Make type pass realize that optimization is safe

Consider:

    f(<<_:8>>) ->
        ok;
    f(A) ->
        try
            << (ok) || <<_:ok>> <= A>>
        after
            ok
        end.

The type analysis pass would falsely assume that the binary generator
`A` would always fail, which would cause the generation of unsafe
code.

lib/compiler/src/beam_ssa_opt.erl

@@ -1660,6 +1660,8 @@ live_opt_is([#b_set{op={succeeded,guard},dst=SuccDst,args=[Dst]}=SuccI,
                     I = I0#b_set{op={bif,is_tuple},dst=SuccDst},
                     live_opt_is([I|Is], Live0, Acc);
                 bs_start_match ->
+                    %% This is safe in Erlang/OTP 27 and later, because match
+                    %% contexts are now mutable sub binaries.
                     [#b_literal{val=new},Bin] = I0#b_set.args,
                     I = I0#b_set{op={bif,is_bitstring},args=[Bin],dst=SuccDst},
                     live_opt_is([I|Is], Live0, Acc);

lib/compiler/src/beam_ssa_type.erl

@@ -2745,8 +2745,8 @@ infer_type({bif,is_atom}, [#b_var{}=Arg], _Ts, _Ds) ->
 infer_type({bif,is_binary}, [#b_var{}=Arg], _Ts, _Ds) ->
     T = {Arg, #t_bitstring{size_unit=8}},
     {[T], [T]};
-infer_type({bif,is_bitstring}, [#b_var{}=Arg], _Ts, _Ds) ->
-    T = {Arg, #t_bitstring{}},
+infer_type({bif,is_bitstring}, [#b_var{}=Arg], Ts, _Ds) ->
+    T = {Arg, beam_types:meet(concrete_type(Arg, Ts), #t_bs_matchable{})},
     {[T], [T]};
 infer_type({bif,is_boolean}, [#b_var{}=Arg], _Ts, _Ds) ->
     T = {Arg, beam_types:make_boolean()},

lib/compiler/test/bs_match_SUITE.erl

@@ -55,7 +55,8 @@
          trim_bs_start_match_resume/1,
          gh_6410/1,bs_match/1,
          binary_aliases/1,gh_6923/1,
-         bs_test_tail/1]).
+         bs_test_tail/1,
+         otp_19019/1]).
 
 -export([coverage_id/1,coverage_external_ignore/2]).
 
@@ -98,7 +99,8 @@ groups() ->
        trim_bs_start_match_resume,
        gh_6410,bs_match,binary_aliases,
        gh_6923,
-       bs_test_tail]}].
+       bs_test_tail,
+       otp_19019]}].
 
 init_per_suite(Config) ->
     test_lib:recompile(?MODULE),
@@ -3255,6 +3257,27 @@ bs_test_tail_float(<<>>) -> [].
 bs_test_partial_tail(<<0:8, T/binary>>) -> bs_test_partial_tail(T);
 bs_test_partial_tail(<<>>) -> ok.
 
+otp_19019(_Config) ->
+    ok = do_otp_19019(id(<<42>>)),
+    <<>> = do_otp_19019(id(<<>>)),
+
+    ok.
+
+do_otp_19019(<<_:8>>) ->
+    ok;
+do_otp_19019(A) ->
+    try
+        %% The `bs_start_match` instruction would be replaced with an
+        %% `is_bitstring/1` test, which is in Erlang/OTP 27 and later is
+        %% safe even if `A` is a match context. However, the type analysis
+        %% pass would assume that the `is_bitstring/1` test would always
+        %% fail.
+        << (ok) || <<_:ok>> <= A>>
+    after
+        ok
+    end.
+
+
 %%% Utilities.
 
 id(I) -> I.