[m-rev.] for review: Prevent float_regs abort on constrained polymorphic inst.

[Peter Wang]

The float_regs pass made a call to inst_expand (which does not touch an
inst with constrained_inst_vars at the top level) was followed by
get_arg_insts (which expects the inst to be expanded), leading to a
compiler abort.

This fixes compilation of tests/valid/constrained_poly_insts.m
when floating point registers are used.

compiler/float_regs.m:
    Replace a call to inst_expand with
    inst_expand_and_remove_constrained_inst_vars.

    Add require_complete_switch scope to a switch.

compiler/inst_util.m:
compiler/prog_mode.m:
    Add comments for would-be callers of inst_expand and get_arg_insts.
---
 compiler/float_regs.m | 19 ++++++++++++++++++-
 compiler/inst_util.m  |  2 ++
 compiler/prog_mode.m  |  3 +++
 3 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/compiler/float_regs.m b/compiler/float_regs.m
index 4f55da795..c4a68f20f 100644
--- a/compiler/float_regs.m
+++ b/compiler/float_regs.m
@@ -834,7 +834,8 @@ insert_reg_wrappers_unify_goal(GoalExpr0, GoalInfo0, Goal, !InstMap, !Info,
         lambda_info_get_module_info(!.Info, ModuleInfo),
         list.length(Args, Arity),
         instmap_lookup_var(!.InstMap, CellVar, CellVarInst0),
-        inst_expand(ModuleInfo, CellVarInst0, CellVarInst),
+        inst_expand_and_remove_constrained_inst_vars(ModuleInfo,
+            CellVarInst0, CellVarInst),
         ( if
             get_arg_insts(CellVarInst, ConsId, Arity, ArgInsts),
             list.map_corresponding(unify_mode_set_rhs_final_inst(ModuleInfo),
@@ -1419,6 +1420,7 @@ search_pred_inst_info(ModuleInfo, Inst, PredOrFunc, Arity, PredInstInfo) :-
     pred_inst_info::out) is semidet.
 
 search_pred_inst_info_2(ModuleInfo, Inst, PredInstInfo) :-
+    require_complete_switch [Inst]
     (
         Inst = any(_, higher_order(PredInstInfo))
     ;
@@ -1427,6 +1429,21 @@ search_pred_inst_info_2(ModuleInfo, Inst, PredInstInfo) :-
         Inst = defined_inst(InstName),
         inst_lookup(ModuleInfo, InstName, InstB),
         search_pred_inst_info_2(ModuleInfo, InstB, PredInstInfo)
+    ;
+        Inst = constrained_inst_vars(_Vars, _SubInst),
+        % This might be necessary if modecheck_higher_order_call is changed
+        % to accept an inst with constrained_inst_vars at the top level:
+        %   search_pred_inst_info_2(ModuleInfo, SubInst, PredInstInfo)
+        fail
+    ;
+        ( Inst = free
+        ; Inst = free(_)
+        ; Inst = bound(_, _, _)
+        ; Inst = not_reached
+        ; Inst = inst_var(_)
+        ; Inst = abstract_inst(_, _)
+        ),
+        fail
     ).
 
 :- pred get_ho_arg_regs(pred_inst_info::in, list(mer_type)::in,
diff --git a/compiler/inst_util.m b/compiler/inst_util.m
index 9d4355890..1ddfb6b9b 100644
--- a/compiler/inst_util.m
+++ b/compiler/inst_util.m
@@ -43,6 +43,8 @@
     % inst_expand(ModuleInfo, Inst0, Inst) checks if the top-level part
     % of the inst is a defined inst, and if so replaces it with the definition.
     %
+    % This leaves insts with constrained_inst_vars at the top level unchanged.
+    %
 :- pred inst_expand(module_info::in, mer_inst::in, mer_inst::out) is det.
 
     % inst_expand_and_remove_constrained_inst_vars is the same as inst_expand
diff --git a/compiler/prog_mode.m b/compiler/prog_mode.m
index 224d20b4c..6c9caa4bc 100644
--- a/compiler/prog_mode.m
+++ b/compiler/prog_mode.m
@@ -136,6 +136,9 @@
     % insts of the arguments of the top level functor, failing if the
     % inst could not be bound to the functor.
     %
+    % Note that inst_expand does not expand insts with constrained_inst_vars
+    % at the top level.
+    %
 :- pred get_arg_insts(mer_inst::in, cons_id::in, arity::in,
     list(mer_inst)::out) is semidet.
 
-- 
2.16.2