Empty enclosing packs in generic lambdas [GH #500]

src/expr.c

@@ -1454,6 +1454,11 @@ changed below to be TRUE for certain kinds of expressions and/or contexts.
 #endif /* MICROSOFT_EXTENSIONS_ALLOWED */
   if (suppress_warning) {
     /* The caller has requested that warnings be suppressed. */
+  } else if (in_generic_lambda_in_real_instantiation()) {
+    /* A hybrid expansion can collapse to an expression with no effect in
+       this prototype instantiation.  Wait for the real instantiation rather
+       than diagnosing that transient form. */
+    suppress_warning = TRUE;
   } else if (expr_stack->is_type_operator_arg_expression) {
     /* decltype expressions are sometimes written to check SFINAE
        conditions, so do not warn inside them. */

src/pending_changes/gh-500.txt

@@ -0,0 +1,14 @@
+[GH #500]
+Empty enclosing packs in generic lambdas
+
+A pack expansion in a generic lambda nested in a template instantiation could
+produce an error node in the IL when it referenced a lambda parameter pack
+before an empty enclosing parameter pack.  Empty enclosing packs are now
+handled correctly regardless of the order of the pack references.  Spurious
+warnings from prototype instantiations in such cases are also suppressed.  For
+example, with --c++20 -tused:
+
+  template<int... Is> void f() {
+    []<int... Js>() { ((Js * Is) * ... * 1); };
+  }
+  template void f<>();  // Previously produced an error node in the IL.

src/scope_stk.c

@@ -12548,7 +12548,7 @@ have a pack expansion so that it can be expanded in a rescan context.
 }  /* add_pack_expansion_descr_to_prototype_arg */
 
 
-static a_boolean in_generic_lambda_in_real_instantiation(void)
+a_boolean in_generic_lambda_in_real_instantiation(void)
 /*
 Return TRUE if we are in the definition of a generic lambda that is nested
 inside a real instantiation.  In such contexts, a kind of "hybrid expansion"
@@ -12836,6 +12836,11 @@ lengths) *err is set to TRUE, FALSE otherwise.
           /* Got an expansion from an enclosing real instantiation and an
              unexpanded pack from a nested generic lambda. */
           has_hybrid_pack_expansion = TRUE;
+          if (elements_for_pack == 0) {
+            /* An empty enclosing pack makes the expansion empty even if an
+               unexpanded pack from the lambda was encountered first. */
+            elements = 0;
+          }  /* if */
         } else if (!is_rescan) {
           pos_st2_error(ec_pack_length_mismatch, &prp->position,
                         prp->symbol->header->identifier,

src/scope_stk.h

@@ -2632,6 +2632,8 @@ extern void end_prescan_context(
 	a_boolean		saved_in_disambiguation,
 	a_boolean		saved_source_sequence_entries_disallowed);
 
+extern a_boolean in_generic_lambda_in_real_instantiation(void);
+
 extern a_boolean in_generic_lambda_in_prototype_instantiation(void);
 
 extern a_routine_ptr enclosing_nonlambda_routine_for_lambda_class(

tests/tests/imported/clang/cpp/SemaCXX/.lambda-pack-expansion.rto/default.1.1.txt

@@ -54,14 +54,4 @@ fe_only -DTEST_NUMBER=1 --clang -tused -e 200 --no_wrap_diagnostics --clang_vers
           baz<TTPs>;
           ^
 
-"Test_name.c", line 130: error: template template parameter "<error>" is not compatible with template template parameter "<unnamed>"
-          baz<TTPs>;
-              ^
-          detected during instantiation of "void GH112352::S::foldExpr2<TTPs...>() [with TTPs=<>]" at line 139
-
-"Test_name.c", line 131: error: too many arguments for template template parameter "TTPs"
-          TTPs<int> D;
-               ^
-          detected during instantiation of "void GH112352::S::foldExpr2<TTPs...>() [with TTPs=<>]" at line 139
-
-5 errors detected in the compilation of "Test_name.c".
+3 errors detected in the compilation of "Test_name.c".

tests/tests/imported/clang/cpp/SemaCXX/.lambda-pack-expansion.rto/edg_x86_32_cp.1.1.txt

@@ -78,14 +78,4 @@ fe_only -DTEST_NUMBER=1 --clang -tused -e 200 --no_wrap_diagnostics --clang_vers
             instantiation of function "lambda []()->void [with Ts=<int, int>]" at line 55
             instantiation of "void GH63677::f<<unnamed>>() [with <unnamed>=int]" at line 58
 
-"Test_name.c", line 130: error: template template parameter "<error>" is not compatible with template template parameter "<unnamed>"
-          baz<TTPs>;
-              ^
-          detected during instantiation of "void GH112352::S::foldExpr2<TTPs...>() [with TTPs=<>]" at line 139
-
-"Test_name.c", line 131: error: too many arguments for template template parameter "TTPs"
-          TTPs<int> D;
-               ^
-          detected during instantiation of "void GH112352::S::foldExpr2<TTPs...>() [with TTPs=<>]" at line 139
-
-9 errors detected in the compilation of "Test_name.c".
+7 errors detected in the compilation of "Test_name.c".

tests/tests/imported/clang/cpp/SemaCXX/.lambda-pack-expansion.rto/edg_x86_64_cp.1.1.txt

@@ -78,14 +78,4 @@ fe_only -DTEST_NUMBER=1 --clang -tused -e 200 --no_wrap_diagnostics --clang_vers
             instantiation of function "lambda []()->void [with Ts=<int, int>]" at line 55
             instantiation of "void GH63677::f<<unnamed>>() [with <unnamed>=int]" at line 58
 
-"Test_name.c", line 130: error: template template parameter "<error>" is not compatible with template template parameter "<unnamed>"
-          baz<TTPs>;
-              ^
-          detected during instantiation of "void GH112352::S::foldExpr2<TTPs...>() [with TTPs=<>]" at line 139
-
-"Test_name.c", line 131: error: too many arguments for template template parameter "TTPs"
-          TTPs<int> D;
-               ^
-          detected during instantiation of "void GH112352::S::foldExpr2<TTPs...>() [with TTPs=<>]" at line 139
-
-9 errors detected in the compilation of "Test_name.c".
+7 errors detected in the compilation of "Test_name.c".

tests/tests/imported/gnu/cpp/cpp1z/.constexpr-if-lambda5.rto/default.1.1.txt

@@ -2,10 +2,14 @@ fe_only -DTEST_NUMBER=1 --gnu_version=999999 -tused -e 200 --no_wrap --no_wrap_d
 "Test_name.c", line 10: warning: expression has no effect
         (..., ts);
               ^
-          detected during instantiation of "void f(Ts...) [with Ts=<int, char>]" at line 16
+          detected during:
+            instantiation of function "lambda [](auto)->auto [with <auto-1>=int]" at line 12
+            instantiation of "void f(Ts...) [with Ts=<int, char>]" at line 16
 
 "Test_name.c", line 10: warning: expression has no effect
         (..., ts);
         ^
-          detected during instantiation of "void f(Ts...) [with Ts=<int, char>]" at line 16
+          detected during:
+            instantiation of function "lambda [](auto)->auto [with <auto-1>=int]" at line 12
+            instantiation of "void f(Ts...) [with Ts=<int, char>]" at line 16
 

tests/tests/imported/gnu/cpp/cpp1z/.fold-lambda4.rto/default.1.1.txt

@@ -3,6 +3,7 @@ fe_only -DTEST_NUMBER=1 --gnu_version=999999 -tused -e 200 --no_wrap --no_wrap_d
               ys;
               ^
           detected during:
-            instantiation of function "lambda [](auto &...)->auto [with <auto-1>=<int>]" at line 26
+            instantiation of function "lambda [](auto)->auto [with <auto-1>=int]" at line 11
+            instantiation of "void X::f(F) [with F=lambda [](auto)->auto]" at line 26
             instantiation of "void bug(Xs...) [with Xs=<X>]" at line 31
 

tests/tests/regressions/.gh-500-warnings.rto/default.1.1.txt

@@ -0,0 +1,8 @@
+fe_only -DTEST_NUMBER=1 -tused --c++20 Test_name.c
+"Test_name.c", line 17: warning: expression has no effect
+      auto l = []<int... S>() { ((S * V) * ... * 1); };
+                                ^
+          detected during:
+            instantiation of function "lambda []()->auto [with S=<3>]" at line 18
+            instantiation of "void real_instantiation::f<V...>() [with V=<2>]" at line 21
+

tests/tests/regressions/.gh-500-warnings.rto/default.2.1.txt

@@ -0,0 +1,8 @@
+fe_only -DTEST_NUMBER=2 -tused --c++20 --gnu_version 150200 Test_name.c
+"Test_name.c", line 17: warning: expression has no effect
+      auto l = []<int... S>() { ((S * V) * ... * 1); };
+                                ^
+          detected during:
+            instantiation of function "lambda []()->auto [with S=<3>]" at line 18
+            instantiation of "void real_instantiation::f<V...>() [with V=<2>]" at line 21
+

tests/tests/regressions/.gh-500-warnings.rto/default.3.1.txt

@@ -0,0 +1,8 @@
+fe_only -DTEST_NUMBER=3 -tused --c++20 --clang_version 220100 Test_name.c
+"Test_name.c", line 17: warning: expression has no effect
+      auto l = []<int... S>() { ((S * V) * ... * 1); };
+                                ^
+          detected during:
+            instantiation of function "lambda []()->auto [with S=<3>]" at line 18
+            instantiation of "void real_instantiation::f<V...>() [with V=<2>]" at line 21
+

tests/tests/regressions/.gh-500-warnings.rto/default.4.1.txt

@@ -0,0 +1,8 @@
+fe_only -DTEST_NUMBER=4 -tused --ms_c++20 --microsoft_version 1950 Test_name.c
+"Test_name.c", line 17: warning: expression has no effect
+      auto l = []<int... S>() { ((S * V) * ... * 1); };
+                                ^
+          detected during:
+            instantiation of function "lambda []()->auto [with S=<3>]" at line 18
+            instantiation of "void real_instantiation::f<V...>() [with V=<2>]" at line 21
+

tests/tests/regressions/.gh-500.rto/default.1.1.txt

@@ -0,0 +1 @@
+fe_only -DTEST_NUMBER=1 --il_display -tused --c++20 Test_name.c

tests/tests/regressions/.gh-500.rto/default.2.1.txt

@@ -0,0 +1 @@
+fe_only -DTEST_NUMBER=2 --il_display -tused --c++20 --gn 150200 Test_name.c

tests/tests/regressions/.gh-500.rto/default.3.1.txt

@@ -0,0 +1 @@
+fe_only -DTEST_NUMBER=3 --il_display -tused --c++20 --clang_version 220100 Test_name.c

tests/tests/regressions/.gh-500.rto/default.4.1.txt

@@ -0,0 +1 @@
+fe_only -DTEST_NUMBER=4 --il_display -tused --ms_c++20 --microsoft_version 1950 Test_name.c

tests/tests/regressions/gh-500-warnings.sft.cpp

@@ -0,0 +1,22 @@
+//type:fp
+//options:--c++20:--c++20 --gnu_version 150200:--c++20 --clang_version 220100:--ms_c++20 --microsoft_version 1950
+//options_all:-tused
+
+namespace suppressed_in_prototype
+{
+  template<int... V> void f() {
+    []<int... S>() { ((S * V) * ... * 1); };
+  }
+
+  template void f<>();
+}
+
+namespace real_instantiation
+{
+  template<int... V> void f() {
+    auto l = []<int... S>() { ((S * V) * ... * 1); };
+    l.template operator()<3>();
+  }
+
+  template void f<2>();
+}

tests/tests/regressions/gh-500.sft.cpp

@@ -0,0 +1,43 @@
+//type:fp
+//options:--c++20:--c++20 --gn 150200:--c++20 --clang_version 220100:--ms_c++20 --microsoft_version 1950
+//options_all:--il_display -tused
+//filter:grep -E '(tk_error|enk_error)'
+
+namespace minimal
+{
+  template<int... Is> void f() {
+    []<int... Js>() { ((Js * Is) * ... * 1); };
+  }
+  template void f<>();
+}
+
+namespace inner_pack_first
+{
+  template<int... V> constexpr auto f() {
+    return []<int... S>() { return ((S * V) * ... * 1); };
+  }
+
+  static_assert(f<>().template operator()<>() == 1);
+  static_assert(f<2>().template operator()<3>() == 6);
+  static_assert(f<2, 3>().template operator()<4, 5>() == 120);
+}
+
+namespace outer_pack_first
+{
+  template<int... V> constexpr auto f() {
+    return []<int... S>() { return ((V * S) * ... * 1); };
+  }
+
+  static_assert(f<>().template operator()<>() == 1);
+  static_assert(f<2>().template operator()<3>() == 6);
+  static_assert(f<2, 3>().template operator()<4, 5>() == 120);
+}
+
+namespace uninstantiated_lambda
+{
+  template<int... V> void f() {
+    []<int... S>() { (void)((S * V) * ... * 1); };
+  }
+
+  void g() { f<>(); }
+}