FIR: add fake source element to implicit function return statement

compiler/fir/raw-fir/light-tree2fir/src/org/jetbrains/kotlin/fir/lightTree/converter/DeclarationsConverter.kt

@@ -1361,8 +1361,9 @@ class DeclarationsConverter(
                 baseSession, baseScopeProvider, stubMode, blockTree, offset = tree.getStartOffset(block), context
             ).convertBlockExpression(blockTree.root)
         } else {
+            val firExpression = buildExpressionStub()
             FirSingleExpressionBlock(
-                buildExpressionStub().toReturn()
+                firExpression.toReturn(baseSource = firExpression.source)
             )
         }
     }

compiler/fir/raw-fir/psi2fir/src/org/jetbrains/kotlin/fir/builder/RawFirBuilder.kt

@@ -236,8 +236,7 @@ class RawFirBuilder(
                 }
                 else -> {
                     val result = { bodyExpression }.toFirExpression("Function has no body (but should)")
-                    // basePsi is null, because 'return' is synthetic & should not be bound to some PSI
-                    FirSingleExpressionBlock(result.toReturn(baseSource = null)) to null
+                    FirSingleExpressionBlock(result.toReturn(baseSource = result.source)) to null
                 }
             }
 

compiler/fir/raw-fir/raw-fir.common/src/org/jetbrains/kotlin/fir/builder/BaseFirBuilder.kt

@@ -139,7 +139,7 @@ abstract class BaseFirBuilder<T>(val baseSession: FirSession, val context: Conte
                 )
             }
 
-            source = baseSource
+            source = baseSource?.fakeElement(FirFakeSourceElementKind.ImplicitReturn)
             result = this@toReturn
             if (labelName == null) {
                 target = context.firFunctionTargets.lastOrNull { !it.isLambda } ?: FirFunctionTarget(labelName, isLambda = false).apply {