Method overwriting by an ambiguity should also invalidate the method …

…cache

fix #21963

Ref #21965
(cherry picked from commit 8bacb65)

src/gf.c

@@ -1162,10 +1162,10 @@ static int check_ambiguous_visitor(jl_typemap_entry_t *oldentry, struct typemap_
             jl_static_show_func_sig(s, isect);
             jl_printf(s, "\nbefore the new definition.\n");
         }
-        return 1;  // there may be multiple ambiguities, keep going
     }
-    else if (closure->after) {
+    if (!msp || closure->after) {
         // record that this method definition is being partially replaced
+        // (either with a real definition, or an ambiguity error)
         if (closure->shadowed == NULL) {
             closure->shadowed = oldentry->func.value;
         }

test/ambiguous.jl

@@ -85,6 +85,16 @@ cfunction(ambig, Int, (UInt8, Int))  # test for a crash (doesn't throw an error)
 ambig(x, y::Integer) = 3
 @test_throws MethodError ambig(2, 0x03)
 
+# Method overwriting by an ambiguity should also invalidate the method cache (#21963)
+ambig(x::Union{Char, Int8}) = 'r'
+@test ambig('c') == 'r'
+@test ambig(Int8(1)) == 'r'
+@test_throws MethodError ambig(Int16(1))
+ambig(x::Union{Char, Int16}) = 's'
+@test_throws MethodError ambig('c')
+@test ambig(Int8(1)) == 'r'
+@test ambig(Int16(1)) == 's'
+
 # Automatic detection of ambiguities
 module Ambig1
 ambig(x, y) = 1