mismatched restriction type in method

Error message

mismatched restriction type in method %v, parameter at index %v. got: %v, want: %v (from method %v). Ensure that all restrictions in an SDF are the same type.

What it means

All SDF restriction-bearing methods must share one restriction type. Beam validates that InitialWatermarkEstimatorState's restriction parameter (index 1) exactly equals the restriction type produced by CreateInitialRestriction (and used by CreateTracker). A mismatch fails validation with this error, which names the reference method that established the expected type.

Solutions

Use the exact same named restriction type (as produced by CreateInitialRestriction and consumed by CreateTracker) in InitialWatermarkEstimatorState.
Eliminate duplicate/same-named restriction types in different packages; pick one canonical type.
Make value/pointer usage consistent across all restriction-bearing methods.
Check for aliases or generics resolving to different underlying named types across methods.

Example fix

// before
func (f *fn) CreateTracker(rest MyRestriction) sdf.SdfBasicTracker { ... }
func (f *fn) InitialWatermarkEstimatorState(rt typex.EventTime, rest *MyRestriction) myState { ... }
// after
func (f *fn) CreateTracker(rest MyRestriction) sdf.SdfBasicTracker { ... }
func (f *fn) InitialWatermarkEstimatorState(rt typex.EventTime, rest MyRestriction) myState { ... }

Defensive patterns

Strategy: validation

Validate before calling

// One canonical restriction type for the whole SDF
type MyRestriction struct{ Start, End int64 }
var _ func(MyRestriction) sdf.SdfBasicTracker = (*fn).CreateTracker
var _ func(string, typex.EventTime, MyRestriction) myState = (*fn).InitialWatermarkEstimatorState

Prevention

Define the restriction type once and reference it everywhere; never create aliases or duplicates.
Be consistent about value vs pointer for the restriction across all methods.
A compile-time assertion on CreateTracker's signature propagates the canonical type.

When it happens

Trigger: Declaring InitialWatermarkEstimatorState with a restriction type different from the one CreateInitialRestriction returns / CreateTracker takes — e.g. a same-named struct in another package, a pointer where others use a value, or a differently parameterized generic restriction.

Common situations: Creating an alias or duplicate restriction struct during refactoring so methods no longer share the identical named type; switching value vs pointer restriction types in some methods; copy-pasting methods between SDFs with same-named but distinct restriction types.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/0e9b948448736b99. Report an issue: GitHub.

Appendix: source

Thrown at sdks/go/pkg/beam/core/graph/fn.go:1284

		switch name {
		case initialWatermarkEstimatorStateName:
			if len(method.Param) != numMainIn+2 {
				err := errors.Errorf("unexpected number of params in method %v. got: %v, want: %v",
					initialWatermarkEstimatorStateName, len(method.Param), numMainIn+2)
				return errors.SetTopLevelMsgf(err, "unexpected number of parameters in method %v. "+
					"got: %v, want: %v. Check that the signature conforms to the expected signature for %v, "+
					"and that elements in SDF method parameters match elements in %v.",
					initialWatermarkEstimatorStateName, len(method.Param), numMainIn+2, initialWatermarkEstimatorStateName, processElementName)
			}
			if method.Param[0].T != typex.EventTimeType {
				err := errors.Errorf("unexpected parameter type in method %v, param %v. got: %v, want: %v",
					initialWatermarkEstimatorStateName, 0, method.Param[0].T, typex.EventTimeType)
				return errors.SetTopLevelMsgf(err, "mismatched event time type in method %v, "+
					"parameter at index %v. got: %v, want: %v.",
					initialWatermarkEstimatorStateName, 0, method.Param[0].T, typex.EventTimeType)
			}
			if method.Param[1].T != restT {
				err := errors.Errorf("mismatched restriction type in method %v, param %v. got: %v, want: %v",
					initialWatermarkEstimatorStateName, 1, method.Param[1].T, restT)
				return errors.SetTopLevelMsgf(err, "mismatched restriction type in method %v, "+
					"parameter at index %v. got: %v, want: %v (from method %v). "+
					"Ensure that all restrictions in an SDF are the same type.",
					initialWatermarkEstimatorStateName, 1, method.Param[1].T, restT, createTrackerName)
			}
			if err := validateSdfElementT(fn, initialWatermarkEstimatorStateName, method, numMainIn, 2); err != nil {
				return err
			}

			if len(method.Ret) != 1 {
				err := errors.Errorf("unexpected number of elements returned in method %v. got: %v, want %v",
					initialWatermarkEstimatorStateName, len(method.Ret), 1)
				return errors.SetTopLevelMsgf(err, "unexpected number of elements returned in method %v. "+
					"got: %v, want %v. Check that the signature conforms to the expected signature for %v.",
					initialWatermarkEstimatorStateName, len(method.Ret), 1, initialWatermarkEstimatorStateName)
			}
			if method.Ret[0].T != watermarkStateT {

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