gdb, fortran: Fix local variable lookup in Fortran parser

This change fixes https://sourceware.org/bugzilla/show_bug.cgi?id=34059.

The Fortran expression parser in GDB ("gdb/f-exp.y") previously performed
symbol lookup in this order when parsing identifiers:

- SEARCH_STRUCT_DOMAIN (types/structs)
- SEARCH_VFT ("C-like" name lookups for variables/types/functions)
- SEARCH_MODULE_DOMAIN (modules)

It searched for "types before variables", causing type names from shared
libraries to shadow local variable names.

For a reproducer like the one given below, the Fortran parser fails to
look up local variable name "array" and treats it as a type value instead
of a variable, because there's a conflicting "array" type from system
libraries.

1 program test
2
3  ! Declare variables used in this test.
4  integer, dimension (-2:2) :: array
5
6  array = 1
7
8  print *, ""           ! Break here
9  print *, array
10
11 end program test

Before the change, GDB shows:

'''
./gdb --data-directory=./data-directory --args /tmp/a.out
GNU gdb (GDB) 18.0.50.20260408-git
Copyright (C) 2026 Free Software Foundation, Inc.
...
Reading symbols from /tmp/a.out...
(gdb) break 8
Breakpoint 1 at 0x11b3: file test.f90, line 8.
(gdb) run
Starting program: /tmp/a.out
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".

Breakpoint 1, test () at test.f90:8
8         print *, ""           ! Break here
(gdb) info locals
array = (1, 1, 1, 1, 1)
(gdb) print array
Attempt to use a type name as an expression
(gdb) ptype array
type = Type array
    Type, C_Union :: :: u
    PTR TO -> ( char :: scratch(0:15 ))
End Type array
'''

This issue is fixed in the Fortran expression parser in GDB ("gdb/f-exp.y")
by prioritizing SEARCH_VFT over other search domains during symbol lookup.
After the change, the 'array' variable is resolved to the correct value.

'''
(gdb) print array
$1 = (1, 1, 1, 1, 1)
(gdb) ptype array
type = integer(kind=4) (-2:2)
'''

Approved-by: Kevin Buettner <kevinb@redhat.com>

gdb/f-exp.y

@@ -1651,8 +1651,8 @@ yylex (void)
     struct block_symbol result;
     const domain_search_flags lookup_domains[] =
     {
-      SEARCH_STRUCT_DOMAIN,
       SEARCH_VFT,
+      SEARCH_STRUCT_DOMAIN,
       SEARCH_MODULE_DOMAIN
     };
     int hextype;

gdb/testsuite/gdb.fortran/var-type-precedence-lib.c

@@ -0,0 +1,30 @@
+/* Copyright 2026 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+/* Define a struct type that will conflict with Fortran variable name.  */
+struct type_shadowing_var
+{
+  int value;
+};
+
+/* Global variables to ensure types are in debug info.  */
+static struct type_shadowing_var global_conflicting_var = {42};
+
+/* Function to ensure library is linked and types are used.  */
+void
+fortran_var_type_order_test (void)
+{
+  global_conflicting_var.value = 42;
+}

gdb/testsuite/gdb.fortran/var-type-precedence.exp

@@ -0,0 +1,60 @@
+# Copyright 2026 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# Test variable lookup when local Fortran variable names conflict with
+# type names from a C shared library.
+
+require allow_fortran_tests
+
+standard_testfile var-type-precedence.f90 var-type-precedence-lib.c
+load_lib fortran.exp
+
+set libname "${testfile}-lib"
+set srcfile_lib ${srcdir}/${subdir}/${srcfile2}
+set binfile_lib [standard_output_file ${libname}.so]
+
+# Compile the C shared library.
+if {[gdb_compile_shlib ${srcfile_lib} ${binfile_lib} \
+	{debug}] != ""} {
+  untested "failed to compile C shared library"
+  return
+}
+
+# Compile the Fortran main program with the C shared library.
+if {[prepare_for_testing "failed to prepare" $testfile \
+	${srcfile} [list debug f90 shlib=${binfile_lib}]]} {
+  return
+}
+
+if {![fortran_runto_main]} {
+  return
+}
+
+set bp_loc [gdb_get_line_number "break-here"]
+gdb_breakpoint "$srcfile:$bp_loc"
+gdb_continue_to_breakpoint "stop-at-bp" ".*$srcfile:$bp_loc.*"
+
+set integer4 [fortran_int4]
+
+# Test that local variable 'type_shadowing_var' is resolved correctly.
+gdb_test "print type_shadowing_var" "= \\(1, 1, 1, 1, 1\\)" \
+    "print local type_shadowing_var variable"
+gdb_test "ptype type_shadowing_var" "type = $integer4 \\(-2:2\\)" \
+    "ptype local type_shadowing_var"
+
+# Verify the C types exist in debug info by checking info types.
+gdb_test "info types type_shadowing_var" \
+    ".*File.*var-type-precedence-lib\\.c:.*struct type_shadowing_var.*" \
+    "C type type_shadowing_var exists in debug info"

gdb/testsuite/gdb.fortran/var-type-precedence.f90

@@ -0,0 +1,40 @@
+! Copyright 2026 Free Software Foundation, Inc.
+!
+! This program is free software; you can redistribute it and/or modify
+! it under the terms of the GNU General Public License as published by
+! the Free Software Foundation; either version 3 of the License, or
+! (at your option) any later version.
+!
+! This program is distributed in the hope that it will be useful,
+! but WITHOUT ANY WARRANTY; without even the implied warranty of
+! MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+! GNU General Public License for more details.
+!
+! You should have received a copy of the GNU General Public License
+! along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+! Source code for var-type-precedence test to verify that local Fortran
+! variables take precedence over conflicting C type names from shared
+! libraries.
+
+program test
+  use iso_c_binding
+  implicit none
+
+  interface
+    subroutine fortran_var_type_order_test () bind (C)
+    end subroutine fortran_var_type_order_test
+  end interface
+
+  ! Declare variables with names that conflict with types in C library.
+  integer, dimension (-2:2) :: type_shadowing_var
+
+  ! Call C library function to ensure it's linked.
+  call fortran_var_type_order_test ()
+
+  type_shadowing_var = 1
+
+  print *, "" ! break-here
+  print *, type_shadowing_var
+
+end program test