[cfe] Update nullabilities after bounds change in instantiations

Closes #59663

Change-Id: Id4eb4d86c463a24ef45f81d065e322a9f688b045
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/403040
Reviewed-by: Johnni Winther <[email protected]>
Commit-Queue: Chloe Stefantsova <[email protected]>

Author: chloestefantsova

pkg/front_end/testcases/general/issue59663.dart

@@ -0,0 +1,14 @@
+// Copyright (c) 2025, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+class A<X> {
+  Function<Y extends X>(Y) foo;
+  A(this.foo);
+}
+
+bar<T extends num>(T t) {}
+
+main() {
+  new A(bar);
+}

pkg/front_end/testcases/general/issue59663.dart.strong.expect

@@ -0,0 +1,18 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+class A<X extends core::Object? = dynamic> extends core::Object {
+  covariant-by-class field <Y extends self::A::X% = dynamic>(Y%) → dynamic foo;
+  constructor •(<Y extends self::A::X% = dynamic>(Y%) → dynamic foo) → self::A<self::A::X%>
+    : self::A::foo = foo, super core::Object::•()
+    ;
+}
+static method bar<T extends core::num>(self::bar::T t) → dynamic {}
+static method main() → dynamic {
+  new self::A::•<core::num>(#C1);
+}
+
+constants  {
+  #C1 = static-tearoff self::bar
+}

pkg/front_end/testcases/general/issue59663.dart.strong.modular.expect

@@ -0,0 +1,18 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+class A<X extends core::Object? = dynamic> extends core::Object {
+  covariant-by-class field <Y extends self::A::X% = dynamic>(Y%) → dynamic foo;
+  constructor •(<Y extends self::A::X% = dynamic>(Y%) → dynamic foo) → self::A<self::A::X%>
+    : self::A::foo = foo, super core::Object::•()
+    ;
+}
+static method bar<T extends core::num>(self::bar::T t) → dynamic {}
+static method main() → dynamic {
+  new self::A::•<core::num>(#C1);
+}
+
+constants  {
+  #C1 = static-tearoff self::bar
+}

pkg/front_end/testcases/general/issue59663.dart.strong.outline.expect

@@ -0,0 +1,13 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+class A<X extends core::Object? = dynamic> extends core::Object {
+  covariant-by-class field <Y extends self::A::X% = dynamic>(Y%) → dynamic foo;
+  constructor •(<Y extends self::A::X% = dynamic>(Y%) → dynamic foo) → self::A<self::A::X%>
+    ;
+}
+static method bar<T extends core::num>(self::bar::T t) → dynamic
+  ;
+static method main() → dynamic
+  ;

pkg/front_end/testcases/general/issue59663.dart.strong.transformed.expect

@@ -0,0 +1,18 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+class A<X extends core::Object? = dynamic> extends core::Object {
+  covariant-by-class field <Y extends self::A::X% = dynamic>(Y%) → dynamic foo;
+  constructor •(<Y extends self::A::X% = dynamic>(Y%) → dynamic foo) → self::A<self::A::X%>
+    : self::A::foo = foo, super core::Object::•()
+    ;
+}
+static method bar<T extends core::num>(self::bar::T t) → dynamic {}
+static method main() → dynamic {
+  new self::A::•<core::num>(#C1);
+}
+
+constants  {
+  #C1 = static-tearoff self::bar
+}

pkg/front_end/testcases/general/issue59663.dart.textual_outline.expect

@@ -0,0 +1,8 @@
+class A<X> {
+  Function<Y extends X>(Y) foo;
+  A(this.foo);
+}
+
+bar<T extends num>(T t) {}
+
+main() {}

pkg/front_end/testcases/general/issue59663.dart.textual_outline_modelled.expect

@@ -0,0 +1,8 @@
+bar<T extends num>(T t) {}
+
+class A<X> {
+  A(this.foo);
+  Function<Y extends X>(Y) foo;
+}
+
+main() {}

pkg/kernel/lib/type_algebra.dart

@@ -305,6 +305,28 @@ FreshStructuralParameters? getFreshStructuralParametersSubstitutingBounds(
       parameterBoundsHaveChanged = true;
     }
 
+    freshTypeParameter.variance =
+        typeParameter.isLegacyCovariant ? null : typeParameter.variance;
+    // Annotations on a type parameter are specific to the declaration of the
+    // type parameter, rather than the type parameter as such, and therefore
+    // should not be copied here.
+  }
+
+  if (parameterBoundsHaveChanged) {
+    // Since the bounds have changed, the nullabilities of fresh structural
+    // parameter types should be updated.
+    freshTypeArguments = [
+      for (int i = 0; i < freshParameters.length; i++)
+        new StructuralParameterType.withDefaultNullability(freshParameters[i])
+    ];
+    instantiator = FunctionTypeInstantiator.fromIterables(
+        typeParameters, freshTypeArguments);
+  }
+
+  for (int i = 0; i < typeParameters.length; ++i) {
+    StructuralParameter typeParameter = typeParameters[i];
+    StructuralParameter freshTypeParameter = freshParameters[i];
+
     // TODO(cstefantsova): Replace the following by an assert checking that the
     // type parameters don't occur in the default types.
     DartType? defaultTypeVisitedByOuter =
@@ -317,13 +339,8 @@ FreshStructuralParameters? getFreshStructuralParametersSubstitutingBounds(
     if (defaultTypeVisitedByThis != null || defaultTypeVisitedByThis != null) {
       parameterBoundsHaveChanged = true;
     }
-
-    freshTypeParameter.variance =
-        typeParameter.isLegacyCovariant ? null : typeParameter.variance;
-    // Annotations on a type parameter are specific to the declaration of the
-    // type parameter, rather than the type parameter as such, and therefore
-    // should not be copied here.
   }
+
   if (!parameterBoundsHaveChanged) {
     return null;
   } else {