reflect/protoreflect: clarify Clear and Remove operation on missing entries

Follow the precedence of Go maps where deletion on a key without an entry in
the map is a noop. Similarly, document that the following methods are safe
to call with entries that do not exist:
* Map.Clear
* KnownFields.Clear
* ExtensionFieldTypes.Remove

Change the implementation for each of these to match the documented behavior.

Change-Id: Ifccff9b7b03baaeffdc366d05f6286ba60e14934
Reviewed-on: https://go-review.googlesource.com/c/148317
Reviewed-by: Herbie Ong <herbie@google.com>

internal/impl/message.go

@@ -310,8 +310,8 @@ type emptyUnknownFields struct{}
 
 func (emptyUnknownFields) Len() int                                          { return 0 }
 func (emptyUnknownFields) Get(pref.FieldNumber) pref.RawFields               { return nil }
-func (emptyUnknownFields) Set(pref.FieldNumber, pref.RawFields)              { /* noop */ }
-func (emptyUnknownFields) Range(func(pref.FieldNumber, pref.RawFields) bool) {}
+func (emptyUnknownFields) Set(pref.FieldNumber, pref.RawFields)              { return } // noop
+func (emptyUnknownFields) Range(func(pref.FieldNumber, pref.RawFields) bool) { return }
 func (emptyUnknownFields) IsSupported() bool                                 { return false }
 
 type emptyExtensionFields struct{}
@@ -319,17 +319,17 @@ type emptyExtensionFields struct{}
 func (emptyExtensionFields) Len() int                                        { return 0 }
 func (emptyExtensionFields) Has(pref.FieldNumber) bool                       { return false }
 func (emptyExtensionFields) Get(pref.FieldNumber) pref.Value                 { return pref.Value{} }
-func (emptyExtensionFields) Set(pref.FieldNumber, pref.Value)                { panic("invalid field") }
-func (emptyExtensionFields) Clear(pref.FieldNumber)                          { panic("invalid field") }
-func (emptyExtensionFields) Mutable(pref.FieldNumber) pref.Mutable           { panic("invalid field") }
-func (emptyExtensionFields) Range(f func(pref.FieldNumber, pref.Value) bool) {}
+func (emptyExtensionFields) Set(pref.FieldNumber, pref.Value)                { panic("extensions not supported") }
+func (emptyExtensionFields) Clear(pref.FieldNumber)                          { return } // noop
+func (emptyExtensionFields) Mutable(pref.FieldNumber) pref.Mutable           { panic("extensions not supported") }
+func (emptyExtensionFields) Range(f func(pref.FieldNumber, pref.Value) bool) { return }
 func (emptyExtensionFields) ExtensionTypes() pref.ExtensionFieldTypes        { return emptyExtensionTypes{} }
 
 type emptyExtensionTypes struct{}
 
 func (emptyExtensionTypes) Len() int                                     { return 0 }
 func (emptyExtensionTypes) Register(pref.ExtensionType)                  { panic("extensions not supported") }
-func (emptyExtensionTypes) Remove(pref.ExtensionType)                    { panic("extensions not supported") }
+func (emptyExtensionTypes) Remove(pref.ExtensionType)                    { return } // noop
 func (emptyExtensionTypes) ByNumber(pref.FieldNumber) pref.ExtensionType { return nil }
 func (emptyExtensionTypes) ByName(pref.FullName) pref.ExtensionType      { return nil }
-func (emptyExtensionTypes) Range(func(pref.ExtensionType) bool)          {}
+func (emptyExtensionTypes) Range(func(pref.ExtensionType) bool)          { return }

reflect/protoreflect/value.go

@@ -88,9 +88,8 @@ type KnownFields interface {
 	// For example, if Mutable is called later, can it reuse memory?
 
 	// Clear clears the field such that a subsequent call to Has reports false.
-	//
-	// It panics if the field number does not correspond with a known field
-	// in MessageDescriptor.Fields or an extension field in ExtensionTypes.
+	// The operation does nothing if the field number does not correspond with
+	// a known field or extension field.
 	Clear(FieldNumber)
 
 	// Mutable returns a reference value for a field with a given field number,
@@ -196,6 +195,7 @@ type ExtensionFieldTypes interface {
 
 	// Remove removes the ExtensionType.
 	// It panics if a value for this extension field is still populated.
+	// The operation does nothing if there is no associated type to remove.
 	Remove(ExtensionType)
 
 	// ByNumber looks up an extension by field number.
@@ -283,6 +283,7 @@ type Map interface {
 	Set(MapKey, Value)
 
 	// Clear clears the entry associated with they given key.
+	// The operation does nothing if there is no entry associated with the key.
 	Clear(MapKey)
 
 	// Mutable returns a Mutable reference for the element with a given key,