[FIR] Check for ConeErrorType in doUnify to prevent endless recursion

^KT-64625 Fixed

analysis/low-level-api-fir/tests/org/jetbrains/kotlin/analysis/low/level/api/fir/diagnostic/compiler/based/DiagnosticCompilerTestFE10TestdataTestGenerated.java

@@ -13031,6 +13031,12 @@ public class DiagnosticCompilerTestFE10TestdataTestGenerated extends AbstractDia
             public void testKt24158() throws Exception {
                 runTest("compiler/testData/diagnostics/tests/exceptions/kt24158.kt");
             }
+
+            @Test
+            @TestMetadata("stackOverflowOnDoUnify.kt")
+            public void testStackOverflowOnDoUnify() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/exceptions/stackOverflowOnDoUnify.kt");
+            }
         }
 
         @Nested

analysis/low-level-api-fir/tests/org/jetbrains/kotlin/analysis/low/level/api/fir/diagnostic/compiler/based/LLFirPreresolvedReversedDiagnosticCompilerFE10TestDataTestGenerated.java

@@ -13031,6 +13031,12 @@ public class LLFirPreresolvedReversedDiagnosticCompilerFE10TestDataTestGenerated
             public void testKt24158() throws Exception {
                 runTest("compiler/testData/diagnostics/tests/exceptions/kt24158.kt");
             }
+
+            @Test
+            @TestMetadata("stackOverflowOnDoUnify.kt")
+            public void testStackOverflowOnDoUnify() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/exceptions/stackOverflowOnDoUnify.kt");
+            }
         }
 
         @Nested

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

@@ -13025,6 +13025,12 @@ public class FirLightTreeOldFrontendDiagnosticsTestGenerated extends AbstractFir
             public void testKt24158() throws Exception {
                 runTest("compiler/testData/diagnostics/tests/exceptions/kt24158.kt");
             }
+
+            @Test
+            @TestMetadata("stackOverflowOnDoUnify.kt")
+            public void testStackOverflowOnDoUnify() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/exceptions/stackOverflowOnDoUnify.kt");
+            }
         }
 
         @Nested

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

@@ -13031,6 +13031,12 @@ public class FirPsiOldFrontendDiagnosticsTestGenerated extends AbstractFirPsiDia
             public void testKt24158() throws Exception {
                 runTest("compiler/testData/diagnostics/tests/exceptions/kt24158.kt");
             }
+
+            @Test
+            @TestMetadata("stackOverflowOnDoUnify.kt")
+            public void testStackOverflowOnDoUnify() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/exceptions/stackOverflowOnDoUnify.kt");
+            }
         }
 
         @Nested

compiler/fir/providers/src/org/jetbrains/kotlin/fir/types/TypeUnification.kt

@@ -26,6 +26,10 @@ fun FirSession.doUnify(
     val originalType = originalTypeProjection.type?.lowerBoundIfFlexible()?.fullyExpandedType(this)
     val typeWithParameters = typeWithParametersProjection.type?.lowerBoundIfFlexible()?.fullyExpandedType(this)
 
+    if (typeWithParameters is ConeErrorType) {
+        return true // Return true to avoid loosing `result` substitution
+    }
+
     if (originalType is ConeIntersectionType) {
         val intersectionResult = mutableMapOf<FirTypeParameterSymbol, ConeTypeProjection>()
         for (intersectedType in originalType.intersectedTypes) {

compiler/testData/diagnostics/tests/exceptions/stackOverflowOnDoUnify.kt

@@ -0,0 +1,8 @@
+// FIR_IDENTICAL
+// ISSUE: KT-64625
+// !DIAGNOSTICS: -DEBUG_INFO_ELEMENT_WITH_ERROR_TYPE
+
+interface A<T1, T2>
+interface B<X> : A<X, <!UNRESOLVED_REFERENCE!>T<!>?>
+
+fun f(x: A<Int, <!UNRESOLVED_REFERENCE!>K<!>?>) = x as B

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

@@ -13031,6 +13031,12 @@ public class DiagnosticTestGenerated extends AbstractDiagnosticTest {
             public void testKt24158() throws Exception {
                 runTest("compiler/testData/diagnostics/tests/exceptions/kt24158.kt");
             }
+
+            @Test
+            @TestMetadata("stackOverflowOnDoUnify.kt")
+            public void testStackOverflowOnDoUnify() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/exceptions/stackOverflowOnDoUnify.kt");
+            }
         }
 
         @Nested