[FIR] Don't constraints for return expressions of lambda if it has Unit return type

compiler/fir/analysis-tests/testData/resolveWithStdlib/inference/builderInferenceAndCoercionToUnit.kt

@@ -0,0 +1,9 @@
+class DropDownComponent<T : Any>(val initialValues: List<T>)
+
+fun test(strings: List<String>) {
+    val dropDown = DropDownComponent(
+        initialValues = buildList {
+            addAll(strings)
+        }
+    )
+}

compiler/fir/analysis-tests/testData/resolveWithStdlib/inference/builderInferenceAndCoercionToUnit.txt

@@ -0,0 +1,16 @@
+FILE: builderInferenceAndCoercionToUnit.kt
+    public final class DropDownComponent<T : R|kotlin/Any|> : R|kotlin/Any| {
+        public constructor<T : R|kotlin/Any|>(initialValues: R|kotlin/collections/List<T>|): R|DropDownComponent<T>| {
+            super<R|kotlin/Any|>()
+        }
+
+        public final val initialValues: R|kotlin/collections/List<T>| = R|<local>/initialValues|
+            public get(): R|kotlin/collections/List<T>|
+
+    }
+    public final fun test(strings: R|kotlin/collections/List<kotlin/String>|): R|kotlin/Unit| {
+        lval dropDown: R|DropDownComponent<kotlin/String>| = R|/DropDownComponent.DropDownComponent|<R|kotlin/String|>(initialValues = R|kotlin/collections/buildList|<R|kotlin/String|>(<L> = buildList@fun R|kotlin/collections/MutableList<kotlin/String>|.<anonymous>(): R|kotlin/Unit| <kind=EXACTLY_ONCE>  {
+            this@R|special/anonymous|.R|FakeOverride<kotlin/collections/MutableList.addAll: R|kotlin/Boolean|>|(R|<local>/strings|)
+        }
+        ))
+    }

compiler/fir/analysis-tests/tests/org/jetbrains/kotlin/fir/FirDiagnosticsWithStdlibTestGenerated.java

@@ -847,6 +847,11 @@ public class FirDiagnosticsWithStdlibTestGenerated extends AbstractFirDiagnostic
             runTest("compiler/fir/analysis-tests/testData/resolveWithStdlib/inference/builderInference.kt");
         }
 
+        @TestMetadata("builderInferenceAndCoercionToUnit.kt")
+        public void testBuilderInferenceAndCoercionToUnit() throws Exception {
+            runTest("compiler/fir/analysis-tests/testData/resolveWithStdlib/inference/builderInferenceAndCoercionToUnit.kt");
+        }
+
         @TestMetadata("builderInferenceFromStdlib.kt")
         public void testBuilderInferenceFromStdlib() throws Exception {
             runTest("compiler/fir/analysis-tests/testData/resolveWithStdlib/inference/builderInferenceFromStdlib.kt");

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

@@ -294,7 +294,7 @@ private enum class VarargMappingState {
 
 private fun FirFunction<*>.indexOf(valueParameter: FirValueParameter): Int = valueParameters.indexOf(valueParameter)
 
-private val ConeKotlinType.isUnit: Boolean
+val ConeKotlinType.isUnit: Boolean
     get() {
         val type = this.lowerBoundIfFlexible()
         if (type.isNullable) return false

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/inference/PostponedArgumentsAnalyzer.kt

@@ -154,13 +154,14 @@ class PostponedArgumentsAnalyzer(
         val checkerSink: CheckerSink = CheckerSinkImpl()
 
         var hasExpressionInReturnArguments = false
+        val lambdaReturnType = lambda.returnType.let(::substitute).takeUnless { it.isUnit }
         returnArguments.forEach {
             if (it !is FirExpression) return@forEach
             hasExpressionInReturnArguments = true
             candidate.resolveArgumentExpression(
                 c.getBuilder(),
                 it,
-                lambda.returnType.let(::substitute),
+                lambdaReturnType,
                 lambda.atom.returnTypeRef, // TODO: proper ref
                 checkerSink,
                 context = resolutionContext,
@@ -169,8 +170,7 @@ class PostponedArgumentsAnalyzer(
             )
         }
 
-        if (!hasExpressionInReturnArguments) {
+        if (!hasExpressionInReturnArguments && lambdaReturnType != null) {
-            val lambdaReturnType = lambda.returnType.let(::substitute)
             /*LambdaArgumentConstraintPosition(lambda)*/
             c.getBuilder().addEqualityConstraint(lambdaReturnType, unitType, SimpleConstraintSystemConstraintPosition)
         }