Remove DCGs that have thrown an exception during term expansion

Some DCG constructs aren't supported and can't be expanded, here we
remove offending DCG rule and don't compile it at all – in a similar
fashion to what we do when incorrect goal was found – whole predicate
isn't getting compiled.

Fixes mthom#2675

src/lib/dcgs.pl

@@ -214,10 +214,13 @@
     dcg_body(GRIf, S0, S1, If),
     dcg_body(GRThen, S1, S, Then).
 
+
+% When DCG expansion throws an exception – remove offending term and rethrow.
+user:term_expansion(throw_dcg_expansion_error(E), _) :-
+    throw(E).
 user:term_expansion(Term0, Term) :-
     nonvar(Term0),
-    dcg_rule(Term0, Term).
-
+    catch(dcg_rule(Term0, Term), E, Term = throw_dcg_expansion_error(E)).
 
 %% seq(Seq)//
 % 

tests/scryer/cli/issues/not_supported_dcg_constructs.in/main.pl

@@ -0,0 +1,6 @@
+:- use_module(library(dcgs)).
+
+d -->
+        (   { true } -> []
+        ;   { true } -> []
+        ).

tests/scryer/cli/issues/not_supported_dcg_constructs.stdout

@@ -0,0 +1 @@
+   error(representation_error(dcg_body),[culprit-({true}->[])]).

tests/scryer/cli/issues/not_supported_dcg_constructs.toml

@@ -0,0 +1,7 @@
+# issue 2675
+args = [
+    "-f",
+    "--no-add-history",
+    "-g", "halt",
+     "main.pl"
+]