gf-cache: avoid listing equivalent types

addresses an issue where we might be find an ambiguity with badly designed methods,
such as the ambiguous Nothing/Missing rules (cf #31602)

src/gf.c

@@ -882,7 +882,7 @@ static jl_method_instance_t *cache_method(
     if (!cache_with_orig && mt) {
         // now examine what will happen if we chose to use this sig in the cache
         // TODO: should we first check `compilationsig <: definition`?
-        temp = ml_matches(mt->defs, 0, compilationsig, -1, 1, world, &min_valid, &max_valid); // TODO: use MAX_UNSPECIALIZED_CONFLICTS?
+        temp = ml_matches(mt->defs, 0, compilationsig, MAX_UNSPECIALIZED_CONFLICTS, 1, world, &min_valid, &max_valid);
         int guards = 0;
         if (temp == jl_false) {
             cache_with_orig = 1;

test/misc.jl

@@ -174,6 +174,9 @@ v11801, t11801 = @timed sin(1)
 
 # interactive utilities
 
+struct ambigconvert; end # inject a problematic `convert` method to ensure it still works
+Base.convert(::Any, v::ambigconvert) = v
+
 import Base.summarysize
 @test summarysize(Core) > (summarysize(Core.Compiler) + Base.summarysize(Core.Intrinsics)) > Core.sizeof(Core)
 @test summarysize(Base) > 100_000 * sizeof(Ptr)