Provide better error hint when UndefVarError results from name clashes

stdlib/REPL/src/REPL.jl

@@ -44,7 +44,13 @@ 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 this name was exported by two different modules, resulting in ambiguity. Try explicitly importing it, 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 +1696,38 @@ 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
+
+    str = try
+        X.x
+        @test false # should not reach this
+    catch e
+        sprint(Base.showerror, e)
+    end
+
+    @test contains(str, "Hint: It looks like this name was exported by two different modules, resulting in ambiguity. Try explicitly importing it, or qualifying the name with the module it should come from.")
+finally
+    empty!(Base.Experimental._hint_handlers)
+end
+
 # Hints for tab completes
 
 fake_repl() do stdin_write, stdout_read, repl