Index: llvm/trunk/lib/Analysis/ScalarEvolutionExpander.cpp
===================================================================
--- llvm/trunk/lib/Analysis/ScalarEvolutionExpander.cpp
+++ llvm/trunk/lib/Analysis/ScalarEvolutionExpander.cpp
@@ -1735,9 +1735,22 @@
          PEnd = Phis.end(); PIter != PEnd; ++PIter) {
     PHINode *Phi = *PIter;
 
+    auto SimplifyPHINode = [&](PHINode *PN) -> Value * {
+      if (Value *V = SimplifyInstruction(PN, DL, &SE.TLI, &SE.DT, &SE.AC))
+        return V;
+      if (!SE.isSCEVable(PN->getType()))
+        return nullptr;
+      auto *Const = dyn_cast<SCEVConstant>(SE.getSCEV(PN));
+      if (!Const)
+        return nullptr;
+      return Const->getValue();
+    };
+
     // Fold constant phis. They may be congruent to other constant phis and
     // would confuse the logic below that expects proper IVs.
-    if (Value *V = SimplifyInstruction(Phi, DL, &SE.TLI, &SE.DT, &SE.AC)) {
+    if (Value *V = SimplifyPHINode(Phi)) {
+      if (V->getType() != Phi->getType())
+        continue;
       Phi->replaceAllUsesWith(V);
       DeadInsts.emplace_back(Phi);
       ++NumElim;
Index: llvm/trunk/test/Transforms/IndVarSimplify/const_phi.ll
===================================================================
--- llvm/trunk/test/Transforms/IndVarSimplify/const_phi.ll
+++ llvm/trunk/test/Transforms/IndVarSimplify/const_phi.ll
@@ -0,0 +1,33 @@
+; RUN: opt < %s -indvars -S | FileCheck %s
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+
+; PR25372
+; We can compute the expression of %phi0 and that is a SCEV
+; constant. However, instcombine can't deduce this, so we can
+; potentially end up trying to handle a constant when replacing
+; congruent IVs.
+
+; CHECK-LABEL: crash
+define void @crash() {
+entry:
+  br i1 false, label %not_taken, label %pre
+
+not_taken:
+  br label %pre
+
+pre:
+; %phi0.pre and %phi1.pre are evaluated by SCEV to constant 0.
+  %phi0.pre = phi i32 [ 0, %entry ], [ 2, %not_taken ]
+  %phi1.pre = phi i32 [ 0, %entry ], [ 1, %not_taken ]
+  br label %loop
+
+loop:
+; %phi0 and %phi1 are evaluated by SCEV to constant 0.
+  %phi0 = phi i32 [ 0, %loop ], [ %phi0.pre, %pre ]
+  %phi1 = phi i32 [ 0, %loop ], [ %phi1.pre, %pre ]
+  br i1 undef, label %exit, label %loop
+
+exit:
+  ret void
+}