'async' reserved in front of function literals

compiler/frontend/src/org/jetbrains/kotlin/types/expressions/FunctionsTypingVisitor.kt

@@ -17,6 +17,7 @@
 package org.jetbrains.kotlin.types.expressions
 
 import com.google.common.collect.Lists
+import com.intellij.psi.PsiElement
 import org.jetbrains.kotlin.builtins.KotlinBuiltIns
 import org.jetbrains.kotlin.descriptors.CallableMemberDescriptor
 import org.jetbrains.kotlin.descriptors.SimpleFunctionDescriptor
@@ -27,6 +28,7 @@ import org.jetbrains.kotlin.diagnostics.DiagnosticUtils
 import org.jetbrains.kotlin.diagnostics.Errors.*
 import org.jetbrains.kotlin.lexer.KtTokens
 import org.jetbrains.kotlin.psi.*
+import org.jetbrains.kotlin.psi.psiUtil.checkReservedPrefixWord
 import org.jetbrains.kotlin.psi.psiUtil.getAnnotationEntries
 import org.jetbrains.kotlin.resolve.*
 import org.jetbrains.kotlin.resolve.BindingContext.EXPECTED_RETURN_TYPE
@@ -53,6 +55,7 @@ internal class FunctionsTypingVisitor(facade: ExpressionTypingInternals) : Expre
             isStatement: Boolean,
             statementScope: LexicalWritableScope? // must be not null if isStatement
     ): KotlinTypeInfo {
+        checkReservedAsync(context, function)
         if (!isStatement) {
             // function expression
             if (!function.getTypeParameters().isEmpty()) {
@@ -131,6 +134,8 @@ internal class FunctionsTypingVisitor(facade: ExpressionTypingInternals) : Expre
     }
 
     override fun visitLambdaExpression(expression: KtLambdaExpression, context: ExpressionTypingContext): KotlinTypeInfo? {
+        checkReservedAsync(context, expression)
+
         if (!expression.getFunctionLiteral().hasBody()) return null
 
         val expectedType = context.expectedType
@@ -153,6 +158,10 @@ internal class FunctionsTypingVisitor(facade: ExpressionTypingInternals) : Expre
         return components.dataFlowAnalyzer.createCheckedTypeInfo(resultType, context, expression)
     }
 
+    private fun checkReservedAsync(context: ExpressionTypingContext, expression: PsiElement) {
+        checkReservedPrefixWord(context.trace, expression, "async", "async block/lambda. Use 'async() { ... }' or 'async(fun...)'")
+    }
+
     private fun createFunctionLiteralDescriptor(
             expression: KtLambdaExpression,
             context: ExpressionTypingContext