[patch, fortran] Fix Bug 103367 - [13/14/15/16 Regression] ICE in gfc_conv_array_initializer

[Jerry D]

The attached patch from Christopher is straight forward and fixes the 
regression.

Regression tested on x86_64. I confirmed the test case ICEs on 13,14, and 15.

I will backport this at after working off more 16 issues.

I will commit this one shortly since it is simple enough.

Regards,

Jerry
From 3f7a5c8d4ad0865465975a17c15c75c613e2764c Mon Sep 17 00:00:00 2001
From: Christopher Albert <alb...@tugraz.at>
Date: Sun, 29 Mar 2026 22:18:43 +0200
Subject: [PATCH] fortran: Fix ICE in gfc_conv_array_initializer with invalid
 index [PR103367]

An undefined variable used as an array index in a parameter initializer
expression reaches gfc_conv_array_initializer after parameter substitution
with an unexpected expression type, hitting gcc_unreachable().

Guard against unexpected expression types by returning a zero-filled
constructor, since the frontend has already diagnosed the error.

	PR fortran/103367

gcc/fortran/ChangeLog:

	* trans-array.cc (gfc_conv_array_initializer): Return empty
	constructor for unexpected expression types after parameter
	substitution.

gcc/testsuite/ChangeLog:

	* gfortran.dg/pr103367.f90: New test.

Signed-off-by: Christopher Albert <alb...@tugraz.at>
---
 gcc/fortran/trans-array.cc             |  9 +++++++++
 gcc/testsuite/gfortran.dg/pr103367.f90 | 14 ++++++++++++++
 2 files changed, 23 insertions(+)
 create mode 100644 gcc/testsuite/gfortran.dg/pr103367.f90

diff --git a/gcc/fortran/trans-array.cc b/gcc/fortran/trans-array.cc
index a2da4fe7c7d..8d41804f5f5 100644
--- a/gcc/fortran/trans-array.cc
+++ b/gcc/fortran/trans-array.cc
@@ -7075,6 +7075,15 @@ gfc_conv_array_initializer (tree type, gfc_expr * expr)
       && expr->symtree->n.sym->value)
     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.  */
+  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);
+
   switch (expr->expr_type)
     {
     case EXPR_CONSTANT:
diff --git a/gcc/testsuite/gfortran.dg/pr103367.f90 b/gcc/testsuite/gfortran.dg/pr103367.f90
new file mode 100644
index 00000000000..c4c860c65ff
--- /dev/null
+++ b/gcc/testsuite/gfortran.dg/pr103367.f90
@@ -0,0 +1,14 @@
+! { dg-do compile }
+!
+! PR fortran/103367
+! An undefined variable in a parameter array initializer reached
+! gfc_conv_array_initializer and hit gcc_unreachable().
+
+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" }
+  print *, y
+end
-- 
2.53.0