Provide better error hint when UndefVarError results from name clas…

…hes (JuliaLang#53469)

We can detect this since we set the `usingfailed` bit when the clash
occurs (to avoid printing the `WARNING` multiple times). In this case,
typos or missing imports (the current message) isn't quite as clear as
it could be, because in fact the name is probably spelled totally right,
it's just that there is a missing explicit import or the name should be
qualified.

This code will stop working if we change the flags in `Core.Binding`,
but the test I added should catch that. However if REPL is supposed to
be independent of Base and not depend on internals there, there could be
an issue. In that case we should probably add an API to Base to inspect
this `usingfailed` bit so we can use it in the REPL.

---------

Co-authored-by: Jameson Nash <vtjnash@gmail.com>
Co-authored-by: Alex Arslan <ararslan@comcast.net>

stdlib/REPL/src/REPL.jl

@@ -44,7 +44,16 @@ function UndefVarError_hint(io::IO, ex::UndefVarError)
                 if C_NULL == owner
                     # No global of this name exists in this module.
                     # This is the common case, so do not print that information.
-                    print(io, "\nSuggestion: check for spelling errors or missing imports.")
+                    # It could be the binding was exported by two modules, which we can detect
+                    # by the `usingfailed` flag in the binding:
+                    if isdefined(bnd, :flags) && Bool(bnd.flags >> 4 & 1) # magic location of the `usingfailed` flag
+                        print(io, "\nHint: It looks like two or more modules export different ",
+                              "bindings with this name, resulting in ambiguity. Try explicitly ",
+                              "importing it from a particular module, or qualifying the name ",
+                              "with the module it should come from.")
+                    else
+                        print(io, "\nSuggestion: check for spelling errors or missing imports.")
+                    end
                     owner = bnd
                 else
                     owner = unsafe_pointer_to_objref(owner)::Core.Binding

stdlib/REPL/test/repl.jl

@@ -1696,6 +1696,35 @@ finally
     empty!(Base.Experimental._hint_handlers)
 end
 
+try # test the functionality of `UndefVarError_hint` against import clashes
+    @assert isempty(Base.Experimental._hint_handlers)
+    Base.Experimental.register_error_hint(REPL.UndefVarError_hint, UndefVarError)
+
+    @eval module X
+
+    module A
+    export x
+    x = 1
+    end # A
+
+    module B
+    export x
+    x = 2
+    end # B
+
+    using .A, .B
+
+    end # X
+
+    expected_message = string("\nHint: It looks like two or more modules export different ",
+                              "bindings with this name, resulting in ambiguity. Try explicitly ",
+                              "importing it from a particular module, or qualifying the name ",
+                              "with the module it should come from.")
+    @test_throws expected_message X.x
+finally
+    empty!(Base.Experimental._hint_handlers)
+end
+
 # Hints for tab completes
 
 fake_repl() do stdin_write, stdout_read, repl