improve type definition super type errors (JuliaLang#34510)

Makes the error message more precise and descriptive for each error
that could occur when defining a new subtype.

Co-authored-by: KristofferC <kristoffer.carlsson@juliacomputing.com>

Author: kamalojasv181

src/builtins.c

@@ -1563,16 +1563,25 @@ JL_CALLABLE(jl_f__primitivetype)
 
 static void jl_set_datatype_super(jl_datatype_t *tt, jl_value_t *super)
 {
-    if (!jl_is_datatype(super) || !jl_is_abstracttype(super) ||
-        tt->super != NULL ||
-        tt->name == ((jl_datatype_t*)super)->name ||
-        jl_is_tuple_type(super) ||
-        jl_is_namedtuple_type(super) ||
-        jl_subtype(super, (jl_value_t*)jl_type_type) ||
-        jl_subtype(super, (jl_value_t*)jl_builtin_type)) {
-        jl_errorf("invalid subtyping in definition of %s",
-                  jl_symbol_name(tt->name->name));
-    }
+    const char *error = NULL;
+    if (!jl_is_datatype(super))
+        error = "can only subtype data types";
+    else if (tt->super != NULL)
+        error = "type already has a supertype";
+    else if (tt->name == ((jl_datatype_t*)super)->name)
+        error = "a type cannot subtype itself";
+    else if (jl_is_tuple_type(super))
+        error = "cannot subtype a tuple type";
+    else if (jl_is_namedtuple_type(super))
+        error = "cannot subtype a named tuple type";
+    else if (jl_subtype(super, (jl_value_t*)jl_type_type))
+        error = "cannot add subtypes to Type";
+    else if (jl_subtype(super, (jl_value_t*)jl_builtin_type))
+        error = "cannot add subtypes to Core.Builtin";
+    else if (!jl_is_abstracttype(super))
+        error = "can only subtype abstract types";
+    if (error)
+         jl_errorf("invalid subtyping in definition of %s: %s.", jl_symbol_name(tt->name->name), error);
     tt->super = (jl_datatype_t*)super;
     jl_gc_wb(tt, tt->super);
 }

test/errorshow.jl

@@ -903,3 +903,21 @@ end
         @test contains(err_str, "maybe you meant `import/using .Bar`")
     end
 end
+
+for (expr, errmsg) in
+    [
+        (:(struct Foo <: 1 end),       "can only subtype data types"),
+        (:(struct Foo <: Float64 end), "can only subtype abstract types"),
+        (:(struct Foo <: Foo end),     "a type cannot subtype itself"),
+        (:(struct Foo <: Tuple{Float64} end), "cannot subtype a tuple type"),
+        (:(struct Foo <: NamedTuple{(:a,), Tuple{Int64}} end), "cannot subtype a named tuple type"),
+        (:(struct Foo <: Type{Float64} end), "cannot add subtypes to Type"),
+        (:(struct Foo <: Type{Float64} end), "cannot add subtypes to Type"),
+        (:(struct Foo <: typeof(Core.apply_type) end), "cannot add subtypes to Core.Builtin"),
+    ]
+    err = try @eval $expr
+    catch e
+        e
+    end
+    @test contains(sprint(showerror, err), errmsg)
+end

test/subtype.jl

@@ -1247,7 +1247,7 @@ end
 
 # Issue #19414
 let ex = try struct A19414 <: Base.AbstractSet end catch e; e end
-    @test isa(ex, ErrorException) && ex.msg == "invalid subtyping in definition of A19414"
+    @test isa(ex, ErrorException) && ex.msg == "invalid subtyping in definition of A19414: can only subtype data types."
 end
 
 # issue #20103, OP and comments