c++: extern thread_local declarations in constexpr [PR104994]

C++14 to C++20 apparently should allow extern thread_local declarations in
constexpr functions, however useless they are there (because accessing
such vars is not valid in a constant expression, perhaps sizeof/decltype).
P2242 changed that for C++23 to passing through declaration but
https://cplusplus.github.io/CWG/issues/2552.html
has been filed for it yesterday.

The following patch implements the proposed wording of CWG 2552 in addition
to fixing the C++14 - C++20 handling bug.
If you'd like instead to keep the current pedantic C++23 wording for now,
that would mean taking out the first hunk (cxx_eval_constant_expression) and
g++.dg/cpp23/constexpr-nonlit2.C hunk.

2022-03-24  Jakub Jelinek  <jakub@redhat.com>

	PR c++/104994
	* constexpr.cc (cxx_eval_constant_expression): Don't diagnose passing
	through extern thread_local declarations.  Change wording from
	declaration to definition.
	(potential_constant_expression_1): Don't diagnose extern thread_local
	declarations.  Change wording from declared to defined.
	* decl.cc (start_decl): Likewise.

	* g++.dg/diagnostic/constexpr1.C: Change expected diagnostic wording
	from declared to defined.
	* g++.dg/cpp23/constexpr-nonlit1.C: Likewise.
	(garply): Change dg-error into dg-bogus.
	* g++.dg/cpp23/constexpr-nonlit2.C: Change expected diagnostic wording
	from declaration to definition.
	* g++.dg/cpp23/constexpr-nonlit6.C: Change expected diagnostic wording
	from declared to defined.
	* g++.dg/cpp23/constexpr-nonlit7.C: New test.
	* g++.dg/cpp2a/constexpr-try5.C: Change expected diagnostic wording
	from declared to defined.
	* g++.dg/cpp2a/consteval3.C: Likewise.

9 files changed, 37 insertions, 30 deletions

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index a3136ce819d..ce4ef8edfac 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -6723,17 +6723,18 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
 	  }
 
 	if (VAR_P (r)
-	    && (TREE_STATIC (r) || CP_DECL_THREAD_LOCAL_P (r))
+	    && (TREE_STATIC (r)
+		|| (CP_DECL_THREAD_LOCAL_P (r) && !DECL_REALLY_EXTERN (r)))
 	    /* Allow __FUNCTION__ etc.  */
 	    && !DECL_ARTIFICIAL (r))
 	  {
 	    if (!ctx->quiet)
 	      {
 		if (CP_DECL_THREAD_LOCAL_P (r))
-		  error_at (loc, "control passes through declaration of %qD "
+		  error_at (loc, "control passes through definition of %qD "
 				 "with thread storage duration", r);
 		else
-		  error_at (loc, "control passes through declaration of %qD "
+		  error_at (loc, "control passes through definition of %qD "
 				 "with static storage duration", r);
 	      }
 	    *non_constant_p = true;
@@ -9188,17 +9189,17 @@ potential_constant_expression_1 (tree t, bool want_rval, bool strict, bool now,
       tmp = DECL_EXPR_DECL (t);
       if (VAR_P (tmp) && !DECL_ARTIFICIAL (tmp))
 	{
-	  if (CP_DECL_THREAD_LOCAL_P (tmp))
+	  if (CP_DECL_THREAD_LOCAL_P (tmp) && !DECL_REALLY_EXTERN (tmp))
 	    {
 	      if (flags & tf_error)
-		error_at (DECL_SOURCE_LOCATION (tmp), "%qD declared "
+		error_at (DECL_SOURCE_LOCATION (tmp), "%qD defined "
 			  "%<thread_local%> in %<constexpr%> context", tmp);
 	      return false;
 	    }
 	  else if (TREE_STATIC (tmp))
 	    {
 	      if (flags & tf_error)
-		error_at (DECL_SOURCE_LOCATION (tmp), "%qD declared "
+		error_at (DECL_SOURCE_LOCATION (tmp), "%qD defined "
 			  "%<static%> in %<constexpr%> context", tmp);
 	      return false;
 	    }
diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index efef1b7370f..68741bbf5d2 100644
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -5774,15 +5774,15 @@ start_decl (const cp_declarator *declarator,
       && cxx_dialect < cxx23)
     {
       bool ok = false;
-      if (CP_DECL_THREAD_LOCAL_P (decl))
+      if (CP_DECL_THREAD_LOCAL_P (decl) && !DECL_REALLY_EXTERN (decl))
 	error_at (DECL_SOURCE_LOCATION (decl),
-		  "%qD declared %<thread_local%> in %qs function only "
+		  "%qD defined %<thread_local%> in %qs function only "
 		  "available with %<-std=c++2b%> or %<-std=gnu++2b%>", decl,
 		  DECL_IMMEDIATE_FUNCTION_P (current_function_decl)
 		  ? "consteval" : "constexpr");
       else if (TREE_STATIC (decl))
 	error_at (DECL_SOURCE_LOCATION (decl),
-		  "%qD declared %<static%> in %qs function only available "
+		  "%qD defined %<static%> in %qs function only available "
 		  "with %<-std=c++2b%> or %<-std=gnu++2b%>", decl,
 		  DECL_IMMEDIATE_FUNCTION_P (current_function_decl)
 		  ? "consteval" : "constexpr");
diff --git a/gcc/testsuite/g++.dg/cpp23/constexpr-nonlit1.C b/gcc/testsuite/g++.dg/cpp23/constexpr-nonlit1.C
index c80ea38732f..c60fb8cdbd8 100644
--- a/gcc/testsuite/g++.dg/cpp23/constexpr-nonlit1.C
+++ b/gcc/testsuite/g++.dg/cpp23/constexpr-nonlit1.C
@@ -23,7 +23,7 @@ baz (int x)
 {
   if (!x)
     return 1;
-  static int a;	// { dg-error "'a' declared 'static' in 'constexpr' function only available with" "" { target c++20_down } }
+  static int a;	// { dg-error "'a' defined 'static' in 'constexpr' function only available with" "" { target c++20_down } }
   return ++a;	// { dg-error "uninitialized variable 'a' in 'constexpr' function" "" { target c++17_down } .-1 }
 }
 
@@ -32,7 +32,7 @@ qux (int x)
 {
   if (!x)
     return 1;
-  thread_local int a;	// { dg-error "'a' declared 'thread_local' in 'constexpr' function only available with" "" { target c++20_down } }
+  thread_local int a;	// { dg-error "'a' defined 'thread_local' in 'constexpr' function only available with" "" { target c++20_down } }
   return ++a;	// { dg-error "uninitialized variable 'a' in 'constexpr' function" "" { target c++17_down } .-1 }
 }
 
@@ -41,7 +41,7 @@ garply (int x)
 {
   if (!x)
     return 1;
-  extern thread_local int a;	// { dg-error "'a' declared 'thread_local' in 'constexpr' function only available with" "" { target c++20_down } }
+  extern thread_local int a;	// { dg-bogus "'thread_local' in 'constexpr' function only available with" "" { target c++20_down } }
   return ++a;
 }
 
diff --git a/gcc/testsuite/g++.dg/cpp23/constexpr-nonlit2.C b/gcc/testsuite/g++.dg/cpp23/constexpr-nonlit2.C
index 0f7b2299e7a..b01f728291d 100644
--- a/gcc/testsuite/g++.dg/cpp23/constexpr-nonlit2.C
+++ b/gcc/testsuite/g++.dg/cpp23/constexpr-nonlit2.C
@@ -24,7 +24,7 @@ baz (int x)
 {
   if (!x)
     return 1;
-  static int a;		// { dg-error "control passes through declaration of 'a' with static storage duration" }
+  static int a;		// { dg-error "control passes through definition of 'a' with static storage duration" }
   return ++a;
 }
 
@@ -33,7 +33,7 @@ qux (int x)
 {
   if (!x)
     return 1;
-  thread_local int a;	// { dg-error "control passes through declaration of 'a' with thread storage duration" }
+  thread_local int a;	// { dg-error "control passes through definition of 'a' with thread storage duration" }
   return ++a;
 }
 
diff --git a/gcc/testsuite/g++.dg/cpp23/constexpr-nonlit6.C b/gcc/testsuite/g++.dg/cpp23/constexpr-nonlit6.C
index 11cb5189c85..fbeb83075b0 100644
--- a/gcc/testsuite/g++.dg/cpp23/constexpr-nonlit6.C
+++ b/gcc/testsuite/g++.dg/cpp23/constexpr-nonlit6.C
@@ -13,13 +13,13 @@ lab:
 constexpr int
 bar ()
 {
-  static int a;		// { dg-error "'a' declared 'static' in 'constexpr' context" }
+  static int a;		// { dg-error "'a' defined 'static' in 'constexpr' context" }
   return ++a;
 }
 
 constexpr int
 baz (int x)
 {
-  thread_local int a;	// { dg-error "'a' declared 'thread_local' in 'constexpr' context" }
+  thread_local int a;	// { dg-error "'a' defined 'thread_local' in 'constexpr' context" }
   return ++a;
 }
diff --git a/gcc/testsuite/g++.dg/cpp23/constexpr-nonlit7.C b/gcc/testsuite/g++.dg/cpp23/constexpr-nonlit7.C
new file mode 100644
index 00000000000..06125756f25
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp23/constexpr-nonlit7.C
@@ -0,0 +1,6 @@
+// PR c++/104994
+// CWG2552
+// { dg-do compile { target c++14 } }
+
+constexpr bool foo () { extern thread_local int t; return true; }
+static constexpr bool a = foo ();
diff --git a/gcc/testsuite/g++.dg/cpp2a/consteval3.C b/gcc/testsuite/g++.dg/cpp2a/consteval3.C
index 8f9316411b5..83463868668 100644
--- a/gcc/testsuite/g++.dg/cpp2a/consteval3.C
+++ b/gcc/testsuite/g++.dg/cpp2a/consteval3.C
@@ -56,8 +56,8 @@ template consteval float f12 (float x); // { dg-error "explicit instantiation sh
 consteval int
 f13 (int x)
 {
-  static int a = 5;		// { dg-error "'a' declared 'static' in 'consteval' function only available with" "" { target c++20_only } }
-				// { dg-error "'a' declared 'static' in 'constexpr' context" "" { target c++23 } .-1 }
-  thread_local int b = 6;	// { dg-error "'b' declared 'thread_local' in 'consteval' function only available with" "" { target c++20_only } }
+  static int a = 5;		// { dg-error "'a' defined 'static' in 'consteval' function only available with" "" { target c++20_only } }
+				// { dg-error "'a' defined 'static' in 'constexpr' context" "" { target c++23 } .-1 }
+  thread_local int b = 6;	// { dg-error "'b' defined 'thread_local' in 'consteval' function only available with" "" { target c++20_only } }
   return x;
 }
diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-try5.C b/gcc/testsuite/g++.dg/cpp2a/constexpr-try5.C
index ed5e40dff95..216634dc56c 100644
--- a/gcc/testsuite/g++.dg/cpp2a/constexpr-try5.C
+++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-try5.C
@@ -5,14 +5,14 @@
 constexpr int foo ()
 try {			// { dg-warning "function-try-block body of 'constexpr' function only available with" "" { target c++17_down } }
   int a;		// { dg-error "uninitialized variable 'a' in 'constexpr' function" "" { target c++17_down } }
-  static double b = 1.0;// { dg-error "'b' declared 'static' in 'constexpr' function only available with" "" { target c++20_down } }
-			// { dg-error "'b' declared 'static' in 'constexpr' context" "" { target c++23 } .-1 }
+  static double b = 1.0;// { dg-error "'b' defined 'static' in 'constexpr' function only available with" "" { target c++20_down } }
+			// { dg-error "'b' defined 'static' in 'constexpr' context" "" { target c++23 } .-1 }
   goto l;		// { dg-error "'goto' in 'constexpr' function only available with" "" { target c++20_down } }
   l:;
   return 0;
 } catch (...) {
   long int c;		// { dg-error "uninitialized variable 'c' in 'constexpr' function" "" { target c++17_down } }
-  static float d = 2.0f;// { dg-error "'d' declared 'static' in 'constexpr' function only available with" "" { target c++20_down } }
+  static float d = 2.0f;// { dg-error "'d' defined 'static' in 'constexpr' function only available with" "" { target c++20_down } }
   goto l2;		// { dg-error "'goto' in 'constexpr' function only available with" "" { target c++20_down } }
   l2:;
   return -1;
@@ -21,20 +21,20 @@ try {			// { dg-warning "function-try-block body of 'constexpr' function only av
 constexpr int bar ()
 {
   int a;		// { dg-error "uninitialized variable 'a' in 'constexpr' function" "" { target c++17_down } }
-  static long double b = 3.0;// { dg-error "'b' declared 'static' in 'constexpr' function only available with" "" { target c++20_down } }
-			// { dg-error "'b' declared 'static' in 'constexpr' context" "" { target c++23 } .-1 }
+  static long double b = 3.0;// { dg-error "'b' defined 'static' in 'constexpr' function only available with" "" { target c++20_down } }
+			// { dg-error "'b' defined 'static' in 'constexpr' context" "" { target c++23 } .-1 }
   goto l;		// { dg-error "'goto' in 'constexpr' function only available with" "" { target c++20_down } }
   l:;
   try {			// { dg-warning "'try' in 'constexpr' function only available with" "" { target c++17_down } }
     short c;		// { dg-error "uninitialized variable 'c' in 'constexpr' function" "" { target c++17_down } }
-    static float d;	// { dg-error "'d' declared 'static' in 'constexpr' function only available with" "" { target c++20_down } }
+    static float d;	// { dg-error "'d' defined 'static' in 'constexpr' function only available with" "" { target c++20_down } }
 			// { dg-error "uninitialized variable 'd' in 'constexpr' function" "" { target c++17_down } .-1 }
     goto l2;		// { dg-error "'goto' in 'constexpr' function only available with" "" { target c++20_down } }
     l2:;
     return 0;
   } catch (int) {
     char e;		// { dg-error "uninitialized variable 'e' in 'constexpr' function" "" { target c++17_down } }
-    static int f = 5;	// { dg-error "'f' declared 'static' in 'constexpr' function only available with" "" { target c++20_down } }
+    static int f = 5;	// { dg-error "'f' defined 'static' in 'constexpr' function only available with" "" { target c++20_down } }
     goto l3;		// { dg-error "'goto' in 'constexpr' function only available with" "" { target c++20_down } }
     l3:;
     return 1;
diff --git a/gcc/testsuite/g++.dg/diagnostic/constexpr1.C b/gcc/testsuite/g++.dg/diagnostic/constexpr1.C
index f2bcec6e4e1..c962a60c847 100644
--- a/gcc/testsuite/g++.dg/diagnostic/constexpr1.C
+++ b/gcc/testsuite/g++.dg/diagnostic/constexpr1.C
@@ -1,7 +1,7 @@
 // { dg-do compile { target c++11 } }
 
-constexpr int foo() { thread_local int i __attribute__((unused)) {}; return 1; }  // { dg-error "40:.i. declared .thread_local." "" { target c++20_down } }
-// { dg-error "40:.i. declared .thread_local. in .constexpr. context" "" { target c++23 } .-1 }
+constexpr int foo() { thread_local int i __attribute__((unused)) {}; return 1; }  // { dg-error "40:.i. defined .thread_local." "" { target c++20_down } }
+// { dg-error "40:.i. defined .thread_local. in .constexpr. context" "" { target c++23 } .-1 }
 
-constexpr int bar() { static int i __attribute__((unused)) {}; return 1; }  // { dg-error "34:.i. declared .static." "" { target c++20_down } }
-// { dg-error "34:.i. declared .static. in .constexpr. context" "" { target c++23 } .-1 }
+constexpr int bar() { static int i __attribute__((unused)) {}; return 1; }  // { dg-error "34:.i. defined .static." "" { target c++20_down } }
+// { dg-error "34:.i. defined .static. in .constexpr. context" "" { target c++23 } .-1 }