Introduce `Unit`-conversions for simple arguments

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

@@ -24444,6 +24444,49 @@ public class FirOldFrontendDiagnosticsTestGenerated extends AbstractFirOldFronte
         }
     }
 
+    @TestMetadata("compiler/testData/diagnostics/tests/unitConversion")
+    @TestDataPath("$PROJECT_ROOT")
+    @RunWith(JUnit3RunnerWithInners.class)
+    public static class UnitConversion extends AbstractFirOldFrontendDiagnosticsTest {
+        private void runTest(String testDataFilePath) throws Exception {
+            KotlinTestUtils.runTest(this::doTest, this, testDataFilePath);
+        }
+
+        public void testAllFilesPresentInUnitConversion() throws Exception {
+            KotlinTestUtils.assertAllTestsPresentByMetadataWithExcluded(this.getClass(), new File("compiler/testData/diagnostics/tests/unitConversion"), Pattern.compile("^(.+)\\.kt$"), Pattern.compile("^(.+)\\.fir\\.kts?$"), true);
+        }
+
+        @TestMetadata("chainedFunSuspendUnitConversion.kt")
+        public void testChainedFunSuspendUnitConversion() throws Exception {
+            runTest("compiler/testData/diagnostics/tests/unitConversion/chainedFunSuspendUnitConversion.kt");
+        }
+
+        @TestMetadata("chainedFunUnitConversion.kt")
+        public void testChainedFunUnitConversion() throws Exception {
+            runTest("compiler/testData/diagnostics/tests/unitConversion/chainedFunUnitConversion.kt");
+        }
+
+        @TestMetadata("chainedUnitSuspendConversion.kt")
+        public void testChainedUnitSuspendConversion() throws Exception {
+            runTest("compiler/testData/diagnostics/tests/unitConversion/chainedUnitSuspendConversion.kt");
+        }
+
+        @TestMetadata("unitConversionCompatibility.kt")
+        public void testUnitConversionCompatibility() throws Exception {
+            runTest("compiler/testData/diagnostics/tests/unitConversion/unitConversionCompatibility.kt");
+        }
+
+        @TestMetadata("unitConversionForAllKinds.kt")
+        public void testUnitConversionForAllKinds() throws Exception {
+            runTest("compiler/testData/diagnostics/tests/unitConversion/unitConversionForAllKinds.kt");
+        }
+
+        @TestMetadata("unitConversionForSubtypes.kt")
+        public void testUnitConversionForSubtypes() throws Exception {
+            runTest("compiler/testData/diagnostics/tests/unitConversion/unitConversionForSubtypes.kt");
+        }
+    }
+
     @TestMetadata("compiler/testData/diagnostics/tests/varargs")
     @TestDataPath("$PROJECT_ROOT")
     @RunWith(JUnit3RunnerWithInners.class)

compiler/resolution/src/org/jetbrains/kotlin/resolve/calls/components/UnitTypeConversions.kt

+/*
+ * Copyright 2010-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.resolve.calls.components
+
+import org.jetbrains.kotlin.builtins.*
+import org.jetbrains.kotlin.descriptors.ParameterDescriptor
+import org.jetbrains.kotlin.resolve.calls.inference.model.LowerPriorityToPreserveCompatibility
+import org.jetbrains.kotlin.resolve.calls.model.KotlinCallArgument
+import org.jetbrains.kotlin.resolve.calls.model.KotlinResolutionCandidate
+import org.jetbrains.kotlin.resolve.calls.model.SimpleKotlinCallArgument
+import org.jetbrains.kotlin.types.KotlinType
+import org.jetbrains.kotlin.types.UnwrappedType
+import org.jetbrains.kotlin.types.isDynamic
+import org.jetbrains.kotlin.types.typeUtil.isUnit
+
+object UnitTypeConversions : ParameterTypeConversion {
+    override fun conversionDefinitelyNotNeeded(
+        candidate: KotlinResolutionCandidate,
+        argument: KotlinCallArgument,
+        expectedParameterType: UnwrappedType
+    ): Boolean {
+        // for callable references and lambdas it already works
+        if (argument !is SimpleKotlinCallArgument) return true
+
+        val receiver = argument.receiver
+        if (receiver.receiverValue.type.hasUnitOrDynamicReturnType()) return true
+        if (receiver.typesFromSmartCasts.any { it.hasUnitOrDynamicReturnType() }) return true
+
+        if (
+            !expectedParameterType.isBuiltinFunctionalType ||
+            !expectedParameterType.getReturnTypeFromFunctionType().isUnit()
+        ) return true
+
+        return false
+    }
+
+    private fun KotlinType.hasUnitOrDynamicReturnType(): Boolean =
+        isFunctionOrKFunctionTypeWithAnySuspendability && arguments.last().type.isUnitOrDynamic()
+
+    private fun KotlinType.isUnitOrDynamic(): Boolean = isUnit() || isDynamic()
+
+    override fun conversionIsNeededBeforeSubtypingCheck(argument: KotlinCallArgument): Boolean =
+        argument is SimpleKotlinCallArgument && argument.receiver.stableType.isFunctionType
+
+    override fun conversionIsNeededAfterSubtypingCheck(argument: KotlinCallArgument): Boolean {
+        if (argument !is SimpleKotlinCallArgument) return false
+
+        var isFunctionTypeOrSubtype = false
+        val hasReturnTypeInSubtypes = argument.receiver.stableType.isFunctionTypeOrSubtype {
+            isFunctionTypeOrSubtype = true
+            it.getReturnTypeFromFunctionType().isUnitOrDynamic()
+        }
+
+        if (!isFunctionTypeOrSubtype) return false
+
+        return !hasReturnTypeInSubtypes
+    }
+
+    override fun convertParameterType(
+        candidate: KotlinResolutionCandidate,
+        argument: KotlinCallArgument,
+        parameter: ParameterDescriptor,
+        expectedParameterType: UnwrappedType
+    ): UnwrappedType? {
+        val nonUnitReturnedParameterType = createFunctionType(
+            candidate.callComponents.builtIns,
+            expectedParameterType.annotations,
+            expectedParameterType.getReceiverTypeFromFunctionType(),
+            expectedParameterType.getValueParameterTypesFromFunctionType().map { it.type },
+            parameterNames = null,
+            candidate.callComponents.builtIns.nullableAnyType,
+            suspendFunction = expectedParameterType.isSuspendFunctionType
+        )
+
+        candidate.addDiagnostic(LowerPriorityToPreserveCompatibility)
+
+        return nonUnitReturnedParameterType
+    }
+
+}
\ No newline at end of file

compiler/resolution/src/org/jetbrains/kotlin/resolve/calls/components/typeConversions.kt

@@ -40,14 +40,23 @@ object TypeConversions {
         )
 
         val suspendConversionData = performConversionBeforeSubtyping(
-            candidate, argument, candidateParameter, samConversionData.convertedType ?: candidateExpectedType, SuspendTypeConversions
+            candidate, argument, candidateParameter,
+            candidateExpectedType = samConversionData.convertedType ?: candidateExpectedType,
+            SuspendTypeConversions
+        )
+
+        val unitConversionData = performConversionBeforeSubtyping(
+            candidate, argument, candidateParameter,
+            candidateExpectedType = suspendConversionData.convertedType ?: samConversionData.convertedType ?: candidateExpectedType,
+            UnitTypeConversions
         )
 
         return ConversionData(
-            convertedType = suspendConversionData.convertedType ?: samConversionData.convertedType,
-            wasConversion = samConversionData.wasConversion || suspendConversionData.wasConversion,
+            convertedType = unitConversionData.convertedType ?: suspendConversionData.convertedType ?: samConversionData.convertedType,
+            wasConversion = samConversionData.wasConversion || suspendConversionData.wasConversion || unitConversionData.wasConversion,
             conversionDefinitelyNotNeeded = samConversionData.conversionDefinitelyNotNeeded &&
-                    suspendConversionData.conversionDefinitelyNotNeeded
+                    suspendConversionData.conversionDefinitelyNotNeeded &&
+                    unitConversionData.conversionDefinitelyNotNeeded
         )
     }
 
@@ -60,10 +69,13 @@ object TypeConversions {
         val samConvertedType = performConversionAfterSubtyping(
             candidate, argument, candidateParameter, candidateExpectedType, SamTypeConversions
         )
-
         if (samConvertedType != null) return samConvertedType
 
-        return performConversionAfterSubtyping(candidate, argument, candidateParameter, candidateExpectedType, SuspendTypeConversions)
+        val suspendConvertedType =
+            performConversionAfterSubtyping(candidate, argument, candidateParameter, candidateExpectedType, SuspendTypeConversions)
+        if (suspendConvertedType != null) return suspendConvertedType
+
+        return performConversionAfterSubtyping(candidate, argument, candidateParameter, candidateExpectedType, UnitTypeConversions)
     }
 
     private fun performConversionAfterSubtyping(

compiler/testData/diagnostics/tests/unitConversion/chainedFunSuspendUnitConversion.fir.kt

+// !LANGUAGE: +UnitConversion +SuspendConversion
+// !DIAGNOSTICS: -UNUSED_PARAMETER -UNUSED_EXPRESSION
+
+fun interface SuspendRunnable {
+    suspend fun run()
+}
+
+fun foo(r: SuspendRunnable) {}
+
+fun bar(): String = ""
+
+abstract class SubInt : () -> Int
+
+fun test(f: () -> String, s: SubInt) {
+    <!INAPPLICABLE_CANDIDATE!>foo<!>(f)
+    <!INAPPLICABLE_CANDIDATE!>foo<!>(s)
+    <!INAPPLICABLE_CANDIDATE!>foo<!>(::bar)
+}
\ No newline at end of file

compiler/testData/diagnostics/tests/unitConversion/chainedFunSuspendUnitConversion.kt

+// !LANGUAGE: +UnitConversion +SuspendConversion
+// !DIAGNOSTICS: -UNUSED_PARAMETER -UNUSED_EXPRESSION
+
+fun interface SuspendRunnable {
+    suspend fun run()
+}
+
+fun foo(r: SuspendRunnable) {}
+
+fun bar(): String = ""
+
+abstract class SubInt : () -> Int
+
+fun test(f: () -> String, s: SubInt) {
+    foo(f)
+    foo(s)
+    foo(::bar)
+}
\ No newline at end of file

compiler/testData/diagnostics/tests/unitConversion/chainedFunSuspendUnitConversion.txt

+package
+
+public fun bar(): kotlin.String
+public fun foo(/*0*/ r: SuspendRunnable): kotlin.Unit
+public fun test(/*0*/ f: () -> kotlin.String, /*1*/ s: SubInt): kotlin.Unit
+
+public abstract class SubInt : () -> kotlin.Int {
+    public constructor SubInt()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public abstract override /*1*/ /*fake_override*/ fun invoke(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public fun interface SuspendRunnable {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public abstract suspend fun run(): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}

compiler/testData/diagnostics/tests/unitConversion/chainedFunUnitConversion.fir.kt

+// !LANGUAGE: +UnitConversion
+// !DIAGNOSTICS: -UNUSED_PARAMETER -UNUSED_EXPRESSION
+
+fun interface KRunnable {
+    fun run()
+}
+
+fun foo(r: KRunnable) {}
+
+abstract class SubInt : () -> Int
+
+fun test(f: () -> Int, s: SubInt) {
+    <!INAPPLICABLE_CANDIDATE!>foo<!>(f)
+    <!INAPPLICABLE_CANDIDATE!>foo<!>(s)
+}
\ No newline at end of file

compiler/testData/diagnostics/tests/unitConversion/chainedFunUnitConversion.kt

+// !LANGUAGE: +UnitConversion
+// !DIAGNOSTICS: -UNUSED_PARAMETER -UNUSED_EXPRESSION
+
+fun interface KRunnable {
+    fun run()
+}
+
+fun foo(r: KRunnable) {}
+
+abstract class SubInt : () -> Int
+
+fun test(f: () -> Int, s: SubInt) {
+    foo(f)
+    foo(s)
+}
\ No newline at end of file

compiler/testData/diagnostics/tests/unitConversion/chainedFunUnitConversion.txt

+package
+
+public fun foo(/*0*/ r: KRunnable): kotlin.Unit
+public fun test(/*0*/ f: () -> kotlin.Int, /*1*/ s: SubInt): kotlin.Unit
+
+public fun interface KRunnable {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public abstract fun run(): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public abstract class SubInt : () -> kotlin.Int {
+    public constructor SubInt()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public abstract override /*1*/ /*fake_override*/ fun invoke(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}

compiler/testData/diagnostics/tests/unitConversion/chainedUnitSuspendConversion.fir.kt

+// !LANGUAGE: +UnitConversion +SuspendConversion
+// !DIAGNOSTICS: -UNUSED_PARAMETER -UNUSED_EXPRESSION
+
+fun foo(f: suspend () -> Unit) {}
+
+fun bar(): String = ""
+
+abstract class SubInt : () -> Int
+
+fun test(g: () -> Double, s: SubInt) {
+    <!INAPPLICABLE_CANDIDATE!>foo<!>(::bar)
+    <!INAPPLICABLE_CANDIDATE!>foo<!>(g)
+    <!INAPPLICABLE_CANDIDATE!>foo<!>(s)
+}
\ No newline at end of file

compiler/testData/diagnostics/tests/unitConversion/chainedUnitSuspendConversion.kt

+// !LANGUAGE: +UnitConversion +SuspendConversion
+// !DIAGNOSTICS: -UNUSED_PARAMETER -UNUSED_EXPRESSION
+
+fun foo(f: suspend () -> Unit) {}
+
+fun bar(): String = ""
+
+abstract class SubInt : () -> Int
+
+fun test(g: () -> Double, s: SubInt) {
+    foo(::bar)
+    foo(g)
+    foo(s)
+}
\ No newline at end of file

compiler/testData/diagnostics/tests/unitConversion/chainedUnitSuspendConversion.txt

+package
+
+public fun bar(): kotlin.String
+public fun foo(/*0*/ f: suspend () -> kotlin.Unit): kotlin.Unit
+public fun test(/*0*/ g: () -> kotlin.Double, /*1*/ s: SubInt): kotlin.Unit
+
+public abstract class SubInt : () -> kotlin.Int {
+    public constructor SubInt()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public abstract override /*1*/ /*fake_override*/ fun invoke(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}

compiler/testData/diagnostics/tests/unitConversion/unitConversionForAllKinds.fir.kt

+// !LANGUAGE: +UnitConversion
+// !DIAGNOSTICS: -UNUSED_PARAMETER -UNUSED_EXPRESSION
+
+fun foo(f: () -> Unit) {}
+fun <T> fooGeneric(f: (T) -> Unit): T = TODO()
+
+fun bar(): String = ""
+fun createCall(): () -> Int = TODO()
+
+fun test(g: () -> String, h: (Float) -> String) {
+    foo(::bar)
+    foo { "something" }
+    foo(g)
+
+    fooGeneric(h)
+}

compiler/testData/diagnostics/tests/unitConversion/unitConversionForAllKinds.kt

+// !LANGUAGE: +UnitConversion
+// !DIAGNOSTICS: -UNUSED_PARAMETER -UNUSED_EXPRESSION
+
+fun foo(f: () -> Unit) {}
+fun <T> fooGeneric(f: (T) -> Unit): T = TODO()
+
+fun bar(): String = ""
+fun createCall(): () -> Int = TODO()
+
+fun test(g: () -> String, h: (Float) -> String) {
+    foo(::bar)
+    foo { "something" }
+    foo(g)
+
+    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Float")!>fooGeneric(h)<!>
+}
\ No newline at end of file

compiler/testData/diagnostics/tests/unitConversion/unitConversionForAllKinds.txt

+package
+
+public fun bar(): kotlin.String
+public fun createCall(): () -> kotlin.Int
+public fun foo(/*0*/ f: () -> kotlin.Unit): kotlin.Unit
+public fun </*0*/ T> fooGeneric(/*0*/ f: (T) -> kotlin.Unit): T
+public fun test(/*0*/ g: () -> kotlin.String, /*1*/ h: (kotlin.Float) -> kotlin.String): kotlin.Unit

compiler/testData/diagnostics/tests/unitConversion/unitConversionForSubtypes.fir.kt

+// !LANGUAGE: +UnitConversion
+// !DIAGNOSTICS: -UNUSED_PARAMETER -UNUSED_EXPRESSION
+
+fun foo(f: (Int, String) -> Unit) {}
+
+abstract class SubInt : (Int, String) -> Int
+abstract class SubIntWrong : (String, String) -> Int
+
+fun test1(s: SubInt, sWrong: SubIntWrong) {
+    <!INAPPLICABLE_CANDIDATE!>foo<!>(s)
+    <!INAPPLICABLE_CANDIDATE!>foo<!>(sWrong)
+
+    val a = "foo"
+    <!INAPPLICABLE_CANDIDATE!>foo<!>(a)
+
+    a as (Int, String) -> String
+    <!INAPPLICABLE_CANDIDATE!>foo<!>(a)
+}
+
+fun <T> test2(x: T) where T : (Int, String) -> Int, T : (Double) -> Int {
+    <!INAPPLICABLE_CANDIDATE!>foo<!>(x)
+}
\ No newline at end of file

compiler/testData/diagnostics/tests/unitConversion/unitConversionForSubtypes.kt

+// !LANGUAGE: +UnitConversion
+// !DIAGNOSTICS: -UNUSED_PARAMETER -UNUSED_EXPRESSION
+
+fun foo(f: (Int, String) -> Unit) {}
+
+abstract class SubInt : (Int, String) -> Int
+abstract class SubIntWrong : (String, String) -> Int
+
+fun test1(s: SubInt, sWrong: SubIntWrong) {
+    foo(s)
+    foo(<!TYPE_MISMATCH!>sWrong<!>)
+
+    val a = "foo"
+    foo(<!TYPE_MISMATCH!>a<!>)
+
+    a <!CAST_NEVER_SUCCEEDS!>as<!> (Int, String) -> String
+    foo(a)
+}
+
+fun <T> test2(x: T) where T : (Int, String) -> Int, T : (Double) -> Int {
+    foo(x)
+}
\ No newline at end of file

compiler/testData/diagnostics/tests/unitConversion/unitConversionForSubtypes.txt

+package
+
+public fun foo(/*0*/ f: (kotlin.Int, kotlin.String) -> kotlin.Unit): kotlin.Unit
+public fun test1(/*0*/ s: SubInt, /*1*/ sWrong: SubIntWrong): kotlin.Unit
+public fun </*0*/ T : (kotlin.Int, kotlin.String) -> kotlin.Int> test2(/*0*/ x: T): kotlin.Unit where T : (kotlin.Double) -> kotlin.Int
+
+public abstract class SubInt : (kotlin.Int, kotlin.String) -> kotlin.Int {
+    public constructor SubInt()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public abstract override /*1*/ /*fake_override*/ fun invoke(/*0*/ p1: kotlin.Int, /*1*/ p2: kotlin.String): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public abstract class SubIntWrong : (kotlin.String, kotlin.String) -> kotlin.Int {
+    public constructor SubIntWrong()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public abstract override /*1*/ /*fake_override*/ fun invoke(/*0*/ p1: kotlin.String, /*1*/ p2: kotlin.String): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}

compiler/tests/org/jetbrains/kotlin/checkers/DiagnosticsTestGenerated.java

@@ -24526,6 +24526,49 @@ public class DiagnosticsTestGenerated extends AbstractDiagnosticsTestWithFirVali
             }
         }
 
+        @TestMetadata("compiler/testData/diagnostics/tests/unitConversion")
+        @TestDataPath("$PROJECT_ROOT")
+        @RunWith(JUnit3RunnerWithInners.class)
+        public static class UnitConversion extends AbstractDiagnosticsTestWithFirValidation {
+            private void runTest(String testDataFilePath) throws Exception {
+                KotlinTestUtils.runTest(this::doTest, this, testDataFilePath);
+            }
+
+            public void testAllFilesPresentInUnitConversion() throws Exception {
+                KotlinTestUtils.assertAllTestsPresentByMetadataWithExcluded(this.getClass(), new File("compiler/testData/diagnostics/tests/unitConversion"), Pattern.compile("^(.*)\\.kts?$"), Pattern.compile("^(.+)\\.fir\\.kts?$"), true);
+            }
+
+            @TestMetadata("chainedFunSuspendUnitConversion.kt")
+            public void testChainedFunSuspendUnitConversion() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/unitConversion/chainedFunSuspendUnitConversion.kt");
+            }
+
+            @TestMetadata("chainedFunUnitConversion.kt")
+            public void testChainedFunUnitConversion() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/unitConversion/chainedFunUnitConversion.kt");
+            }
+
+            @TestMetadata("chainedUnitSuspendConversion.kt")
+            public void testChainedUnitSuspendConversion() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/unitConversion/chainedUnitSuspendConversion.kt");
+            }
+
+            @TestMetadata("unitConversionCompatibility.kt")
+            public void testUnitConversionCompatibility() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/unitConversion/unitConversionCompatibility.kt");
+            }
+
+            @TestMetadata("unitConversionForAllKinds.kt")
+            public void testUnitConversionForAllKinds() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/unitConversion/unitConversionForAllKinds.kt");
+            }
+
+            @TestMetadata("unitConversionForSubtypes.kt")
+            public void testUnitConversionForSubtypes() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/unitConversion/unitConversionForSubtypes.kt");
+            }
+        }
+
         @TestMetadata("compiler/testData/diagnostics/tests/varargs")
         @TestDataPath("$PROJECT_ROOT")
         @RunWith(JUnit3RunnerWithInners.class)

compiler/tests/org/jetbrains/kotlin/checkers/javac/DiagnosticsUsingJavacTestGenerated.java

@@ -24446,6 +24446,49 @@ public class DiagnosticsUsingJavacTestGenerated extends AbstractDiagnosticsUsing
             }
         }
 
+        @TestMetadata("compiler/testData/diagnostics/tests/unitConversion")
+        @TestDataPath("$PROJECT_ROOT")
+        @RunWith(JUnit3RunnerWithInners.class)
+        public static class UnitConversion extends AbstractDiagnosticsUsingJavacTest {
+            private void runTest(String testDataFilePath) throws Exception {
+                KotlinTestUtils.runTest(this::doTest, this, testDataFilePath);
+            }
+
+            public void testAllFilesPresentInUnitConversion() throws Exception {
+                KotlinTestUtils.assertAllTestsPresentByMetadataWithExcluded(this.getClass(), new File("compiler/testData/diagnostics/tests/unitConversion"), Pattern.compile("^(.+)\\.kt$"), Pattern.compile("^(.+)\\.fir\\.kts?$"), true);
+            }
+
+            @TestMetadata("chainedFunSuspendUnitConversion.kt")
+            public void testChainedFunSuspendUnitConversion() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/unitConversion/chainedFunSuspendUnitConversion.kt");
+            }
+
+            @TestMetadata("chainedFunUnitConversion.kt")
+            public void testChainedFunUnitConversion() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/unitConversion/chainedFunUnitConversion.kt");
+            }
+
+            @TestMetadata("chainedUnitSuspendConversion.kt")
+            public void testChainedUnitSuspendConversion() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/unitConversion/chainedUnitSuspendConversion.kt");
+            }
+
+            @TestMetadata("unitConversionCompatibility.kt")
+            public void testUnitConversionCompatibility() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/unitConversion/unitConversionCompatibility.kt");
+            }
+
+            @TestMetadata("unitConversionForAllKinds.kt")
+            public void testUnitConversionForAllKinds() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/unitConversion/unitConversionForAllKinds.kt");
+            }
+
+            @TestMetadata("unitConversionForSubtypes.kt")
+            public void testUnitConversionForSubtypes() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/unitConversion/unitConversionForSubtypes.kt");
+            }
+        }
+
         @TestMetadata("compiler/testData/diagnostics/tests/varargs")
         @TestDataPath("$PROJECT_ROOT")
         @RunWith(JUnit3RunnerWithInners.class)

compiler/util/src/org/jetbrains/kotlin/config/LanguageVersionSettings.kt

@@ -153,6 +153,7 @@ enum class LanguageFeature(
     FunctionReferenceWithDefaultValueAsOtherType(sinceVersion = KOTLIN_1_4),
     NonStrictOnlyInputTypesChecks(sinceVersion = KOTLIN_1_4),
     SuspendConversion(sinceVersion = KOTLIN_1_4, defaultState = State.DISABLED),
+    UnitConversion(sinceVersion = KOTLIN_1_4, defaultState = State.DISABLED),
     OverloadResolutionByLambdaReturnType(sinceVersion = KOTLIN_1_4),
 
     BooleanElvisBoundSmartCasts(sinceVersion = KOTLIN_1_3, defaultState = State.DISABLED), // see KT-26357 for details

core/descriptors/src/org/jetbrains/kotlin/builtins/functionTypes.kt

@@ -54,6 +54,9 @@ val KotlinType.isSuspendFunctionTypeOrSubtype: Boolean
 val KotlinType.isBuiltinFunctionalTypeOrSubtype: Boolean
     get() = isTypeOrSubtypeOf { it.isBuiltinFunctionalType }
 
+fun KotlinType.isFunctionTypeOrSubtype(predicate: (KotlinType) -> Boolean): Boolean =
+    isTypeOrSubtypeOf { it.isFunctionType && predicate(it) }
+
 val KotlinType.isFunctionType: Boolean
     get() = constructor.declarationDescriptor?.getFunctionalClassKind() == FunctionClassDescriptor.Kind.Function