[dart2wasm] Fix null checks being added on non-nullable value types.

Bug: #59840
Change-Id: Ifaac6430eabfee21e4971b3dd864e22f65c80adf
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/402980
Reviewed-by: Martin Kustermann <[email protected]>
Commit-Queue: Nate Biggs <[email protected]>

Author: natebiggs

pkg/dart2wasm/lib/code_generator.dart

@@ -2561,20 +2561,25 @@ abstract class AstCodeGenerator
 
   @override
   w.ValueType visitNullCheck(NullCheck node, w.ValueType expectedType) {
-    return _nullCheck(node.operand, translator.throwNullCheckError);
-  }
-
-  w.ValueType _nullCheck(Expression operand, Procedure errorProcedure) {
-    w.ValueType operandType = translator.translateType(dartTypeOf(operand));
+    w.ValueType operandType =
+        translator.translateType(dartTypeOf(node.operand));
     w.ValueType nonNullOperandType = operandType.withNullability(false);
+
+    // In rare cases the operand is non-nullable but TFA doesn't optimize away
+    // the null check. If the operand is an unboxed type, the br_on_non_null
+    // would fail to compile.
+    if (!operandType.nullable) {
+      translateExpression(node.operand, operandType);
+      return nonNullOperandType;
+    }
     w.Label nullCheckBlock = b.block(const [], [nonNullOperandType]);
-    translateExpression(operand, operandType);
+    translateExpression(node.operand, operandType);
 
-    // We lower a null check to a br_on_non_null, throwing a [TypeError] in the
-    // null case.
+    // We lower a null check to a br_on_non_null, throwing a [TypeError] in
+    // the null case.
     b.br_on_non_null(nullCheckBlock);
     call(translator.stackTraceCurrent.reference);
-    call(errorProcedure.reference);
+    call(translator.throwNullCheckError.reference);
     b.unreachable();
     b.end();
     return nonNullOperandType;

tests/web/wasm/regress_59840_test.dart

@@ -0,0 +1,22 @@
+// 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.
+
+void main() {
+  test(something: 1.0, other: 1.0);
+}
+
+void test({double? something, double? other}) {
+  // This asserts prevents the null check below from being removed.
+  assert(
+    (something == null && other == null) ||
+        (something != null && other != null),
+  );
+
+  if (something != null) {
+    print(something);
+    // With assertions 'other' will be an unboxed double. Ensure there is no
+    // null check added.
+    print(other!);
+  }
+}