Make assignment (match) in comprehension work as strict generator

erts/emulator/test/bs_utf_SUITE.erl

@@ -359,8 +359,8 @@ utf32_illegal_sequences(Config) when is_list(Config) ->
     utf32_fail_range(16#D800, 16#DFFF),		%Reserved for UTF-16.
     utf32_fail_range(-100, -1),
 
-    <<>> = id(<< 0 || <<X/utf32>> <= <<"àxxx">>, _ = X >>),
-    <<>> = id(<< 0 || <<X/little-utf32>> <= <<"àxxx">>, _ = X >>),
+    <<>> = id(<< 0 || <<X/utf32>> <= <<"àxxx">>, true =:= X >>),
+    <<>> = id(<< 0 || <<X/little-utf32>> <= <<"àxxx">>, true =:= X >>),
 
     ok.
 

lib/compiler/src/v3_core.erl

@@ -2090,6 +2090,7 @@ get_nomatch_total(NomatchModes) ->
             end
         end.
 
+is_generator({match,_,_,_}) -> true;
 is_generator({generate,_,_,_}) -> true;
 is_generator({generate_strict,_,_,_}) -> true;
 is_generator({b_generate,_,_,_}) -> true;
@@ -2189,6 +2190,8 @@ get_qual_anno(Abstract) -> element(2, Abstract).
 %% generator(Line, Generator, Guard, State) -> {Generator',State}.
 %%  Transform a given generator into its #igen{} representation.
 
+generator(Line, {match,L,P,E}, Gs, StrictPats, St0) ->
+    generator(Line, {generate_strict,L,P,{cons,L,E,{nil,L}}}, Gs, StrictPats, St0);
 generator(Line, {Generate,Lg,P0,E}, Gs, StrictPats, St0)
   when Generate =:= generate;
        Generate =:= generate_strict ->

lib/compiler/test/beam_validator_SUITE_data/singleton_inference.erl

@@ -1,7 +1,17 @@
+%% %CopyrightBegin%
+%%
+%% SPDX-License-Identifier: Apache-2.0
+%%
+%% Copyright Ericsson AB 1998-2025. All Rights Reserved.
+%%
+%% %CopyrightEnd%
 -module(singleton_inference).
 -export([test/0]).
 
 test() ->
     {'EXIT',{{badmatch,true}, _}} =
-        catch [0 || (X = (true or (X = is_port(node()))))],
+        catch case X = (true or (X = is_port(node()))) of
+                  true -> 1;
+                  false -> 0
+              end,
     ok.

lib/compiler/test/bs_bincomp_SUITE.erl

@@ -261,7 +261,7 @@ inconsistent_types_2() ->
                    <<
                      Y ||
                        _ <- Y,
-                       (not ((false = Y) = (Y /= []))), (_ = Y)
+                       (not ((false = Y) = (Y /= []))), true =:= (_ = Y)
                    >>
            end
     >>.

lib/compiler/test/receive_SUITE.erl

@@ -192,7 +192,7 @@ coverage(Config) when is_list(Config) ->
 
     %% Cover code for handling a non-boolean `br` in beam_ssa_dead.
     self() ! whatever,
-    {'EXIT',{{badmatch,_},_}} = (catch [a || other = receive whatever -> false end]),
+    {'EXIT',{{badmatch,_},_}} = (catch [a || true =:= (other = receive whatever -> false end)]),
 
     %% Cover code in beam_ssa_pre_codegen.
     self() ! 0,

lib/compiler/test/trycatch_SUITE.erl

@@ -1175,7 +1175,7 @@ grab_bag_3() ->
     try 2 of
         true ->
             <<
-              "" || [V0] = door
+              "" || true =:= ([V0] = door)
             >>
     catch
         error:true:V0 ->

lib/debugger/test/int_eval_SUITE_data/my_int_eval_module.erl

@@ -232,10 +232,10 @@ otp_8310() ->
     a = if (false orelse a) =:= a -> a; true -> b end,
     F1 = fun() -> a end,
     {'EXIT',{{bad_filter,a},_}} =
-        (catch {a, [X || X <- [1,2,3], _ = F1()]}),
+        (catch {a, [X || X <- [1,2,3], F1()]}),
     F2 = fun() -> << 3:8 >> end,
     {'EXIT',{{bad_filter,<<3>>},_}} =
-        (catch {a, << << X >> || << X >> <= << 7:8 >>,_ = F2() >>}),
+        (catch {a, << << X >> || << X >> <= << 7:8 >>, F2() >>}),
     {'EXIT',{{bad_generator,a},_}} =
         (catch {a, [X || X <- a]}),
     {'EXIT',{{bad_generator,b},_}} =

lib/stdlib/src/erl_features.erl

@@ -76,6 +76,14 @@ feature_specs() ->
             experimental => 25,
             approved => 27,
             keywords => ['maybe', 'else'],
+            type => extension},
+      compr_assign =>
+          #{short => "Assignment in comprehensions (EEP77)",
+            description =>
+                "Implementation of '=' assignments in comprehensions proposed in EEP77.",
+            status => experimental,
+            experimental => 29,
+            keywords => [],
             type => extension}}.
 
 %% Return all currently known features.

lib/stdlib/src/erl_lint.erl

@@ -405,6 +405,13 @@ format_error_1(update_literal) ->
     ~"expression updates a literal";
 format_error_1(illegal_zip_generator) ->
     ~"only generators are allowed in a zip generator.";
+format_error_1(compr_assign) ->
+    ~"""
+     matches using '=' are not allowed in comprehension qualifiers
+     unless the experimental 'compr_assign' language feature is enabled.
+     With 'compr_assign' enabled, a match 'P = E' will behave as a
+     strict generator 'P <-:- [E]'."
+     """;
 %% --- patterns and guards ---
 format_error_1(illegal_map_assoc_in_pattern) -> ~"illegal pattern, did you mean to use `:=`?";
 format_error_1(illegal_pattern) -> ~"illegal pattern";
@@ -4114,12 +4121,20 @@ lc_quals([F|Qs], Vt, Uvt, St0) ->
     Info = is_guard_test2_info(St0),
     {Fvt,St1} = case is_guard_test2(F, Info) of
 		    true -> guard_test(F, Vt, St0);
-		    false -> expr(F, Vt, St0)
+		    false -> expr(F, Vt, check_compr_assign(F, St0))
 		end,
     lc_quals(Qs, vtupdate(Fvt, Vt), Uvt, St1);
 lc_quals([], Vt, Uvt, St) ->
     {Vt, Uvt, St}.
 
+check_compr_assign({match,Anno,_,_}, St) ->
+    case lists:member(compr_assign, erl_features:enabled()) of
+        true -> St;
+        false -> add_error(Anno, compr_assign, St)
+    end;
+check_compr_assign(_, St) ->
+    St.
+
 is_guard_test2_info(#lint{records=RDs,locals=Locals,imports=Imports}) ->
     {RDs,fun(FA) ->
 		 is_local_function(Locals, FA) orelse

lib/stdlib/test/erl_lint_SUITE.erl

@@ -365,9 +365,9 @@ unused_vars_warn_lc(Config) when is_list(Config) ->
                   [Z || Z <- (Y = X), % Y unused.
                        Y > X]; % Y unbound.
               k(X) ->
-                  [Y || Y = X > 3, Z = X]; % Z unused.
+                  [Y || true =:= (Y = X > 3), true =:= (Z = X)]; % Z unused.
               k(X) ->
-                  [Z || Y = X > 3, Z = X]. % Y unused.
+                  [Z || true =:= (Y = X > 3), true =:= (Z = X)]. % Y unused.
            ">>,
            [warn_unused_vars],
            {error,[{{8,21},erl_lint,{unbound_var,'Y'}},
@@ -376,8 +376,8 @@ unused_vars_warn_lc(Config) when is_list(Config) ->
                    {{4,34},erl_lint,{unused_var,'Y'}},
                    {{8,34},erl_lint,{unused_var,'Y'}},
                    {{10,31},erl_lint,{unused_var,'Y'}},
-                   {{13,36},erl_lint,{unused_var,'Z'}},
-                   {{15,25},erl_lint,{unused_var,'Y'}}]}},
+                   {{13,57},erl_lint,{unused_var,'Z'}},
+                   {{15,35},erl_lint,{unused_var,'Y'}}]}},
 
           {lc14,
            <<"lc2() ->

lib/stdlib/test/qlc_SUITE.erl

@@ -2977,14 +2977,14 @@ lookup2(Config) when is_list(Config) ->
        <<"%% Only guards are inspected. No lookup.
           etsc(fun(E) ->
                  Q = qlc:q([{X,Y} || {X,Y} <- ets:table(E),
-                                     Y = (X =:= 3)]),
+                                     true =:= (Y = (X =:= 3))]),
                  {'EXIT', {{badmatch,false},_}} = (catch qlc:e(Q))
          end, [{false,3},{true,3}])">>,
 
        <<"%% Only guards are inspected. No lookup.
           etsc(fun(E) ->
                  Q = qlc:q([{X,Y} || {X,Y} <- ets:table(E),
-                                     Y = (X =:= 3)]),
+                                     true =:= (Y = (X =:= 3))]),
                  {'EXIT', {{badmatch,false},_}} = (catch qlc:e(Q))
          end, [{3,true},{4,true}])">>,
 
@@ -2995,7 +2995,7 @@ lookup2(Config) when is_list(Config) ->
           true = ets:insert(E2, [{true,1},{false,2}]),
           Q = qlc:q([{X,Z} || {_,X} <- ets:table(E1),
                               {Y,Z} <- ets:table(E2),
-                              Y = (X =:= 3)]),
+                              true =:= (Y = (X =:= 3))]),
           {'EXIT', {{badmatch,false},_}} = (catch qlc:e(Q)),
           ets:delete(E1),
           ets:delete(E2)">>,

system/doc/reference_manual/features.md

@@ -133,3 +133,6 @@ The following configurable features exist:
   [`maybe`](expressions.md#maybe) expression proposed in
   [EEP 49](https://www.erlang.org/eeps/eep-0049).
   It was approved in Erlang/OTP 27.
+- **`compr_assign` (experimental)** - Implementation of `Pattern = Expr`
+  assignments in [comprehensions](expressions.md#comprehensions) 
+  proposed in [EEP 77](https://www.erlang.org/eeps/eep-0077).