Minor: detect 'returns()' more accurately

Before, we've assumed that if call has no expression arguments, then
it's a 'returns()' call.

Now we check that we've resolved to overload of 'returns' from
ContractsDsl with zero parameters.

compiler/frontend/src/org/jetbrains/kotlin/contracts/parsing/PsiContractsUtils.kt

@@ -76,6 +76,10 @@ fun DeclarationDescriptor.isReturnsEffectDescriptor(): Boolean = equalsDslDescri
 
 fun DeclarationDescriptor.isReturnsNotNullDescriptor(): Boolean = equalsDslDescriptor(RETURNS_NOT_NULL)
 
+fun DeclarationDescriptor.isReturnsWildcardDescriptor(): Boolean = equalsDslDescriptor(RETURNS) &&
+        this is FunctionDescriptor &&
+        valueParameters.isEmpty()
+
 fun DeclarationDescriptor.isEffectDescriptor(): Boolean = equalsDslDescriptor(EFFECT)
 
 fun DeclarationDescriptor.isCallsInPlaceEffectDescriptor(): Boolean = equalsDslDescriptor(CALLS_IN_PLACE)

compiler/frontend/src/org/jetbrains/kotlin/contracts/parsing/effects/PsiReturnsEffectParser.kt

@@ -20,9 +20,7 @@ import org.jetbrains.kotlin.contracts.description.EffectDeclaration
 import org.jetbrains.kotlin.contracts.description.ReturnsEffectDeclaration
 import org.jetbrains.kotlin.contracts.description.expressions.ConstantReference
 import org.jetbrains.kotlin.contracts.parsing.*
-import org.jetbrains.kotlin.diagnostics.Errors
 import org.jetbrains.kotlin.psi.KtExpression
-import org.jetbrains.kotlin.resolve.BindingTrace
 import org.jetbrains.kotlin.resolve.calls.callUtil.getResolvedCall
 
 internal class PsiReturnsEffectParser(
@@ -34,25 +32,20 @@ internal class PsiReturnsEffectParser(
         val resolvedCall = expression.getResolvedCall(callContext.bindingContext) ?: return null
         val descriptor = resolvedCall.resultingDescriptor
 
-        if (descriptor.isReturnsNotNullDescriptor())
-            return ReturnsEffectDeclaration(ConstantReference.NOT_NULL)
+        if (descriptor.isReturnsNotNullDescriptor()) return ReturnsEffectDeclaration(ConstantReference.NOT_NULL)
+        if (descriptor.isReturnsWildcardDescriptor()) return ReturnsEffectDeclaration(ConstantReference.WILDCARD)
 
         if (!descriptor.isReturnsEffectDescriptor()) return null
 
         val argumentExpression = resolvedCall.firstArgumentAsExpressionOrNull()
-        val constantValue = if (argumentExpression == null) {
-            ConstantReference.WILDCARD
-        } else {
-            // Note that we distinguish absence of an argument and unparsed argument
-            val constant = contractParserDispatcher.parseConstant(argumentExpression)
-            if (constant == null) {
-                collector.badDescription(
-                    "only true/false/null constants in Returns-effect are currently supported",
-                    argumentExpression
-                )
-                return null
-            }
-            constant
+        val constantValue = if (argumentExpression != null) contractParserDispatcher.parseConstant(argumentExpression) else null
+
+        if (constantValue == null) {
+            collector.badDescription(
+                "only true/false/null constants in Returns-effect are currently supported",
+                argumentExpression ?: expression
+            )
+            return null
         }
 
         return ReturnsEffectDeclaration(constantValue)