Add language feature for suspend functions in fun interfaces

Since in 1.5 JVM_IR is enabled by default and supports suspend functions
in fun interfaces, it is safe to enable the feature by default as well.

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

@@ -10054,6 +10054,28 @@ public class FirOldFrontendDiagnosticsTestGenerated extends AbstractFirDiagnosti
                     runTest("compiler/testData/diagnostics/tests/functionLiterals/return/unresolvedReferenceInReturnBlock.kt");
                 }
             }
+
+            @Nested
+            @TestMetadata("compiler/testData/diagnostics/tests/functionLiterals/suspend")
+            @TestDataPath("$PROJECT_ROOT")
+            public class Suspend extends AbstractFirDiagnosticTest {
+                @Test
+                public void testAllFilesPresentInSuspend() throws Exception {
+                    KtTestUtil.assertAllTestsPresentByMetadataWithExcluded(this.getClass(), new File("compiler/testData/diagnostics/tests/functionLiterals/suspend"), Pattern.compile("^(.+)\\.kt$"), Pattern.compile("^(.+)\\.fir\\.kts?$"), true);
+                }
+
+                @Test
+                @TestMetadata("disabled.kt")
+                public void testDisabled() throws Exception {
+                    runTest("compiler/testData/diagnostics/tests/functionLiterals/suspend/disabled.kt");
+                }
+
+                @Test
+                @TestMetadata("enabled.kt")
+                public void testEnabled() throws Exception {
+                    runTest("compiler/testData/diagnostics/tests/functionLiterals/suspend/enabled.kt");
+                }
+            }
         }
 
         @Nested

compiler/frontend/src/org/jetbrains/kotlin/resolve/checkers/FunInterfaceDeclarationChecker.kt

@@ -6,6 +6,7 @@
 package org.jetbrains.kotlin.resolve.checkers
 
 import com.intellij.psi.PsiElement
+import org.jetbrains.kotlin.config.LanguageFeature
 import org.jetbrains.kotlin.descriptors.ClassDescriptor
 import org.jetbrains.kotlin.descriptors.DeclarationDescriptor
 import org.jetbrains.kotlin.descriptors.FunctionDescriptor
@@ -60,7 +61,10 @@ class FunInterfaceDeclarationChecker : DeclarationChecker {
     ) {
         val ktFunction = abstractMember.source.getPsi() as? KtNamedFunction
 
-        if (abstractMember.isSuspend) {
+        if (abstractMember.isSuspend &&
+            !(context.languageVersionSettings.supportsFeature(LanguageFeature.SuspendFunctionsInFunInterfaces) &&
+                    context.languageVersionSettings.supportsFeature(LanguageFeature.JvmIrEnabledByDefault))
+        ) {
             val reportOn = ktFunction?.modifierList?.getModifier(KtTokens.SUSPEND_KEYWORD) ?: funInterfaceKeyword
             context.trace.report(Errors.FUN_INTERFACE_WITH_SUSPEND_FUNCTION.on(reportOn))
             return

compiler/testData/diagnostics/tests/functionLiterals/suspend/disabled.fir.kt

+// LANGUAGE: -SuspendFunctionsInFunInterfaces, +JvmIrEnabledByDefault
+
+fun interface I {
+    suspend fun foo()
+}
\ No newline at end of file

compiler/testData/diagnostics/tests/functionLiterals/suspend/disabled.kt

+// LANGUAGE: -SuspendFunctionsInFunInterfaces, +JvmIrEnabledByDefault
+
+fun interface I {
+    <!FUN_INTERFACE_WITH_SUSPEND_FUNCTION!>suspend<!> fun foo()
+}
\ No newline at end of file

compiler/testData/diagnostics/tests/functionLiterals/suspend/disabled.txt

+package
+
+public fun interface I {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract suspend fun foo(): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}

compiler/testData/diagnostics/tests/functionLiterals/suspend/enabled.kt

+// FIR_IDENTICAL
+// LANGUAGE: +SuspendFunctionsInFunInterfaces, +JvmIrEnabledByDefault
+
+fun interface I {
+    suspend fun foo()
+}
\ No newline at end of file

compiler/testData/diagnostics/tests/functionLiterals/suspend/enabled.txt

+package
+
+public fun interface I {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract suspend fun foo(): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}

compiler/tests-common-new/tests-gen/org/jetbrains/kotlin/test/runners/DiagnosticTestGenerated.java

@@ -10060,6 +10060,28 @@ public class DiagnosticTestGenerated extends AbstractDiagnosticTest {
                     runTest("compiler/testData/diagnostics/tests/functionLiterals/return/unresolvedReferenceInReturnBlock.kt");
                 }
             }
+
+            @Nested
+            @TestMetadata("compiler/testData/diagnostics/tests/functionLiterals/suspend")
+            @TestDataPath("$PROJECT_ROOT")
+            public class Suspend extends AbstractDiagnosticTest {
+                @Test
+                public void testAllFilesPresentInSuspend() throws Exception {
+                    KtTestUtil.assertAllTestsPresentByMetadataWithExcluded(this.getClass(), new File("compiler/testData/diagnostics/tests/functionLiterals/suspend"), Pattern.compile("^(.*)\\.kts?$"), Pattern.compile("^(.+)\\.fir\\.kts?$"), true);
+                }
+
+                @Test
+                @TestMetadata("disabled.kt")
+                public void testDisabled() throws Exception {
+                    runTest("compiler/testData/diagnostics/tests/functionLiterals/suspend/disabled.kt");
+                }
+
+                @Test
+                @TestMetadata("enabled.kt")
+                public void testEnabled() throws Exception {
+                    runTest("compiler/testData/diagnostics/tests/functionLiterals/suspend/enabled.kt");
+                }
+            }
         }
 
         @Nested

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

@@ -198,6 +198,7 @@ enum class LanguageFeature(
 
     InlineClasses(sinceVersion = KOTLIN_1_3, defaultState = State.ENABLED_WITH_WARNING, kind = UNSTABLE_FEATURE),
     JvmInlineValueClasses(sinceVersion = KOTLIN_1_5, defaultState = State.ENABLED, kind = OTHER),
+    SuspendFunctionsInFunInterfaces(sinceVersion = KOTLIN_1_5, defaultState = State.ENABLED, kind = OTHER),
     ;
 
     val presentableName: String