Don't skip constraints from equations with the kind equal

The problem was that we didn't incorporate T == Foo into K <: Inv<out T>.
 It happened because of optimisation that isn't applicable here as we
 already have projection in the initial type

 #KT-39777 Fixed

compiler/fir/analysis-tests/tests/org/jetbrains/kotlin/fir/FirOldFrontendDiagnosticsTestGenerated.java

@@ -10006,6 +10006,11 @@ public class FirOldFrontendDiagnosticsTestGenerated extends AbstractFirOldFronte
             runTest("compiler/testData/diagnostics/tests/inference/dependantOnVarianceNullable.kt");
         }
 
+        @TestMetadata("equalitySubstitutionInsideNonInvariantType.kt")
+        public void testEqualitySubstitutionInsideNonInvariantType() throws Exception {
+            runTest("compiler/testData/diagnostics/tests/inference/equalitySubstitutionInsideNonInvariantType.kt");
+        }
+
         @TestMetadata("expectedTypeAdditionalTest.kt")
         public void testExpectedTypeAdditionalTest() throws Exception {
             runTest("compiler/testData/diagnostics/tests/inference/expectedTypeAdditionalTest.kt");

compiler/resolution/src/org/jetbrains/kotlin/resolve/calls/inference/components/ConstraintIncorporator.kt

@@ -221,9 +221,10 @@ class ConstraintIncorporator(
         val isFromVariableFixation = baseConstraint.position.from is FixVariableConstraintPosition
                 || otherConstraint.position.from is FixVariableConstraintPosition
 
-        if (!containsConstrainingTypeWithoutProjection(newConstraint, otherConstraint)
+        if (!otherConstraint.kind.isEqual() &&
-            && !isUsefulForNullabilityConstraint
+            !isUsefulForNullabilityConstraint &&
-            && !isFromVariableFixation
+            !isFromVariableFixation &&
+            !containsConstrainingTypeWithoutProjection(newConstraint, otherConstraint)
         ) return
 
         if (trivialConstraintTypeInferenceOracle.isGeneratedConstraintTrivial(

compiler/testData/diagnostics/tests/inference/equalitySubstitutionInsideNonInvariantType.kt

@@ -0,0 +1,17 @@
+// FIR_IDENTICAL
+// !DIAGNOSTICS: -UNUSED_PARAMETER
+
+fun <K> materialize(): K = TODO()
+
+class Foo
+class Inv<T>
+
+fun <T> test1(x: Inv<out T>) {}
+fun <T> test2(x: Inv<in T>) {}
+fun <T> test3(x: Inv<T>) {}
+
+fun main() {
+    test1<Foo>(materialize())
+    test2<Foo>(materialize())
+    test3<Foo>(materialize())
+}

compiler/testData/diagnostics/tests/inference/equalitySubstitutionInsideNonInvariantType.txt

@@ -0,0 +1,21 @@
+package
+
+public fun main(): kotlin.Unit
+public fun </*0*/ K> materialize(): K
+public fun </*0*/ T> test1(/*0*/ x: Inv<out T>): kotlin.Unit
+public fun </*0*/ T> test2(/*0*/ x: Inv<in T>): kotlin.Unit
+public fun </*0*/ T> test3(/*0*/ x: Inv<T>): kotlin.Unit
+
+public final class Foo {
+    public constructor Foo()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public final class Inv</*0*/ T> {
+    public constructor Inv</*0*/ T>()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}

compiler/tests/org/jetbrains/kotlin/checkers/DiagnosticsTestGenerated.java

@@ -10013,6 +10013,11 @@ public class DiagnosticsTestGenerated extends AbstractDiagnosticsTestWithFirVali
                 runTest("compiler/testData/diagnostics/tests/inference/dependantOnVarianceNullable.kt");
             }
 
+            @TestMetadata("equalitySubstitutionInsideNonInvariantType.kt")
+            public void testEqualitySubstitutionInsideNonInvariantType() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/inference/equalitySubstitutionInsideNonInvariantType.kt");
+            }
+
             @TestMetadata("expectedTypeAdditionalTest.kt")
             public void testExpectedTypeAdditionalTest() throws Exception {
                 runTest("compiler/testData/diagnostics/tests/inference/expectedTypeAdditionalTest.kt");

compiler/tests/org/jetbrains/kotlin/checkers/javac/DiagnosticsUsingJavacTestGenerated.java

@@ -10008,6 +10008,11 @@ public class DiagnosticsUsingJavacTestGenerated extends AbstractDiagnosticsUsing
                 runTest("compiler/testData/diagnostics/tests/inference/dependantOnVarianceNullable.kt");
             }
 
+            @TestMetadata("equalitySubstitutionInsideNonInvariantType.kt")
+            public void testEqualitySubstitutionInsideNonInvariantType() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/inference/equalitySubstitutionInsideNonInvariantType.kt");
+            }
+
             @TestMetadata("expectedTypeAdditionalTest.kt")
             public void testExpectedTypeAdditionalTest() throws Exception {
                 runTest("compiler/testData/diagnostics/tests/inference/expectedTypeAdditionalTest.kt");