[PATCH, fortran] Followup on PR103367

From 7e53117e487fac5a1d4f209b40070921ae8a0416 Mon Sep 17 00:00:00 2001
From: Christopher Albert <[email protected]>
Date: Wed, 8 Apr 2026 22:37:11 +0200
Subject: [PATCH] fortran: Diagnose invalid array initializer after parameter
 substitution [PR103367]

Keep the trunk fallback from r16-8509, but turn it into a real
constant-expression error instead of silently returning an empty
constructor.  This avoids the ICE from PR103367 without regressing
the more specific diagnostics discussed in the Bugzilla follow-up.

	PR fortran/103367

gcc/fortran/ChangeLog:

	* trans-array.cc (gfc_conv_array_initializer): Emit an error for
	invalid residual initializer expressions before returning a safe
	empty constructor.

gcc/testsuite/ChangeLog:

	* gfortran.dg/pr103367.f90: Expect a constant-expression error and
	prune the legacy-extension warning.

Signed-off-by: Christopher Albert <[email protected]>
---
 gcc/fortran/trans-array.cc             | 10 +++++++---
 gcc/testsuite/gfortran.dg/pr103367.f90 |  7 ++++---
 2 files changed, 11 insertions(+), 6 deletions(-)

diff --git a/gcc/fortran/trans-array.cc b/gcc/fortran/trans-array.cc
index ed299c8b21b..b57c18aba00 100644
--- a/gcc/fortran/trans-array.cc
+++ b/gcc/fortran/trans-array.cc
@@ -7099,13 +7099,17 @@ gfc_conv_array_initializer (tree type, gfc_expr * expr)
     expr = expr->symtree->n.sym->value;
 
   /* After parameter substitution the expression should be a constant, array
-     constructor, structure constructor, or NULL.  Anything else means the
-     frontend already diagnosed an error; return a zero-filled array.  */
+     constructor, structure constructor, or NULL.  Anything else is invalid
+     and must not ICE later in lowering.  */
   if (expr->expr_type != EXPR_CONSTANT
       && expr->expr_type != EXPR_STRUCTURE
       && expr->expr_type != EXPR_ARRAY
       && expr->expr_type != EXPR_NULL)
-    return build_constructor (type, NULL);
+    {
+      gfc_error ("Array initializer at %L does not reduce to a constant "
+		 "expression", &expr->where);
+      return build_constructor (type, NULL);
+    }
 
   switch (expr->expr_type)
     {
diff --git a/gcc/testsuite/gfortran.dg/pr103367.f90 b/gcc/testsuite/gfortran.dg/pr103367.f90
index c4c860c65ff..e7c89b41a7c 100644
--- a/gcc/testsuite/gfortran.dg/pr103367.f90
+++ b/gcc/testsuite/gfortran.dg/pr103367.f90
@@ -1,14 +1,15 @@
 ! { dg-do compile }
 !
 ! PR fortran/103367
-! An undefined variable in a parameter array initializer reached
-! gfc_conv_array_initializer and hit gcc_unreachable().
+! Invalid initialization expressions must not ICE if they survive
+! semantic checking and reach array initializer lowering.
 
 program p
   type t
     integer :: a(1,2) = 3
   end type
   type(t), parameter :: x(1) = t(4)
-  integer :: y(1,2) = (x(b)%a)  ! { dg-warning "Legacy Extension" }
+  integer :: y(1,2) = (x(b)%a) ! { dg-error "does not reduce to a constant expression" }
   print *, y
 end
+! { dg-prune-output "Legacy Extension: REAL array index" }
-- 
2.53.0