fix getting field docstrings for parametric structs (#59725)

Author: KristofferC

stdlib/REPL/src/docview.jl

@@ -640,7 +640,7 @@ function _repl(x, brief::Bool=true, mod::Module=Main, internal_accesses::Union{N
     docs = esc(:(@doc $x))
     docs = if isfield(x)
         quote
-            if isa($(esc(x.args[1])), DataType)
+            if $(esc(x.args[1])) isa Type
                 fielddoc($(esc(x.args[1])), $(esc(x.args[2])))
             else
                 $docs
@@ -684,6 +684,7 @@ function fielddoc(binding::Binding, field::Symbol)
 end
 
 # As with the additional `doc` methods, this converts an object to a `Binding` first.
+fielddoc(obj::UnionAll, field::Symbol) = fielddoc(Base.unwrap_unionall(obj), field)
 fielddoc(object, field::Symbol) = fielddoc(aliasof(object, typeof(object)), field)
 
 

stdlib/REPL/test/docview.jl

@@ -111,6 +111,24 @@ end
     @test endswith(get_help_standard("Int.not_a_field"), "`$Int` has no fields.\n")
 end
 
+@testset "Parametric struct field help (#59524)" begin
+    "NonParametricStruct docstring"
+    struct NonParametricStruct
+        "field_x docstring"
+        field_x::Float64
+    end
+
+    "ParametricStruct docstring"
+    struct ParametricStruct{T<:Real}
+        "field_y docstring"
+        field_y::T
+    end
+
+    @test occursin("field_x docstring", get_help_standard("NonParametricStruct.field_x"))
+    @test occursin("field_y docstring", get_help_standard("ParametricStruct.field_y"))
+    @test endswith(get_help_standard("ParametricStruct.not_a_field"), "ParametricStruct` has field `field_y`.\n")
+end
+
 module InternalWarningsTests
 
     module A