fix missing methods in ml_matches results (JuliaLang#50962)

This was resulting in it being too aggressive at filtering out
"duplicate" results, resulting in possible inference mistakes or missing
guardsig entries.

Fixes:
JuliaLang#50722 (comment)

Author: vtjnash

src/gf.c

@@ -3370,7 +3370,7 @@ static int sort_mlmatches(jl_array_t *t, size_t idx, arraylist_t *visited, array
             continue; // already part of this cycle
         jl_method_match_t *matc2 = (jl_method_match_t*)jl_array_ptr_ref(t, childidx);
         jl_method_t *m2 = matc2->method;
-        int subt2 = matc2->fully_covers != NOT_FULLY_COVERS; // jl_subtype((jl_value_t*)type, (jl_value_t*)m2->sig)
+        int subt2 = matc2->fully_covers == FULLY_COVERS; // jl_subtype((jl_value_t*)type, (jl_value_t*)m2->sig)
         // TODO: we could change this to jl_has_empty_intersection(ti, (jl_value_t*)matc2->spec_types);
         // since we only care about sorting of the intersections the user asked us about
         if (!subt2 && jl_has_empty_intersection(m2->sig, m->sig))
@@ -3515,7 +3515,7 @@ static int sort_mlmatches(jl_array_t *t, size_t idx, arraylist_t *visited, array
                     size_t idx2 = (size_t)stack->items[j];
                     jl_method_match_t *matc2 = (jl_method_match_t*)jl_array_ptr_ref(t, idx2);
                     jl_method_t *m2 = matc2->method;
-                    int subt2 = matc2->fully_covers != NOT_FULLY_COVERS; // jl_subtype((jl_value_t*)type, (jl_value_t*)m2->sig)
+                    int subt2 = matc2->fully_covers == FULLY_COVERS; // jl_subtype((jl_value_t*)type, (jl_value_t*)m2->sig)
                     // if their intersection contributes to the ambiguity cycle
                     // and the contribution of m is fully ambiguous with the portion of the cycle from m2
                     if (subt2 || jl_subtype((jl_value_t*)ti, m2->sig)) {

test/ambiguous.jl

@@ -288,6 +288,30 @@ for f in (Ambig8.f, Ambig8.g)
     @test f(Int8(0)) == 4
     @test_throws MethodError f(0)
     @test_throws MethodError f(pi)
+    let ambig = Ref{Int32}(0)
+        ms = Base._methods_by_ftype(Tuple{typeof(f), Union{Int,AbstractIrrational}}, nothing, 10, Base.get_world_counter(), false, Ref{UInt}(typemin(UInt)), Ref{UInt}(typemax(UInt)), ambig)
+        @test ms isa Vector
+        @test length(ms) == 2
+        @test ambig[] == 1
+    end
+    let ambig = Ref{Int32}(0)
+        ms = Base._methods_by_ftype(Tuple{typeof(f), Union{Int,AbstractIrrational}}, nothing, -1, Base.get_world_counter(), false, Ref{UInt}(typemin(UInt)), Ref{UInt}(typemax(UInt)), ambig)
+        @test ms isa Vector
+        @test length(ms) == 2
+        @test ambig[] == 1
+    end
+    let ambig = Ref{Int32}(0)
+        ms = Base._methods_by_ftype(Tuple{typeof(f), Union{Int,AbstractIrrational}}, nothing, 10, Base.get_world_counter(), true, Ref{UInt}(typemin(UInt)), Ref{UInt}(typemax(UInt)), ambig)
+        @test ms isa Vector
+        @test length(ms) == 3
+        @test ambig[] == 1
+    end
+    let ambig = Ref{Int32}(0)
+        ms = Base._methods_by_ftype(Tuple{typeof(f), Union{Int,AbstractIrrational}}, nothing, -1, Base.get_world_counter(), true, Ref{UInt}(typemin(UInt)), Ref{UInt}(typemax(UInt)), ambig)
+        @test ms isa Vector
+        @test length(ms) == 3
+        @test ambig[] == 1
+    end
 end
 
 module Ambig9