fix #25678: return matters for generated functions (#40778)

Just explicitely check for `CodeInfo` objects and use an explicit
`return` in this case inside the `@generated` macro.

Co-authored-by: Jeff Bezanson <[email protected]>

Author: simeonschaub

base/expr.jl

@@ -464,7 +464,10 @@ macro generated(f)
                          Expr(:block,
                               lno,
                               Expr(:if, Expr(:generated),
-                                   body,
+                                   # https://github.com/JuliaLang/julia/issues/25678
+                                   Expr(:block,
+                                        :(local tmp = $body),
+                                        :(if tmp isa Core.CodeInfo; return tmp; else tmp; end)),
                                    Expr(:block,
                                         Expr(:meta, :generated_only),
                                         Expr(:return, nothing))))))

test/syntax.jl

@@ -2959,3 +2959,10 @@ end
     ex.args = fill!(Vector{Any}(undef, 600000), 1)
     @test_throws ErrorException("syntax: expression too large") eval(ex)
 end
+
+# issue 25678
+@generated f25678(x::T) where {T} = code_lowered(sin, Tuple{x})[]
+@test f25678(pi/6) === sin(pi/6)
+
+@generated g25678(x) = return :x
+@test g25678(7) === 7