Always interpret in current world (#478)

* always interpret in current world

* add a test

* julia 1.0 compat

Author: pfitzseb

src/utils.jl

@@ -25,14 +25,20 @@ function to_function(@nospecialize(x))
     isa(x, GlobalRef) ? getfield(x.mod, x.name) : x
 end
 
+@static if isdefined(Base, :get_world_counter)
+    import Base: get_world_counter
+else
+    get_world_counter() = ccall(:jl_get_world_counter, UInt, ())
+end
+
 """
     method = whichtt(tt)
 
 Like `which` except it operates on the complete tuple-type `tt`.
 """
 function whichtt(@nospecialize(tt))
     # TODO: provide explicit control over world age? In case we ever need to call "old" methods.
-    m = ccall(:jl_gf_invoke_lookup, Any, (Any, UInt), tt, typemax(UInt))
+    m = ccall(:jl_gf_invoke_lookup, Any, (Any, UInt), tt, get_world_counter())
     m === nothing && return nothing
     isa(m, Method) && return m
     return m.func::Method

test/interpret.jl

@@ -758,3 +758,28 @@ end
 
     @test @interpret(g()) === true
 end
+
+const override_world = typemax(Csize_t) - 1
+macro unreachable(ex)
+    quote
+        world_counter = cglobal(:jl_world_counter, Csize_t)
+        regular_world = unsafe_load(world_counter)
+
+        $(Expr(:tryfinally, # don't introduce scope
+            quote
+                unsafe_store!(world_counter, $(override_world-1))
+                $(esc(ex))
+            end,
+            quote
+                unsafe_store!(world_counter, regular_world)
+            end
+        ))
+    end
+end
+
+@testset "unreachable worlds" begin
+    foobar() = 42
+    @unreachable foobar() = "nope"
+
+    @test @interpret(foobar()) == foobar()
+end