sys_core_fold: Eliminate warnings for unused terms in effect context

The optimization introduced in 0a0d39d351fc could cause spurious
warnings of the type: "a term is constructed, but never used".
That would happen for constructs in effect context.

To avoid those warnings, we will need to apply warning suppression
also in effect context.

2 files changed, 30 insertions, 11 deletions

diff --git a/lib/compiler/src/sys_core_fold.erl b/lib/compiler/src/sys_core_fold.erl
index 4068d60ec4..37006cd8ce 100644
--- a/lib/compiler/src/sys_core_fold.erl
+++ b/lib/compiler/src/sys_core_fold.erl
@@ -2493,7 +2493,7 @@ opt_simple_let_2(Let0, Vs0, Arg0, Body, PrevBody, Ctxt, Sub) ->
 		    Arg1;
 		false ->
 		    %% let <Var> = Arg in <OtherVar>  ==>  seq Arg OtherVar
-		    Arg = maybe_suppress_warnings(Arg1, Vs0, PrevBody, Ctxt),
+		    Arg = maybe_suppress_warnings(Arg1, Vs0, PrevBody),
 		    expr(#c_seq{arg=Arg,body=Body}, Ctxt,
 			 sub_new_preserve_types(Sub))
 	    end;
@@ -2501,7 +2501,7 @@ opt_simple_let_2(Let0, Vs0, Arg0, Body, PrevBody, Ctxt, Sub) ->
 	    %% No variables left.
 	    Body;
 	{Vs,Arg1,#c_literal{}} ->
-	    Arg = maybe_suppress_warnings(Arg1, Vs, PrevBody, Ctxt),
+	    Arg = maybe_suppress_warnings(Arg1, Vs, PrevBody),
 	    E = case Ctxt of
 		    effect ->
 			%% Throw away the literal body.
@@ -2520,7 +2520,7 @@ opt_simple_let_2(Let0, Vs0, Arg0, Body, PrevBody, Ctxt, Sub) ->
 	    %%        seq Arg BodyWithoutVar
 	    case is_any_var_used(Vs, Body) of
 		false ->
-		    Arg = maybe_suppress_warnings(Arg1, Vs, PrevBody, Ctxt),
+		    Arg = maybe_suppress_warnings(Arg1, Vs, PrevBody),
 		    expr(#c_seq{arg=Arg,body=Body}, Ctxt,
 			 sub_new_preserve_types(Sub));
 		true ->
@@ -2530,7 +2530,7 @@ opt_simple_let_2(Let0, Vs0, Arg0, Body, PrevBody, Ctxt, Sub) ->
 	    end
     end.
 
-%% maybe_suppress_warnings(Arg, [#c_var{}], PreviousBody, Context) -> Arg'
+%% maybe_suppress_warnings(Arg, [#c_var{}], PreviousBody) -> Arg'
 %%  Try to suppress false warnings when a variable is not used.
 %%  For instance, we don't expect a warning for useless building in:
 %%
@@ -2541,10 +2541,7 @@ opt_simple_let_2(Let0, Vs0, Arg0, Body, PrevBody, Ctxt, Sub) ->
 %%  referenced in the original unoptimized code. If they were, we will
 %%  consider the warning false and suppress it.
 
-maybe_suppress_warnings(Arg, _, _, effect) ->
-    %% Don't suppress any warnings in effect context.
-    Arg;
-maybe_suppress_warnings(Arg, Vs, PrevBody, value) ->
+maybe_suppress_warnings(Arg, Vs, PrevBody) ->
     case should_suppress_warning(Arg) of
 	true ->
 	    Arg;				%Already suppressed.
@@ -2568,8 +2565,16 @@ suppress_warning([H|T]) ->
 		true ->
 		    suppress_warning(cerl:data_es(H) ++ T);
 		false ->
-		    Arg = cerl:set_ann(H, [compiler_generated]),
-		    cerl:c_seq(Arg, suppress_warning(T))
+		    %% Some other thing, such as a function call.
+		    %% This cannot be the compiler's fault, so the
+		    %% warning should not be suppressed. We must
+		    %% be careful not to destroy tail-recursion.
+		    case T of
+			[] ->
+			    H;
+			[_|_] ->
+			    cerl:c_seq(H, suppress_warning(T))
+		    end
 	    end
     end;
 suppress_warning([]) -> void().
diff --git a/lib/compiler/test/warnings_SUITE.erl b/lib/compiler/test/warnings_SUITE.erl
index 471ded219f..ca6946e3cd 100644
--- a/lib/compiler/test/warnings_SUITE.erl
+++ b/lib/compiler/test/warnings_SUITE.erl
@@ -281,6 +281,7 @@ bad_arith(Config) when is_list(Config) ->
 	     {3,sys_core_fold,{eval_failure,badarith}},
 	     {9,sys_core_fold,nomatch_guard},
 	     {9,sys_core_fold,{eval_failure,badarith}},
+	     {9,sys_core_fold,{no_effect,{erlang,is_integer,1}}},
 	     {10,sys_core_fold,nomatch_guard},
 	     {10,sys_core_fold,{eval_failure,badarith}},
 	     {15,sys_core_fold,{eval_failure,badarith}}
@@ -369,7 +370,7 @@ files(Config) when is_list(Config) ->
 
 %% Test warnings for term construction and BIF calls in effect context.
 effect(Config) when is_list(Config) ->
-    Ts = [{lc,
+    Ts = [{effect,
 	   <<"
              t(X) ->
                case X of
@@ -475,6 +476,19 @@ effect(Config) when is_list(Config) ->
              m9(Bs) ->
                 [{B,ok} = {B,foo:bar(B)} || B <- Bs],
                 ok.
+
+             m10(ConfigTableSize) ->
+               case ConfigTableSize of
+                 apa ->
+                   CurrentConfig = {id(camel_phase3),id(sms)},
+                   case CurrentConfig of
+                     {apa, bepa} -> ok;
+		     _ -> ok
+	           end
+               end,
+               ok.
+
+             id(I) -> I.
              ">>,
 	   [],
 	   {warnings,[{5,sys_core_fold,{no_effect,{erlang,is_integer,1}}},