[gcc r12-1676] [Ada] Compiler crash on sliding of fixed-lower-bound object in Loop_Invariant

[gcc r12-1676] [Ada] Compiler crash on sliding of fixed-lower-bound object in Loop_Invariant

[Pierre-Marie de Rodat]

https://gcc.gnu.org/g:eb077d7047629d5704338270ca57b64272627268

commit r12-1676-geb077d7047629d5704338270ca57b64272627268
Author: Gary Dismukes <dismukes@adacore.com>
Date:   Thu Mar 25 13:37:58 2021 -0400

    [Ada] Compiler crash on sliding of fixed-lower-bound object in Loop_Invariant
    
    gcc/ada/
    
            * exp_util.adb (Expand_Sliding_Conversion): Only perform
            expansion when Expander_Active is True. Add a comment about this
            and refine existing comment regarding string literals.

Diff:
---
 gcc/ada/exp_util.adb | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/gcc/ada/exp_util.adb b/gcc/ada/exp_util.adb
index ed1a016be48..d02e587d853 100644
--- a/gcc/ada/exp_util.adb
+++ b/gcc/ada/exp_util.adb
@@ -5343,10 +5343,15 @@ package body Exp_Util is
       All_FLBs_Match : Boolean := True;
 
    begin
-      --  Sliding should never be needed for string literals, because they have
-      --  their bounds set according to the applicable index constraint.
-
-      if Nkind (N) /= N_String_Literal then
+      --  This procedure is called during semantic analysis, and we only expand
+      --  a sliding conversion when Expander_Active, to avoid doing it during
+      --  preanalysis (which can lead to problems with the target subtype not
+      --  getting properly expanded during later full analysis). Also, sliding
+      --  should never be needed for string literals, because their bounds are
+      --  determined directly based on the fixed lower bound of Arr_Typ and
+      --  their length.
+
+      if Expander_Active and then Nkind (N) /= N_String_Literal then
          Constraints := New_List;
 
          Act_Subt  := Get_Actual_Subtype (N);