improve compile-time method matching logic

don't consider typevars unmatched if we can prove that they would be
matched by any actual call.

fixes #5575

slightly expand some array constructor signatures so `Type{<:Top}` matches
them, not just `Type{<:Any}`.

base/array.jl

@@ -166,7 +166,7 @@ cell(dims::(Integer...)) = Array(Any, convert((Int...), dims))
 
 for (fname, felt) in ((:zeros,:zero), (:ones,:one))
     @eval begin
-        ($fname){T}(::Type{T}, dims...)  = fill!(Array(T, dims...), ($felt)(T))
+        ($fname)(T::Type, dims...)       = fill!(Array(T, dims...), ($felt)(T))
         ($fname)(dims...)                = fill!(Array(Float64, dims...), ($felt)(Float64))
         ($fname){T}(A::AbstractArray{T}) = fill!(similar(A), ($felt)(T))
     end

src/gf.c

@@ -1580,6 +1580,28 @@ DLLEXPORT jl_tuple_t *jl_match_method(jl_value_t *type, jl_value_t *sig,
     return result;
 }
 
+// Determine whether a typevar exists inside at most one DataType.
+// These are the typevars that will always be matched by any matching
+// arguments.
+static int tvar_exists_at_top_level(jl_value_t *tv, jl_tuple_t *sig, int attop)
+{
+    int i, l=jl_tuple_len(sig);
+    for(i=0; i < l; i++) {
+        jl_value_t *a = jl_tupleref(sig, i);
+        if (jl_is_vararg_type(a))
+            a = jl_tparam0(a);
+        if (a == tv)
+            return 1;
+        if (jl_is_tuple(a) && tvar_exists_at_top_level(tv, (jl_tuple_t*)a, attop))
+            return 1;
+        if (attop && jl_is_datatype(a)) {
+            if (tvar_exists_at_top_level(tv, ((jl_datatype_t*)a)->parameters, 0))
+                return 1;
+        }
+    }
+    return 0;
+}
+
 // returns a match as (argtypes, static_params, Method)
 static jl_value_t *ml_matches(jl_methlist_t *ml, jl_value_t *type,
                               jl_sym_t *name, int lim)
@@ -1641,7 +1663,10 @@ static jl_value_t *ml_matches(jl_methlist_t *ml, jl_value_t *type,
                 int matched_all_typevars = 1;
                 size_t l = jl_tuple_len(env);
                 for(i=1; i < l; i+=2) {
-                    if (jl_is_typevar(jl_tupleref(env,i))) {
+                    if (jl_is_typevar(jl_tupleref(env,i)) &&
+                        // if tvar is at the top level it will definitely be matched.
+                        // see issue #5575
+                        !tvar_exists_at_top_level(jl_tupleref(env,i-1), ml->sig, 1)) {
                         matched_all_typevars = 0;
                         break;
                     }

src/jltypes.c

@@ -3179,8 +3179,7 @@ void jl_init_types(void)
 
     // Type{T}
     jl_typetype_tvar = jl_new_typevar(jl_symbol("T"),
-                                      (jl_value_t*)jl_bottom_type,
-                                      jl_top_type);
+                                      (jl_value_t*)jl_bottom_type,jl_top_type);
     jl_typetype_type = (jl_datatype_t*)jl_apply_type((jl_value_t*)jl_type_type,
                                                      jl_tuple(1,jl_typetype_tvar));