Add self-referencing struct shim

Part of JuliaLang/julia#56497 that I ignored before.  I have doubts about how
    long this solution will stay in the base repository, and how complete it
    is (doesn't seem to work with M1.M2.S), but we are testing for it here.

Also change the expected value of a test changed in the same PR.

Author: mlechu

src/desugaring.jl

@@ -3854,6 +3854,24 @@ function _constructor_min_initalized(ex::SyntaxTree)
     end
 end
 
+# Let S be a struct we're defining in module M.  Below is a hack to allow its
+# field types to refer to S as M.S.  See #56497.
+function insert_struct_shim(ctx, fieldtypes, name)
+    function replace_type(ex)
+        if kind(ex) == K"." &&
+            numchildren(ex) == 2 &&
+            kind(ex[2]) == K"Symbol" &&
+            ex[2].name_val == name.name_val
+            @ast ctx ex [K"call" "struct_name_shim"::K"core" ex[1] ex[2] ctx.mod::K"Value" name]
+        elseif numchildren(ex) > 0
+            @ast ctx ex [ex.kind map(replace_type, children(ex))...]
+        else
+            ex
+        end
+    end
+    map(replace_type, fieldtypes)
+end
+
 function expand_struct_def(ctx, ex, docs)
     @chk numchildren(ex) == 2
     type_sig = ex[1]
@@ -3986,7 +4004,7 @@ function expand_struct_def(ctx, ex, docs)
                     # TODO: if there is a previous compatible definition, re-use params. See #57253
                     false::K"Bool"
                     newtype_var
-                    [K"call" "svec"::K"core" field_types...]
+                    [K"call" "svec"::K"core" insert_struct_shim(ctx, field_types, struct_name)...]
                 ]
                 [K"constdecl"
                     global_struct_name

test/typedefs.jl

@@ -251,7 +251,7 @@ end
 """)
 @test fieldtypes(test_mod.M36104.T36104) == (Vector{test_mod.M36104.T36104},)
 @test_throws ErrorException("expected") JuliaLowering.include_string(test_mod, """struct X36104; x::error("expected"); end""")
-@test isdefined(test_mod, :X36104)
+@test !isdefined(test_mod, :X36104)
 JuliaLowering.include_string(test_mod, "struct X36104; x::Int; end")
 @test fieldtypes(test_mod.X36104) == (Int,)
 JuliaLowering.include_string(test_mod, "primitive type P36104 8 end")