docs: fix up generated function examples in metaprogramming.md (#51386)

The last example was untested and has been non-functional actually.

doc/src/manual/metaprogramming.md

@@ -1340,8 +1340,7 @@ julia> function sub2ind_loop(dims::NTuple{N}, I::Integer...) where N
  ind = I[i]-1 + dims[i]*ind
  end
  return ind + 1
- end
-sub2ind_loop (generic function with 1 method)
+ end;
 
 julia> sub2ind_loop((3, 5), 1, 2)
 4
@@ -1380,8 +1379,7 @@ julia> @generated function sub2ind_gen(dims::NTuple{N}, I::Integer...) where N
  ex = :(I[$i] - 1 + dims[$i] * $ex)
  end
  return :($ex + 1)
- end
-sub2ind_gen (generic function with 1 method)
+ end;
 
 julia> sub2ind_gen((3, 5), 1, 2)
 4
@@ -1392,20 +1390,21 @@ julia> sub2ind_gen((3, 5), 1, 2)
 An easy way to find out is to extract the body into another (regular) function:
 
 ```jldoctest sub2ind_gen2
-julia> @generated function sub2ind_gen(dims::NTuple{N}, I::Integer...) where N
- return sub2ind_gen_impl(dims, I...)
- end
-sub2ind_gen (generic function with 1 method)
-
 julia> function sub2ind_gen_impl(dims::Type{T}, I...) where T <: NTuple{N,Any} where N
  length(I) == N || return :(error("partial indexing is unsupported"))
  ex = :(I[$N] - 1)
  for i = (N - 1):-1:1
  ex = :(I[$i] - 1 + dims[$i] * $ex)
  end
  return :($ex + 1)
- end
-sub2ind_gen_impl (generic function with 1 method)
+ end;
+
+julia> @generated function sub2ind_gen(dims::NTuple{N}, I::Integer...) where N
+ return sub2ind_gen_impl(dims, I...)
+ end;
+
+julia> sub2ind_gen((3, 5), 1, 2)
+4
 ```
 
 We can now execute `sub2ind_gen_impl` and examine the expression it returns:
@@ -1434,25 +1433,34 @@ To solve this problem, the language provides syntax for writing normal, non-gene
 alternative implementations of generated functions.
 Applied to the `sub2ind` example above, it would look like this:
 
-```julia
-function sub2ind_gen(dims::NTuple{N}, I::Integer...) where N
- if N != length(I)
- throw(ArgumentError("Number of dimensions must match number of indices."))
- end
- if @generated
- ex = :(I[$N] - 1)
- for i = (N - 1):-1:1
- ex = :(I[$i] - 1 + dims[$i] * $ex)
- end
- return :($ex + 1)
- else
- ind = I[N] - 1
- for i = (N - 1):-1:1
- ind = I[i] - 1 + dims[i]*ind
- end
- return ind + 1
- end
-end
+```jldoctest sub2ind_gen_opt
+julia> function sub2ind_gen_impl(dims::Type{T}, I...) where T <: NTuple{N,Any} where N
+ ex = :(I[$N] - 1)
+ for i = (N - 1):-1:1
+ ex = :(I[$i] - 1 + dims[$i] * $ex)
+ end
+ return :($ex + 1)
+ end;
+
+julia> function sub2ind_gen_fallback(dims::NTuple{N}, I) where N
+ ind = I[N] - 1
+ for i = (N - 1):-1:1
+ ind = I[i] - 1 + dims[i]*ind
+ end
+ return ind + 1
+ end;
+
+julia> function sub2ind_gen(dims::NTuple{N}, I::Integer...) where N
+ length(I) == N || error("partial indexing is unsupported")
+ if @generated
+ return sub2ind_gen_impl(dims, I...)
+ else
+ return sub2ind_gen_fallback(dims, I)
+ end
+ end;
+
+julia> sub2ind_gen((3, 5), 1, 2)
+4
 ```
 
 Internally, this code creates two implementations of the function: a generated one where