KT-61728 [LL API] Add extra test for smartcast inside let

analysis/low-level-api-fir/testdata/contextCollector/smartCasts/insideLetOnNullableReceiver.kt

@@ -0,0 +1,7 @@
+fun foo(param: String?): String {
+    param?.let {
+        return <expr>param</expr>
+    }
+
+    return ""
+}

analysis/low-level-api-fir/testdata/contextCollector/smartCasts/insideLetOnNullableReceiver.txt

@@ -0,0 +1,40 @@
+Tower Data Context:
+    Element 0
+        Scope: FirDefaultStarImportingScope
+    Element 1
+        Scope: FirDefaultSimpleImportingScope
+    Element 2
+        Scope: FirExplicitStarImportingScope
+    Element 3
+        Scope: FirDefaultSimpleImportingScope
+    Element 4
+        Scope: FirDefaultSimpleImportingScope
+    Element 5
+        Scope: FirPackageMemberScope
+    Element 6
+        Scope: FirExplicitSimpleImportingScope
+    Element 7
+        Scope: FirLocalScope
+            Properties:
+                FirValueParameterSymbol param: R|kotlin/String?|
+    Element 8
+        Scope: FirLocalScope
+    Element 9
+        Scope: FirLocalScope
+            Properties:
+                FirValueParameterSymbol it: R|kotlin/String|
+    Element 10
+        Scope: FirLocalScope
+Smart Casts:
+    FirValueParameterSymbol param: R|kotlin/String?|
+    Types:
+        kotlin/Any
+
+FILE: [ResolvedTo(IMPORTS)] insideLetOnNullableReceiver.kt
+    public final [ResolvedTo(BODY_RESOLVE)] fun foo([ResolvedTo(BODY_RESOLVE)] param: R|kotlin/String?|): R|kotlin/String| {
+        R|<local>/param|?.{ $subj$.R|kotlin/let|<R|kotlin/String|, R|kotlin/Nothing|>(<L> = [ResolvedTo(BODY_RESOLVE)] let@fun <anonymous>([ResolvedTo(BODY_RESOLVE)] it: R|kotlin/String|): R|kotlin/Nothing| <inline=Inline, kind=EXACTLY_ONCE>  {
+            ^foo R|<local>/param|
+        }
+        ) }
+        ^foo String()
+    }

analysis/low-level-api-fir/tests/org/jetbrains/kotlin/analysis/low/level/api/fir/ContextCollectorSourceTestGenerated.java

@@ -257,6 +257,12 @@ public class ContextCollectorSourceTestGenerated extends AbstractContextCollecto
             runTest("analysis/low-level-api-fir/testdata/contextCollector/smartCasts/extensionReceiver.kt");
         }
 
+        @Test
+        @TestMetadata("insideLetOnNullableReceiver.kt")
+        public void testInsideLetOnNullableReceiver() throws Exception {
+            runTest("analysis/low-level-api-fir/testdata/contextCollector/smartCasts/insideLetOnNullableReceiver.kt");
+        }
+
         @Test
         @TestMetadata("insideLoop.kt")
         public void testInsideLoop() throws Exception {