[gcc r15-10552] c++/modules: Stream all REQUIRES_EXPR_PARMS [PR122789]

[Nathaniel Shead]

https://gcc.gnu.org/g:dabaca84b2bce49d2c5e9c98f88cf13ea61a7604

commit r15-10552-gdabaca84b2bce49d2c5e9c98f88cf13ea61a7604
Author: Nathaniel Shead <nathanieloshead@gmail.com>
Date:   Sun Nov 23 23:24:39 2025 +1100

    c++/modules: Stream all REQUIRES_EXPR_PARMS [PR122789]
    
    We don't generally stream the TREE_CHAIN of a DECL, as this would cause
    us to unnecessarily walk into the next member in its scope chain any
    time it was referenced by an expression.
    
    Unfortunately, REQUIRES_EXPR_PARMS is a tree chain of PARM_DECLs, so we
    were only ever streaming the first parameter.  This meant that when a
    parameter's type could not be tsubst'd we would ICE instead of returning
    false.
    
    This patch special-cases REQUIRES_EXPR to always stream the chain of
    decls in its first operand.  As a drive-by improvement we also remove a
    fixme about checking uncontexted PARM_DECLs.
    
            PR c++/122789
    
    gcc/cp/ChangeLog:
    
            * module.cc (trees_out::core_vals): Treat REQUIRES_EXPR
            specially and stream the chained decls of its first operand.
            (trees_in::core_vals): Likewise.
            (trees_out::tree_node): Check the PARM_DECLs we see are what we
            expect.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/modules/concept-12_a.C: New test.
            * g++.dg/modules/concept-12_b.C: New test.
    
    Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
    Reviewed-by: Jason Merrill <jason@redhat.com>
    (cherry picked from commit 53b4e1d951eaf5bbb2fcedfdd156d80a4160878a)

Diff:
---
 gcc/cp/module.cc                            | 37 +++++++++++++++++++----------
 gcc/testsuite/g++.dg/modules/concept-12_a.C |  6 +++++
 gcc/testsuite/g++.dg/modules/concept-12_b.C |  5 ++++
 3 files changed, 36 insertions(+), 12 deletions(-)

diff --git a/gcc/cp/module.cc b/gcc/cp/module.cc
index 8490b64e2330..9b1283692d1a 100644
--- a/gcc/cp/module.cc
+++ b/gcc/cp/module.cc
@@ -6407,13 +6407,19 @@ trees_out::core_vals (tree t)
 	if (has_warning_spec (t))
 	  u (get_warning_spec (t));
 
-      /* Walk in forward order, as (for instance) REQUIRES_EXPR has a
-         bunch of unscoped parms on its first operand.  It's safer to
-         create those in order.  */
       bool vl = TREE_CODE_CLASS (code) == tcc_vl_exp;
-      for (unsigned limit = (vl ? VL_EXP_OPERAND_LENGTH (t)
-			     : TREE_OPERAND_LENGTH (t)),
-	     ix = unsigned (vl); ix != limit; ix++)
+      unsigned limit = (vl ? VL_EXP_OPERAND_LENGTH (t)
+			: TREE_OPERAND_LENGTH (t));
+      unsigned ix = unsigned (vl);
+      if (code == REQUIRES_EXPR)
+	{
+	  /* The first operand of a REQUIRES_EXPR is a tree chain
+	     of PARM_DECLs.  We need to stream this separately as
+	     otherwise we would only stream the first one.  */
+	  chained_decls (REQUIRES_EXPR_PARMS (t));
+	  ++ix;
+	}
+      for (; ix != limit; ix++)
 	WT (TREE_OPERAND (t, ix));
     }
   else
@@ -6989,9 +6995,15 @@ trees_in::core_vals (tree t)
 	put_warning_spec (t, u ());
 
       bool vl = TREE_CODE_CLASS (code) == tcc_vl_exp;
-      for (unsigned limit = (vl ? VL_EXP_OPERAND_LENGTH (t)
-			     : TREE_OPERAND_LENGTH (t)),
-	     ix = unsigned (vl); ix != limit; ix++)
+      unsigned limit = (vl ? VL_EXP_OPERAND_LENGTH (t)
+			: TREE_OPERAND_LENGTH (t));
+      unsigned ix = unsigned (vl);
+      if (code == REQUIRES_EXPR)
+	{
+	  REQUIRES_EXPR_PARMS (t) = chained_decls ();
+	  ++ix;
+	}
+      for (; ix != limit; ix++)
 	RTU (TREE_OPERAND (t, ix));
     }
 
@@ -10022,9 +10034,10 @@ trees_out::tree_node (tree t)
 	      break;
 
 	    case PARM_DECL:
-	      /* REQUIRES_EXPRs have a tree list of uncontexted
-		 PARM_DECLS.  It'd be nice if they had a
-		 distinguishing flag to double check.  */
+	      /* REQUIRES_EXPRs have a chain of uncontexted PARM_DECLS,
+		 and an implicit this parm in an NSDMI has no context.  */
+	      gcc_checking_assert (CONSTRAINT_VAR_P (t)
+				   || DECL_NAME (t) == this_identifier);
 	      break;
 	    }
 	  goto by_value;
diff --git a/gcc/testsuite/g++.dg/modules/concept-12_a.C b/gcc/testsuite/g++.dg/modules/concept-12_a.C
new file mode 100644
index 000000000000..9d370d35bab6
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/concept-12_a.C
@@ -0,0 +1,6 @@
+// PR c++/122789
+// { dg-additional-options "-fmodules -fconcepts" }
+// { dg-module-cmi M }
+
+export module M;
+template <typename T> constexpr bool b = requires(int, typename T::U x) { x; };
diff --git a/gcc/testsuite/g++.dg/modules/concept-12_b.C b/gcc/testsuite/g++.dg/modules/concept-12_b.C
new file mode 100644
index 000000000000..83353caeacca
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/concept-12_b.C
@@ -0,0 +1,5 @@
+// PR c++/122789
+// { dg-additional-options "-fmodules -fconcepts" }
+
+module M;
+static_assert(!b<int>);