From type/Parameter

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method type_captures(Parameter:D: --> List:D)

Returns a list of variable names of type captures associated with this parameter. Type captures define a type name within the attached code, which is an alias to the type gleaned from the argument during a call.

sub a(::T ::U $x) { T.say }
a(8);                                       # OUTPUT: «(Int)␤» 
say &a.signature.params[0].type_captures;   # OUTPUT: «(T U)␤» 
sub b($x) { $x.^name.say }
a(8);                                       # OUTPUT: «Int␤»

The type used may change from call to call. Once they are defined, type captures can be used wherever you would use a type, even later in the same signature:

sub c(::T $x, T $y, $z) { my T $zz = $z };
c(4, 5, 6);          # OK 
 
try c(4, 5, "six");
given $! { .message.say };
# OUTPUT: «Type check failed in assignment to $zz; expected Int but got Str ("six")␤» 
 
try c("four", 5, "six");
given $! { .message.say };
# OUTPUT: «Type check failed in binding to parameter '$y'; expected Str but got Int (5)␤» 

Type captures may be used at the same time as nominal type constraint.

sub d(::T Numeric $x, T $y) {};
d(4, 5);            # OK 
d(4e0, 5e0);        # OK 
 
try d(4e0, 5);
given $! { .message.say };
# OUTPUT: «Type check failed in binding to parameter '$y'; expected Num but got Int (5)␤» 
 
try d("four", "five");
given $! { .message.say };
# OUTPUT: «Type check failed in binding to parameter '$x'; expected Numeric but got Str ("four")␤»