Avoid redefining types in sigs mode (#910)

Fixes #903

This is split out from #894.

Author: timholy

Project.toml

@@ -1,6 +1,6 @@
 name = "Revise"
 uuid = "295af30f-e4ad-537b-8983-00126c2a3abe"
-version = "3.7.3"
+version = "3.7.4"
 
 [deps]
 CodeTracking = "da1fd8a2-8d9e-5ec2-8556-3022fb5608a2"

src/lowered.jl

@@ -452,8 +452,12 @@ function methods_by_execution!(@nospecialize(recurse), methodinfo, docexprs, fra
                     end
                 end
             elseif LoweredCodeUtils.get_lhs_rhs(stmt) !== nothing
-                # If we're here, either isrequired[pc] is true, or the mode forces us to eval assignments
-                pc = step_expr!(recurse, frame, stmt, true)
+                if mode === :sigs && stmt.head === :const && (a = stmt.args[1]) isa GlobalRef && @invokelatest(isdefined(mod, a.name))
+                    # avoid redefining types unless we have to
+                    pc = next_or_nothing!(frame)
+                else
+                    pc = step_expr!(recurse, frame, stmt, true)
+                end
             elseif head === :call
                 f = @lookup(frame, stmt.args[1])
                 if isdefined(Core, :_defaultctors) && f === Core._defaultctors && length(stmt.args) == 3