Fix a bug reported by Peter Ross.

Estimated hours taken: 6
Branches: main, release

Fix a bug reported by Peter Ross.

compiler/typeclasses.m:
	When recovering after an unsatisfiable constraint, don't merely
	delete the existing unproven constraints.  Instead construct new
	hlds_constraints from scratch.  This ensures that the invariants
	on it are maintained.

tests/invalid/Mmakefile:
tests/invalid/unsatisfiable_constraint_bug.err_exp:
tests/invalid/unsatisfiable_constraint_bug.m:
	Regression test.

compiler/typeclasses.m

@@ -126,10 +126,13 @@ perform_context_reduction(OrigTypeAssignSet, !Info) :-
         Spec = report_unsatisfiable_constraints(!.Info, TypeAssignSet0),
         typecheck_info_add_error(Spec, !Info),
         DeleteConstraints = (pred(TA0::in, TA::out) is det :-
+            % Make a new hlds_constraints structure for the type assign,
+            % with the same assumed constraints but all unproven constraints
+            % deleted.
             type_assign_get_typeclass_constraints(TA0, Constraints0),
-            Constraints = (Constraints0
-                ^ unproven := [])
-                ^ redundant := multi_map.init,
+            type_assign_get_typevarset(TA0, TVarSet),
+            make_hlds_constraints(ClassTable, TVarSet, [],
+                Constraints0 ^ assumed, Constraints),
             type_assign_set_typeclass_constraints(Constraints, TA0, TA)
         ),
         list.map(DeleteConstraints, OrigTypeAssignSet, NewTypeAssignSet),

tests/invalid/Mmakefile

@@ -209,6 +209,7 @@ SINGLEMODULE= \
 	uniq_mutable \
 	uniq_neg \
 	unsatisfiable_constraint \
+	unsatisfiable_constraint_bug \
 	unsatisfiable_super \
 	user_eq_dummy \
 	uu_type \

tests/invalid/unsatisfiable_constraint_bug.err_exp

@@ -0,0 +1,9 @@
+unsatisfiable_constraint_bug.m:023: In clause for predicate `test_stream'/5:
+unsatisfiable_constraint_bug.m:023:   unsatisfiable typeclass constraint:
+unsatisfiable_constraint_bug.m:023:   `stream.writer(S, float, B)'.
+unsatisfiable_constraint_bug.m:025: In clause for predicate `test_stream'/5:
+unsatisfiable_constraint_bug.m:025:   unsatisfiable typeclass constraint:
+unsatisfiable_constraint_bug.m:025:   `stream.reader(S, float, B, E)'.
+unsatisfiable_constraint_bug.m:026: In clause for predicate `test_stream'/5:
+unsatisfiable_constraint_bug.m:026:   unsatisfiable typeclass constraint:
+unsatisfiable_constraint_bug.m:026:   `stream.reader(S, float, State, E)'.

tests/invalid/unsatisfiable_constraint_bug.m

@@ -0,0 +1,33 @@
+% The code caused the compiler the abort, rather than report an error
+% for the unsatisfiable constraints.
+
+% Software Error: map.lookup: key not found
+% Key Type: term.var(parse_tree.prog_data.prog_var_type)
+% Key Value: var(6)
+% Value Type: parse_tree.prog_data.mer_type
+
+:- module unsatisfiable_constraint_bug.
+:- interface.
+:- import_module io.
+:- import_module stream.
+
+:- pred test_stream(S::in, B::di, B::uo, io::di, io::uo) is det
+        <= (reader(S, int, B, E), writer(S, int, B)).
+            % XXX add these two lines and the program compiles
+            % reader(S, float, B, E), writer(S, float, B)).
+
+:- implementation.
+
+test_stream(S, !Buffer, !IO) :-
+	stream.put(S, 10, !Buffer),
+	stream.put(S, 3.14, !Buffer),
+	stream.get(S, ResultA : stream.result(int, E), !Buffer),
+	stream.get(S, ResultB : stream.result(float, E), !Buffer),
+	stream.get(S, ResultC : stream.result(float, E), !Buffer),
+	io.write(ResultA, !IO),
+	io.nl(!IO),
+	io.write(ResultB, !IO),
+	io.nl(!IO),
+	io.write(ResultC, !IO),
+	io.nl(!IO).
+