From type/X/Method/InvalidQualifier

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Returns the invocant of the failed, qualified method call

From type/Parameter

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method invocant(Parameter:D: --> Bool:D)

Returns True if the parameter is the invocant parameter.

my Signature $sig = :($i : Str $x is rw, Bool :$is-named);
say $sig.params[0].invocant;                       # OUTPUT: «True␤» 
say $sig.params[1].invocant;                       # OUTPUT: «False␤»