Implement full support for CWG 2355 ("Deducing noexcept-specifiers"). [GH edgcpp#197]

Previously this was a GCC/Clang emulation feature.

Author: cpplearner

src/Changes

@@ -1,3 +1,13 @@
+10/1/26  [GH #197]
+Deducing noexcept-specifiers
+
+Previously, the front end emulated Clang and GCC's ability to deduce simple
+dependent noexcept-specifiers (see the entry for EDGcpfe/18317 of 9/6/17).
+Meanwhile, the standard has been changed through Core issue 2355 to make that
+behavior standard-mandated.  It is now the default behavior, except in
+Microsoft mode with microsoft_version < 1951.
+
+
 Version 7.0, September 28, 2026
 
 9/15/26  [EDGcpfe/28958]

src/cmd_line.c

@@ -3237,7 +3237,7 @@ option values if they were not already set by a command line option.
             exc_spec_in_func_type = TRUE;
           }  /* if */
 #endif /* EXC_SPEC_IN_FUNC_TYPE_ENABLING_POSSIBLE */
-          deduction_from_exc_spec_allowed = FALSE;
+          deduction_from_exc_spec_allowed = microsoft_version >= 1951;
           if (!option_kind_used[(int)optk_aligned_new]) {
             overaligned_allocation_enabled = TRUE;
           }  /* if */
@@ -5928,13 +5928,7 @@ before this routine is called.
       warn_on_deduced_return_types = TRUE;
     }  /* if */
   }  /* if */
-  if (cpp17_mode) {
-    /* g++ and clang allow deduction from the noexcept flag of a function
-       parameter. */
-    if (exc_spec_in_func_type) {
-      deduction_from_exc_spec_allowed = TRUE;
-    }  /* if */
-  } else {
+  if (!cpp17_mode) {
     /* g++ and clang do not implement this as a DR in C++14 mode.  Note that
        clang has this disabled by default even in C++17 mode (as of 6.0.1).
        But we don't have a separate option to enable this, so we still
@@ -13281,7 +13275,7 @@ variables declared in cmd_line.h.
   implicit_noexcept_enabled = FALSE;
   core_constant_expr_is_noexcept = FALSE;
   exc_spec_in_func_type = FALSE;
-  deduction_from_exc_spec_allowed = FALSE;
+  deduction_from_exc_spec_allowed = TRUE;
   delegating_constructors_enabled = FALSE;
   inheriting_constructors_enabled = FALSE;
   constexpr_enabled = FALSE;

src/templates.c

@@ -13889,9 +13889,6 @@ A nontype template parameter is potentially being deduced from a noexcept
 specifier.  For example:
 
 	template<bool B> void f(int (*)() noexcept(B));
-
-This is currently nonstandard, but accepted by GCC and Clang, and the omission
-in the standard is perhaps accidental.
 */
 {
   a_boolean				match = FALSE;
@@ -13910,8 +13907,7 @@ in the standard is perhaps accidental.
     if (deduction_from_exc_spec_allowed &&
         (flags & MTT_ALLOW_STRICTER_NOEXCEPT) == 0 &&
         !constant_bool_value_known_at_compile_time(t_cp)) {
-      /* Some compilers allow deduction from the noexcept flag of a function
-         type.  Do this deduction, if needed. */
+      /* Perform deduction from the noexcept flag of a function type. */
       a_constant_ptr	cp;
       a_boolean		cp_is_local = FALSE;
       if (esp != NULL && esp->variant.noexcept_arg != NULL) {

tests/expectations/edg_x86_32/cwg.log

@@ -1,6 +1,3 @@
-2355a.sft.cpp:cp:-DTEST_NUMBER=1:FAIL:
-2355c.sft.cpp:cp:-DTEST_NUMBER=1:FAIL:
-2355d.sft.cpp:fn:-DTEST_NUMBER=1:FAIL:
 P2615R1a.sft.cpp:fp:-DTEST_NUMBER=1:FAIL:
 P2647R1b.sft.cpp:fp:-DTEST_NUMBER=1:FAIL:
 cwg1028c.sft.cpp:fp:-DTEST_NUMBER=1:FAIL:

tests/expectations/edg_x86_32_cp/cwg.log

@@ -1,6 +1,3 @@
-2355a.sft.cpp:cp:-DTEST_NUMBER=1:FAIL:
-2355c.sft.cpp:cp:-DTEST_NUMBER=1:FAIL:
-2355d.sft.cpp:fn:-DTEST_NUMBER=1:FAIL:
 P2615R1a.sft.cpp:fp:-DTEST_NUMBER=1:FAIL:
 P2647R1b.sft.cpp:fp:-DTEST_NUMBER=1:FAIL:
 cwg1028c.sft.cpp:fp:-DTEST_NUMBER=1:FAIL:

tests/expectations/edg_x86_64/cwg.log

@@ -1,6 +1,3 @@
-2355a.sft.cpp:cp:-DTEST_NUMBER=1:FAIL:
-2355c.sft.cpp:cp:-DTEST_NUMBER=1:FAIL:
-2355d.sft.cpp:fn:-DTEST_NUMBER=1:FAIL:
 P2615R1a.sft.cpp:fp:-DTEST_NUMBER=1:FAIL:
 P2647R1b.sft.cpp:fp:-DTEST_NUMBER=1:FAIL:
 cwg1028c.sft.cpp:fp:-DTEST_NUMBER=1:FAIL:

tests/expectations/edg_x86_64_cp/cwg.log

@@ -1,6 +1,3 @@
-2355a.sft.cpp:cp:-DTEST_NUMBER=1:FAIL:
-2355c.sft.cpp:cp:-DTEST_NUMBER=1:FAIL:
-2355d.sft.cpp:fn:-DTEST_NUMBER=1:FAIL:
 P2615R1a.sft.cpp:fp:-DTEST_NUMBER=1:FAIL:
 P2647R1b.sft.cpp:fp:-DTEST_NUMBER=1:FAIL:
 cwg1028c.sft.cpp:fp:-DTEST_NUMBER=1:FAIL:

tests/tests/cwg/.2355a.rto/default.1.1.txt

@@ -1,18 +1 @@
-cpfe -DTEST_NUMBER=1 --c++20 -tused -A Test_name.c
-"Test_name.c", line 8: error: no instance of function template "f1" matches the argument list
-            argument types are: (void ())
-      f1(g1);    // OK: E is false
-      ^
-"Test_name.c", line 2: note: candidate function template "f1" failed deduction
-    template <bool E> void f1(void (*)() noexcept(E));
-                           ^
-
-"Test_name.c", line 9: error: no instance of function template "f1" matches the argument list
-            argument types are: (void () noexcept)
-      f1(g2);    // OK: E is true
-      ^
-"Test_name.c", line 2: note: candidate function template "f1" failed deduction
-    template <bool E> void f1(void (*)() noexcept(E));
-                           ^
-
-2 errors detected in the compilation of "Test_name.c".
+cpfe -DTEST_NUMBER=1 -tused --c++17 -A Test_name.c

tests/tests/cwg/.2355a.rto/default.10.1.txt

@@ -0,0 +1,18 @@
+fe_only -DTEST_NUMBER=10 -tused --ms_c++14 --microsoft_version 1951 Test_name.c
+"Test_name.c", line 9: error: no instance of function template "f1" matches the argument list
+            argument types are: (void ())
+      f1(g1);    // OK: E is false
+      ^
+"Test_name.c", line 3: note: candidate function template "f1" failed deduction
+    template <bool E> void f1(void (*)() noexcept(E));
+                           ^
+
+"Test_name.c", line 10: error: no instance of function template "f1" matches the argument list
+            argument types are: (void ())
+      f1(g2);    // OK: E is true
+      ^
+"Test_name.c", line 3: note: candidate function template "f1" failed deduction
+    template <bool E> void f1(void (*)() noexcept(E));
+                           ^
+
+2 errors detected in the compilation of "Test_name.c".

tests/tests/cwg/.2355a.rto/default.2.1.txt

@@ -0,0 +1 @@
+cpfe -DTEST_NUMBER=2 -tused --c++17 --gn 160200 Test_name.c