diff --git a/compiler/fir/analysis-tests/testData/resolve/samConstructors/genericSam.txt b/compiler/fir/analysis-tests/testData/resolve/samConstructors/genericSam.txt
index 6d3cc04eef8..303583bb3ba 100644
--- a/compiler/fir/analysis-tests/testData/resolve/samConstructors/genericSam.txt
+++ b/compiler/fir/analysis-tests/testData/resolve/samConstructors/genericSam.txt
@@ -1,6 +1,6 @@
 FILE: main.kt
     public final fun main(): R|kotlin/Unit| {
-        R|/MyFunction|<R|kotlin/Int|, R|kotlin/String|>(<L> = MyFunction@fun <anonymous>(x: R|kotlin/Int|): R|kotlin/String| {
+        R|/MyFunction|<R|kotlin/Int|, R|kotlin/String|>(<L> = MyFunction@fun <anonymous>(x: R|ft<kotlin/Int, kotlin/Int?>!|): R|kotlin/String| {
             ^ R|<local>/x|.R|kotlin/Int.toInt|().R|kotlin/Any.toString|()
         }
         )
@@ -8,7 +8,7 @@ FILE: main.kt
             ^ R|<local>/x|.R|kotlin/Any.toString|()
         }
         )
-        R|/MyFunction|<R|kotlin/Any?|, R|ft<kotlin/String, kotlin/String?>!|>(<L> = MyFunction@fun <anonymous>(x: R|kotlin/Any?|): R|kotlin/String| {
+        R|/MyFunction|<R|kotlin/Any?|, R|ft<kotlin/String, kotlin/String?>!|>(<L> = MyFunction@fun <anonymous>(x: R|ft<kotlin/Any?, kotlin/Any?>?|): R|kotlin/String| {
             ^ String()
         }
         )
diff --git a/compiler/fir/analysis-tests/testData/resolve/samConstructors/genericSamInferenceFromExpectType.txt b/compiler/fir/analysis-tests/testData/resolve/samConstructors/genericSamInferenceFromExpectType.txt
index e468bdcc691..edea02d289b 100644
--- a/compiler/fir/analysis-tests/testData/resolve/samConstructors/genericSamInferenceFromExpectType.txt
+++ b/compiler/fir/analysis-tests/testData/resolve/samConstructors/genericSamInferenceFromExpectType.txt
@@ -6,11 +6,11 @@ FILE: main.kt
     public final fun <X, Y> foo3(f: R|MyFunction<X, Y>|, x: R|X|): R|kotlin/Unit| {
     }
     public final fun main(): R|kotlin/Unit| {
-        R|/foo1|(R|/MyFunction|<R|kotlin/Int|, R|kotlin/String|>(<L> = MyFunction@fun <anonymous>(x: R|kotlin/Int|): R|kotlin/String| {
+        R|/foo1|(R|/MyFunction|<R|kotlin/Int|, R|kotlin/String|>(<L> = MyFunction@fun <anonymous>(x: R|ft<kotlin/Int, kotlin/Int?>!|): R|kotlin/String| {
             ^ R|<local>/x|.R|kotlin/Int.toInt|().R|kotlin/Any.toString|()
         }
         ))
-        R|/foo2|(R|/MyFunction|<R|kotlin/Number|, R|ft<kotlin/String, kotlin/String?>!|>(<L> = MyFunction@fun <anonymous>(x: R|kotlin/Number|): R|kotlin/String| {
+        R|/foo2|(R|/MyFunction|<R|kotlin/Number|, R|ft<kotlin/String, kotlin/String?>!|>(<L> = MyFunction@fun <anonymous>(x: R|ft<kotlin/Number, kotlin/Number?>!|): R|kotlin/String| {
             ^ R|<local>/x|.R|kotlin/Number.toInt|().R|kotlin/Any.toString|()
         }
         ))
@@ -18,7 +18,7 @@ FILE: main.kt
             ^ R|<local>/x|.R|kotlin/Any.toString|()
         }
         ))
-        R|/foo3|<R|kotlin/Int|, R|ft<kotlin/String, kotlin/String?>!|>(R|/MyFunction|<R|kotlin/Int|, R|ft<kotlin/String, kotlin/String?>!|>(<L> = MyFunction@fun <anonymous>(x: R|kotlin/Int|): R|kotlin/String| {
+        R|/foo3|<R|kotlin/Int|, R|ft<kotlin/String, kotlin/String?>!|>(R|/MyFunction|<R|kotlin/Int|, R|ft<kotlin/String, kotlin/String?>!|>(<L> = MyFunction@fun <anonymous>(x: R|ft<kotlin/Int, kotlin/Int?>!|): R|kotlin/String| {
             ^ R|<local>/x|.R|kotlin/Int.plus|(Int(1)).R|kotlin/Any.toString|()
         }
         ), Int(1))
diff --git a/compiler/fir/analysis-tests/testData/resolve/samConversions/genericSam.txt b/compiler/fir/analysis-tests/testData/resolve/samConversions/genericSam.txt
index 90579edbf31..39caae7fff8 100644
--- a/compiler/fir/analysis-tests/testData/resolve/samConversions/genericSam.txt
+++ b/compiler/fir/analysis-tests/testData/resolve/samConversions/genericSam.txt
@@ -12,7 +12,7 @@ FILE: main.kt
             ^ R|<local>/x|.R|kotlin/Any.toString|()
         }
         )
-        Q|JavaUsage|.R|/JavaUsage.foo3|<R|kotlin/Int|, R|ft<kotlin/String, kotlin/String?>!|>(foo3@fun <anonymous>(x: R|kotlin/Int|): R|kotlin/String| {
+        Q|JavaUsage|.R|/JavaUsage.foo3|<R|kotlin/Int|, R|ft<kotlin/String, kotlin/String?>!|>(foo3@fun <anonymous>(x: R|ft<kotlin/Int, kotlin/Int?>!|): R|kotlin/String| {
             ^ R|<local>/x|.R|kotlin/Int.plus|(Int(1)).R|kotlin/Any.toString|()
         }
         , Int(1))
diff --git a/compiler/fir/analysis-tests/testData/resolveWithStdlib/inference/flexibleTypeInSystem.kt b/compiler/fir/analysis-tests/testData/resolveWithStdlib/inference/flexibleTypeInSystem.kt
new file mode 100644
index 00000000000..248d78fa271
--- /dev/null
+++ b/compiler/fir/analysis-tests/testData/resolveWithStdlib/inference/flexibleTypeInSystem.kt
@@ -0,0 +1,17 @@
+// ISSUE: KT-37623
+// FULL_JDK
+// FILE: JavaEnum.java
+
+public enum JavaEnum {
+    A;
+}
+
+// FILE: main.kt
+
+import java.util.EnumMap
+
+val enumMap = EnumMap(
+    mapOf(
+        JavaEnum.A to "A"
+    )
+)
\ No newline at end of file
diff --git a/compiler/fir/analysis-tests/testData/resolveWithStdlib/inference/flexibleTypeInSystem.txt b/compiler/fir/analysis-tests/testData/resolveWithStdlib/inference/flexibleTypeInSystem.txt
new file mode 100644
index 00000000000..2e77b232377
--- /dev/null
+++ b/compiler/fir/analysis-tests/testData/resolveWithStdlib/inference/flexibleTypeInSystem.txt
@@ -0,0 +1,3 @@
+FILE: main.kt
+    public final val enumMap: R|java/util/EnumMap<ft<JavaEnum, JavaEnum?>!, ft<kotlin/String, kotlin/String?>!>| = R|java/util/EnumMap.EnumMap|<R|ft<JavaEnum, JavaEnum?>!|, R|ft<kotlin/String, kotlin/String?>!|>(R|kotlin/collections/mapOf|<R|ft<JavaEnum, JavaEnum?>!|, R|kotlin/String|>(Q|JavaEnum|.R|/JavaEnum.A|.R|kotlin/to|<R|JavaEnum|, R|kotlin/String|>(String(A))))
+        public get(): R|java/util/EnumMap<ft<JavaEnum, JavaEnum?>!, ft<kotlin/String, kotlin/String?>!>|
diff --git a/compiler/fir/analysis-tests/testData/resolveWithStdlib/j+k/FunctionTypeInJava.txt b/compiler/fir/analysis-tests/testData/resolveWithStdlib/j+k/FunctionTypeInJava.txt
index 87fa9c4604a..b0e5c16b064 100644
--- a/compiler/fir/analysis-tests/testData/resolveWithStdlib/j+k/FunctionTypeInJava.txt
+++ b/compiler/fir/analysis-tests/testData/resolveWithStdlib/j+k/FunctionTypeInJava.txt
@@ -17,7 +17,7 @@ FILE: main.kt
         }
 
         Q|JavaClass|.R|/JavaClass.foo2|(R|<local>/y|)
-        Q|JavaClass|.R|/JavaClass.foo3|<R|kotlin/Int|>(foo3@fun <anonymous>(it: R|kotlin/Int|): R|ft<kotlin/String, kotlin/String?>!| {
+        Q|JavaClass|.R|/JavaClass.foo3|<R|kotlin/Int|>(foo3@fun <anonymous>(it: R|ft<kotlin/Int, kotlin/Int?>!|): R|ft<kotlin/String, kotlin/String?>!| {
             ^ R|<local>/it|.R|kotlin/Int.plus|(Int(4)).R|kotlin/Any.toString|()
         }
         , Int(5))
diff --git a/compiler/fir/analysis-tests/testData/resolveWithStdlib/javaLangComparator.txt b/compiler/fir/analysis-tests/testData/resolveWithStdlib/javaLangComparator.txt
index ac8bda89f68..71c47b33f06 100644
--- a/compiler/fir/analysis-tests/testData/resolveWithStdlib/javaLangComparator.txt
+++ b/compiler/fir/analysis-tests/testData/resolveWithStdlib/javaLangComparator.txt
@@ -1,6 +1,6 @@
 FILE: javaLangComparator.kt
     public final fun test_2(list: R|kotlin/collections/List<kotlin/Int>|): R|kotlin/Unit| {
-        lval comp: R|java/util/Comparator<kotlin/Int>| = Q|java/util|.R|java/util/Comparator|<R|kotlin/Int|>(<L> = Comparator@fun <anonymous>(x: R|kotlin/Int|, y: R|kotlin/Int|): R|kotlin/Int| {
+        lval comp: R|java/util/Comparator<kotlin/Int>| = Q|java/util|.R|java/util/Comparator|<R|kotlin/Int|>(<L> = Comparator@fun <anonymous>(x: R|ft<kotlin/Int, kotlin/Int?>!|, y: R|ft<kotlin/Int, kotlin/Int?>!|): R|kotlin/Int| {
             ^ Int(1)
         }
         )
diff --git a/compiler/fir/analysis-tests/testData/resolveWithStdlib/kotlinComparatorAlias.txt b/compiler/fir/analysis-tests/testData/resolveWithStdlib/kotlinComparatorAlias.txt
index 359c24ba395..6fd1c9eda35 100644
--- a/compiler/fir/analysis-tests/testData/resolveWithStdlib/kotlinComparatorAlias.txt
+++ b/compiler/fir/analysis-tests/testData/resolveWithStdlib/kotlinComparatorAlias.txt
@@ -1,6 +1,6 @@
 FILE: kotlinComparatorAlias.kt
     public final fun test_1(): R|kotlin/Unit| {
-        lval comp: R|java/util/Comparator<kotlin/Int>| = R|java/util/Comparator|<R|kotlin/Int|>(<L> = Comparator@fun <anonymous>(x: R|kotlin/Int|, y: R|kotlin/Int|): R|kotlin/Int| {
+        lval comp: R|java/util/Comparator<kotlin/Int>| = R|java/util/Comparator|<R|kotlin/Int|>(<L> = Comparator@fun <anonymous>(x: R|ft<kotlin/Int, kotlin/Int?>!|, y: R|ft<kotlin/Int, kotlin/Int?>!|): R|kotlin/Int| {
             ^ Int(1)
         }
         )
diff --git a/compiler/fir/analysis-tests/tests/org/jetbrains/kotlin/fir/FirDiagnosticsWithStdlibTestGenerated.java b/compiler/fir/analysis-tests/tests/org/jetbrains/kotlin/fir/FirDiagnosticsWithStdlibTestGenerated.java
index 80f6bab254b..186c2897041 100644
--- a/compiler/fir/analysis-tests/tests/org/jetbrains/kotlin/fir/FirDiagnosticsWithStdlibTestGenerated.java
+++ b/compiler/fir/analysis-tests/tests/org/jetbrains/kotlin/fir/FirDiagnosticsWithStdlibTestGenerated.java
@@ -556,6 +556,11 @@ public class FirDiagnosticsWithStdlibTestGenerated extends AbstractFirDiagnostic
         public void testComplexConstraintSystem() throws Exception {
             runTest("compiler/fir/analysis-tests/testData/resolveWithStdlib/inference/complexConstraintSystem.kt");
         }
+
+        @TestMetadata("flexibleTypeInSystem.kt")
+        public void testFlexibleTypeInSystem() throws Exception {
+            runTest("compiler/fir/analysis-tests/testData/resolveWithStdlib/inference/flexibleTypeInSystem.kt");
+        }
     }
 
     @TestMetadata("compiler/fir/analysis-tests/testData/resolveWithStdlib/j+k")
diff --git a/compiler/fir/cones/src/org/jetbrains/kotlin/fir/types/ConeTypes.kt b/compiler/fir/cones/src/org/jetbrains/kotlin/fir/types/ConeTypes.kt
index 91c24e9bb6b..f92a94e90e1 100644
--- a/compiler/fir/cones/src/org/jetbrains/kotlin/fir/types/ConeTypes.kt
+++ b/compiler/fir/cones/src/org/jetbrains/kotlin/fir/types/ConeTypes.kt
@@ -107,7 +107,7 @@ open class ConeFlexibleType(val lowerBound: ConeKotlinType, val upperBound: Cone
     }
 
     override val typeArguments: Array<out ConeTypeProjection>
-        get() = emptyArray()
+        get() = lowerBound.typeArguments
 
     override val nullability: ConeNullability
         get() = lowerBound.nullability.takeIf { it == upperBound.nullability } ?: ConeNullability.UNKNOWN
diff --git a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/types/ConeInferenceContext.kt b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/types/ConeInferenceContext.kt
index 1dfad011c54..b15d4fa777e 100644
--- a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/types/ConeInferenceContext.kt
+++ b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/types/ConeInferenceContext.kt
@@ -288,6 +288,7 @@ interface ConeInferenceContext : TypeSystemInferenceExtensionContext, ConeTypeCo
         return object : AbstractConeSubstitutor(),
             TypeSubstitutorMarker {
             override fun substituteType(type: ConeKotlinType): ConeKotlinType? {
+                if (type !is ConeLookupTagBasedType) return null
                 val new = map[type.typeConstructor()] ?: return null
                 return (new as ConeKotlinType).approximateIntegerLiteralType().updateNullabilityIfNeeded(type)
             }
diff --git a/compiler/testData/diagnostics/testsWithStdLib/typealias/typeAliasSamAdapterConstructors.fir.kt b/compiler/testData/diagnostics/testsWithStdLib/typealias/typeAliasSamAdapterConstructors.fir.kt
index 64b9eb0da5b..42d31a64950 100644
--- a/compiler/testData/diagnostics/testsWithStdLib/typealias/typeAliasSamAdapterConstructors.fir.kt
+++ b/compiler/testData/diagnostics/testsWithStdLib/typealias/typeAliasSamAdapterConstructors.fir.kt
@@ -5,4 +5,4 @@ typealias ComparatorStrT = ComparatorT<String>
 
 val test1 = RunnableT { }
 val test2 = ComparatorT<String> { s1, s2 -> s1.compareTo(s2) }
-val test3 = ComparatorStrT { s1, s2 -> s1.<!UNRESOLVED_REFERENCE!>compareTo<!>(s2) }
+val test3 = ComparatorStrT { s1, s2 -> s1.<!INAPPLICABLE_CANDIDATE!>compareTo<!>(s2) }
diff --git a/compiler/testData/diagnostics/testsWithStdLib/typealias/typeAliasSamAdapterConstructors2.fir.kt b/compiler/testData/diagnostics/testsWithStdLib/typealias/typeAliasSamAdapterConstructors2.fir.kt
index b353439a94d..63c2639f042 100644
--- a/compiler/testData/diagnostics/testsWithStdLib/typealias/typeAliasSamAdapterConstructors2.fir.kt
+++ b/compiler/testData/diagnostics/testsWithStdLib/typealias/typeAliasSamAdapterConstructors2.fir.kt
@@ -23,5 +23,5 @@ typealias C2<T> = JHost.Consumer2<T, T>
 val test1 = R { }
 val test2 = C<String> { s -> println(s.length) }
 val test3 = CStr { s -> <!AMBIGUITY!>println<!>(s.<!UNRESOLVED_REFERENCE!>length<!>) }
-val test4 = CStrList { ss -> <!UNRESOLVED_REFERENCE, UNRESOLVED_REFERENCE, UNRESOLVED_REFERENCE!>for (s in ss) { <!AMBIGUITY!>println<!>(s.<!UNRESOLVED_REFERENCE!>length<!>) }<!> }
+val test4 = CStrList { ss -> <!INAPPLICABLE_CANDIDATE, UNRESOLVED_REFERENCE, UNRESOLVED_REFERENCE!>for (s in ss) { <!AMBIGUITY!>println<!>(s.<!UNRESOLVED_REFERENCE!>length<!>) }<!> }
 val test5 = <!INAPPLICABLE_CANDIDATE!>C2<!><Int> { a, b -> val x: Int = a <!AMBIGUITY!>+<!> b; println(x)}
diff --git a/compiler/testData/ir/irText/expressions/sam/samConstructors.fir.txt b/compiler/testData/ir/irText/expressions/sam/samConstructors.fir.txt
index 642f9c5110a..5fa6c8181bf 100644
--- a/compiler/testData/ir/irText/expressions/sam/samConstructors.fir.txt
+++ b/compiler/testData/ir/irText/expressions/sam/samConstructors.fir.txt
@@ -25,12 +25,12 @@ FILE fqName:<root> fileName:/samConstructors.kt
       RETURN type=kotlin.Nothing from='public final fun test4 (): java.util.Comparator<kotlin.Int> declared in <root>'
         CALL 'public final fun Comparator <T> (block: kotlin.Function2<T of java.util.Comparator?, T of java.util.Comparator?, kotlin.Int>): java.util.Comparator<T of java.util.Comparator> declared in java.util' type=java.util.Comparator<kotlin.Int> origin=null
           <T>: kotlin.Int
-          block: FUN_EXPR type=kotlin.Function2<kotlin.Int, kotlin.Int, kotlin.Int> origin=LAMBDA
-            FUN LOCAL_FUNCTION_FOR_LAMBDA name:<anonymous> visibility:local modality:FINAL <> (a:kotlin.Int, b:kotlin.Int) returnType:kotlin.Int
-              VALUE_PARAMETER name:a index:0 type:kotlin.Int
-              VALUE_PARAMETER name:b index:1 type:kotlin.Int
+          block: FUN_EXPR type=kotlin.Function2<kotlin.Int?, kotlin.Int?, kotlin.Int> origin=LAMBDA
+            FUN LOCAL_FUNCTION_FOR_LAMBDA name:<anonymous> visibility:local modality:FINAL <> (a:kotlin.Int?, b:kotlin.Int?) returnType:kotlin.Int
+              VALUE_PARAMETER name:a index:0 type:kotlin.Int?
+              VALUE_PARAMETER name:b index:1 type:kotlin.Int?
               BLOCK_BODY
-                RETURN type=kotlin.Nothing from='local final fun <anonymous> (a: kotlin.Int, b: kotlin.Int): kotlin.Int declared in <root>.test4'
+                RETURN type=kotlin.Nothing from='local final fun <anonymous> (a: kotlin.Int?, b: kotlin.Int?): kotlin.Int declared in <root>.test4'
                   CALL 'public final fun minus (other: kotlin.Int): kotlin.Int [operator] declared in kotlin.Int' type=kotlin.Int origin=MINUS
-                    $this: GET_VAR 'a: kotlin.Int declared in <root>.test4.<anonymous>' type=kotlin.Int origin=null
-                    other: GET_VAR 'b: kotlin.Int declared in <root>.test4.<anonymous>' type=kotlin.Int origin=null
+                    $this: GET_VAR 'a: kotlin.Int? declared in <root>.test4.<anonymous>' type=kotlin.Int? origin=null
+                    other: GET_VAR 'b: kotlin.Int? declared in <root>.test4.<anonymous>' type=kotlin.Int? origin=null
diff --git a/compiler/testData/ir/irText/expressions/sam/samConversionToGeneric.fir.txt b/compiler/testData/ir/irText/expressions/sam/samConversionToGeneric.fir.txt
index 38d505d14cb..aa03b1c864d 100644
--- a/compiler/testData/ir/irText/expressions/sam/samConversionToGeneric.fir.txt
+++ b/compiler/testData/ir/irText/expressions/sam/samConversionToGeneric.fir.txt
@@ -4,12 +4,12 @@ FILE fqName:<root> fileName:/samConversionToGeneric.kt
       RETURN type=kotlin.Nothing from='public final fun test1 (): <root>.J<kotlin.String> declared in <root>'
         CALL 'public final fun J <T> (block: kotlin.Function1<T of <root>.J?, T of <root>.J?>): <root>.J<T of <root>.J> declared in <root>' type=<root>.J<kotlin.String> origin=null
           <T>: kotlin.String
-          block: FUN_EXPR type=kotlin.Function1<kotlin.String, kotlin.String> origin=LAMBDA
-            FUN LOCAL_FUNCTION_FOR_LAMBDA name:<anonymous> visibility:local modality:FINAL <> (x:kotlin.String) returnType:kotlin.String
-              VALUE_PARAMETER name:x index:0 type:kotlin.String
+          block: FUN_EXPR type=kotlin.Function1<kotlin.String?, kotlin.String?> origin=LAMBDA
+            FUN LOCAL_FUNCTION_FOR_LAMBDA name:<anonymous> visibility:local modality:FINAL <> (x:kotlin.String?) returnType:kotlin.String?
+              VALUE_PARAMETER name:x index:0 type:kotlin.String?
               BLOCK_BODY
-                RETURN type=kotlin.Nothing from='local final fun <anonymous> (x: kotlin.String): kotlin.String declared in <root>.test1'
-                  GET_VAR 'x: kotlin.String declared in <root>.test1.<anonymous>' type=kotlin.String origin=null
+                RETURN type=kotlin.Nothing from='local final fun <anonymous> (x: kotlin.String?): kotlin.String? declared in <root>.test1'
+                  GET_VAR 'x: kotlin.String? declared in <root>.test1.<anonymous>' type=kotlin.String? origin=null
   FUN name:test2 visibility:public modality:FINAL <> () returnType:<root>.J<kotlin.String?>
     BLOCK_BODY
       RETURN type=kotlin.Nothing from='public final fun test2 (): <root>.J<kotlin.String?> declared in <root>'