolveWithStdlib/j+k/mapMerge.kt

@@ -0,0 +1,5 @@
+// FULL_JDK
+
+fun foo(x: MutableMap<String, List<String>>) {
+    x.merge("", listOf("")) { a, b -> a + b }
+}

compiler/fir/resolve/testData/resolveWithStdlib/j+k/mapMerge.txt

@@ -0,0 +1,7 @@
+FILE: mapMerge.kt
+    public final fun foo(x: R|kotlin/collections/MutableMap<kotlin/String, kotlin/collections/List<kotlin/String>>|): R|kotlin/Unit| {
+        R|<local>/x|.R|FakeOverride<java/util/Map.merge: R|kotlin/collections/List<kotlin/String>?|>|(String(), R|kotlin/collections/listOf|<R|kotlin/String|>(String()), <L> = merge@fun <anonymous>(a: R|ft<kotlin/collections/List<kotlin/String>, kotlin/collections/List<kotlin/String>?>!|, b: R|ft<kotlin/collections/List<kotlin/String>, kotlin/collections/List<kotlin/String>?>!|): R|ft<kotlin/collections/List<kotlin/String>, kotlin/collections/List<kotlin/String>?>!| {
+            ^ R|<local>/a|.R|kotlin/collections/plus|<R|kotlin/String|>(R|<local>/b|)
+        }
+        )
+    }

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

@@ -645,6 +645,11 @@ public class FirDiagnosticsWithStdlibTestGenerated extends AbstractFirDiagnostic
             runTest("compiler/fir/resolve/testData/resolveWithStdlib/j+k/MapEntry.kt");
         }
 
+        @TestMetadata("mapMerge.kt")
+        public void testMapMerge() throws Exception {
+            runTest("compiler/fir/resolve/testData/resolveWithStdlib/j+k/mapMerge.kt");
+        }
+
         @TestMetadata("MyException.kt")
         public void testMyException() throws Exception {
             runTest("compiler/fir/resolve/testData/resolveWithStdlib/j+k/MyException.kt");