FIR: discriminate candidates with suspend conversion

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

@@ -240,7 +240,6 @@ fun Candidate.resolvePlainArgumentType(
 
     // If the argument is of functional type and the expected type is a suspend function type, we need to do "suspend conversion."
     // TODO: should refer to LanguageVersionSettings.SuspendConversion
-    // TODO: should prefer another candidate without suspend conversion when ambiguous
     if (expectedType?.isSuspendFunctionType(session) == true &&
         argumentTypeForApplicabilityCheck.isBuiltinFunctionalType(session) &&
         !argumentTypeForApplicabilityCheck.isSuspendFunctionType(session)
@@ -253,6 +252,7 @@ fun Candidate.resolvePlainArgumentType(
                 isKFunctionType = argumentTypeForApplicabilityCheck.isKFunctionType(session)
             )
         substitutor.substituteOrSelf(argumentTypeForApplicabilityCheck)
+        usesSuspendConversion = true
     }
 
     checkApplicabilityForArgumentType(

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

@@ -97,6 +97,7 @@ class Candidate(
     var resultingTypeForCallableReference: ConeKotlinType? = null
     var outerConstraintBuilderEffect: (ConstraintSystemOperation.() -> Unit)? = null
     var usesSAM: Boolean = false
+    var usesSuspendConversion: Boolean = false
 
     var argumentMapping: Map<FirExpression, FirValueParameter>? = null
     lateinit var typeArgumentMapping: TypeArgumentMapping

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

@@ -42,7 +42,7 @@ class ConeOverloadConflictResolver(
                 candidates
 
         return chooseMaximallySpecificCandidates(
-            fixedCandidates, discriminateGenerics, discriminateAbstracts, discriminateSAMs = true
+            fixedCandidates, discriminateGenerics, discriminateAbstracts, discriminateSAMs = true, discriminateSuspendConversions = true
         )
     }
 
@@ -63,7 +63,8 @@ class ConeOverloadConflictResolver(
         candidates: Set<Candidate>,
         discriminateGenerics: Boolean,
         discriminateAbstracts: Boolean,
-        discriminateSAMs: Boolean
+        discriminateSAMs: Boolean,
+        discriminateSuspendConversions: Boolean,
     ): Set<Candidate> {
         findMaximallySpecificCall(candidates, false)?.let { return setOf(it) }
 
@@ -78,7 +79,23 @@ class ConeOverloadConflictResolver(
                 0, candidates.size -> {
                 }
                 else -> return chooseMaximallySpecificCandidates(
-                    filtered, discriminateGenerics, discriminateAbstracts, discriminateSAMs = false
+                    filtered, discriminateGenerics, discriminateAbstracts, discriminateSAMs = false, discriminateSuspendConversions
+                )
+            }
+        }
+
+        if (discriminateSuspendConversions) {
+            val filtered = candidates.filterTo(mutableSetOf()) { !it.usesSuspendConversion }
+            when (filtered.size) {
+                1 -> return filtered
+                0, candidates.size -> {
+                }
+                else -> return chooseMaximallySpecificCandidates(
+                    filtered,
+                    discriminateGenerics,
+                    discriminateAbstracts,
+                    discriminateSAMs = false,
+                    discriminateSuspendConversions = false
                 )
             }
         }
@@ -90,7 +107,11 @@ class ConeOverloadConflictResolver(
                 0, candidates.size -> {
                 }
                 else -> return chooseMaximallySpecificCandidates(
-                    filtered, discriminateGenerics, discriminateAbstracts = false, discriminateSAMs = false
+                    filtered,
+                    discriminateGenerics,
+                    discriminateAbstracts = false,
+                    discriminateSAMs = false,
+                    discriminateSuspendConversions = false
                 )
             }
         }

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

@@ -226,7 +226,7 @@ internal object CheckCallableReferenceExpectedType : CheckerStage() {
             is FirVariable<*> -> createKPropertyType(fir, resultingReceiverType, returnTypeRef)
             else -> ConeKotlinErrorType(ConeSimpleDiagnostic("Unknown callable kind: ${fir::class}", DiagnosticKind.UnknownCallableKind))
         }.let(candidate.substitutor::substituteOrSelf)
-
+        candidate.usesSuspendConversion = requireSuspendConversion
         candidate.resultingTypeForCallableReference = resultingType
         candidate.outerConstraintBuilderEffect = fun ConstraintSystemOperation.() {
             addOtherSystem(candidate.system.asReadOnlyStorage())

compiler/testData/diagnostics/tests/suspendConversion/overloadResolutionBySuspendModifier.fir.kt

@@ -14,6 +14,6 @@ fun test1() {
 
 // candidate without suspend conversions is more specific
 fun test2(f: () -> Int, g: suspend () -> Int) {
-    <!AMBIGUITY!>foo<!>(f)
+    foo(f)
     foo(g)
 }

compiler/testData/diagnostics/testsWithStdLib/coroutines/suspendFunctionType/functionVsSuspendFunction.fir.kt

@@ -15,5 +15,5 @@ fun test4(): suspend () -> Unit = useSuspendFn {}
 fun test5() = useSuspendFn {}
 
 fun test5(sfn: suspend () -> Unit) = ambiguous(sfn)
-fun test6(fn: () -> Unit) = <!AMBIGUITY!>ambiguous<!>(fn)
+fun test6(fn: () -> Unit) = ambiguous(fn)
 fun test7(): () -> Unit = <!AMBIGUITY!>ambiguous<!> {}