fix a regression in parametric dispatch with unions

This is a bit of an undisciplined hack, but it should also be quite
harmless. No reason not to get Union{T,Void} working ASAP.

Author: JeffBezanson

src/jltypes.c

@@ -369,9 +369,16 @@ static jl_value_t *intersect_union(jl_uniontype_t *a, jl_value_t *b,
                                    cenv_t *penv, cenv_t *eqc, variance_t var)
 {
     int eq0 = eqc->n, co0 = penv->n;
-    jl_svec_t *t = jl_alloc_svec(jl_svec_len(a->types));
+    size_t i, l = jl_svec_len(a->types);
+    // shortcut an easy case: union contains type b
+    if (!jl_is_typevar(b)) {
+        for(i=0; i < l; i++) {
+            if (jl_svecref(a->types,i) == b)
+                return b;
+        }
+    }
+    jl_svec_t *t = jl_alloc_svec(l);
     JL_GC_PUSH1(&t);
-    size_t i, l=jl_svec_len(t);
     for(i=0; i < l; i++) {
         int eq_l = eqc->n, co_l = penv->n;
         jl_value_t *ti = jl_type_intersect(jl_svecref(a->types,i), b,

test/core.jl

@@ -3619,3 +3619,9 @@ end
 @test @m8846(a) === (:a, 0)
 @test @m8846(a,1) === (:a, 1)
 @test_throws MethodError eval(:(@m8846(a,b,c)))
+
+# a simple case of parametric dispatch with unions
+let foo{T}(x::Union{T,Void},y::Union{T,Void}) = 1
+    @test foo(1, nothing) === 1
+    @test_throws MethodError foo(nothing, nothing)  # can't determine T
+end