Index: lib/CodeGen/GlobalISel/Utils.cpp
===================================================================
--- lib/CodeGen/GlobalISel/Utils.cpp
+++ lib/CodeGen/GlobalISel/Utils.cpp
@@ -53,6 +53,14 @@
          "PhysReg not implemented");
 
   const TargetRegisterClass *RegClass = TII.getRegClass(II, OpIdx, &TRI, MF);
+  // Some of the target independent instructions, like COPY, may not impose any
+  // register class constraints on some of their operands:
+  if (!RegClass) {
+    assert(!isTargetSpecificOpcode(II.getOpcode()) &&
+           "Only target independent instructions are allowed to have operands "
+           "with no register class constraints");
+    return Reg;
+  }
   return constrainRegToClass(MRI, TII, RBI, InsertPt, Reg, *RegClass);
 }
 
@@ -60,6 +68,8 @@
                                             const TargetInstrInfo &TII,
                                             const TargetRegisterInfo &TRI,
                                             const RegisterBankInfo &RBI) {
+  assert(!isPreISelGenericOpcode(I.getOpcode()) &&
+         "A selected instruction is expected");
   MachineBasicBlock &MBB = *I.getParent();
   MachineFunction &MF = *MBB.getParent();
   MachineRegisterInfo &MRI = MF.getRegInfo();
Index: lib/Target/ARM/ARMLegalizerInfo.cpp
===================================================================
--- lib/Target/ARM/ARMLegalizerInfo.cpp
+++ lib/Target/ARM/ARMLegalizerInfo.cpp
@@ -132,6 +132,9 @@
   setAction({G_PTRTOINT, s32}, Legal);
   setAction({G_PTRTOINT, 1, p0}, Legal);
 
+  setAction({G_FPTOSI, s32}, Legal);
+  setAction({G_FPTOSI, 1, s32}, Legal);
+
   for (unsigned Op : {G_ASHR, G_LSHR, G_SHL})
     setAction({Op, s32}, Legal);
 
Index: test/CodeGen/ARM/GlobalISel/arm-select-copy_to_regclass-of-fptosi.mir
===================================================================
--- /dev/null
+++ test/CodeGen/ARM/GlobalISel/arm-select-copy_to_regclass-of-fptosi.mir
@@ -0,0 +1,27 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -mtriple armv7-gnueabihf -run-pass instruction-select -verify-machineinstrs -o - %s | FileCheck %s
+---
+# Test that we constrain register classes of nested instructions built from Dst Pattern
+#
+# G_FPTOSI selects to (COPY_TO_REGCLASS (VTOSIZS SPR:$a), GPR), where
+# COPY_TO_REGCLASS doesn't constrain its source register class. It exposes the bug as
+# we create a tmp reg for VTOSIZS' result and don't constrain its register class as
+# COPY_TO_REGCLASS' source (which is fine) nor as VTOSIZS' destination (which is not).
+#
+# https://bugs.llvm.org/show_bug.cgi?id=35965
+name:            test_fptosi
+legalized:       true
+regBankSelected: true
+body:             |
+  bb.1:
+    ; CHECK-LABEL: name: test_fptosi
+    ; CHECK: [[COPY:%[0-9]+]]:spr = COPY %s0
+    ; CHECK: [[VTOSIZS:%[0-9]+]]:spr = VTOSIZS [[COPY]], 14, %noreg
+    ; CHECK: [[COPY1:%[0-9]+]]:gpr = COPY [[VTOSIZS]]
+    ; CHECK: %r0 = COPY [[COPY1]]
+    ; CHECK: MOVPCLR 14, %noreg, implicit %r0
+    %0:fprb(s32) = COPY %s0
+    %1:gprb(s32) = G_FPTOSI %0(s32)
+    %r0 = COPY %1(s32)
+    MOVPCLR 14, %noreg, implicit %r0
+...
Index: test/TableGen/GlobalISelEmitter.td
===================================================================
--- test/TableGen/GlobalISelEmitter.td
+++ test/TableGen/GlobalISelEmitter.td
@@ -308,6 +308,7 @@
 // CHECK-NEXT:    GIR_ComplexRenderer, /*InsnID*/1, /*RendererID*/1,
 // CHECK-NEXT:    GIR_ComplexSubOperandRenderer, /*InsnID*/1, /*RendererID*/2, /*SubOperand*/0, // src5a
 // CHECK-NEXT:    GIR_ComplexSubOperandRenderer, /*InsnID*/1, /*RendererID*/2, /*SubOperand*/1, // src5b
+// CHECK-NEXT:    GIR_ConstrainSelectedInstOperands, /*InsnID*/1,
 // CHECK-NEXT:    GIR_BuildMI, /*InsnID*/0, /*Opcode*/MyTarget::INSN3,
 // CHECK-NEXT:    GIR_Copy, /*NewInsnID*/0, /*OldInsnID*/0, /*OpIdx*/0, // dst
 // CHECK-NEXT:    GIR_Copy, /*NewInsnID*/0, /*OldInsnID*/0, /*OpIdx*/1, // src1
Index: utils/TableGen/GlobalISelEmitter.cpp
===================================================================
--- utils/TableGen/GlobalISelEmitter.cpp
+++ utils/TableGen/GlobalISelEmitter.cpp
@@ -610,8 +610,8 @@
 /// Generates code to check that a match rule matches.
 class RuleMatcher : public Matcher {
 public:
-  using ActionVec = std::vector<std::unique_ptr<MatchAction>>;
-  using action_iterator = ActionVec::iterator;
+  using ActionList = std::list<std::unique_ptr<MatchAction>>;
+  using action_iterator = ActionList::iterator;
 
 protected:
   /// A list of matchers that all need to succeed for the current rule to match.
@@ -622,7 +622,7 @@
 
   /// A list of actions that need to be taken when all predicates in this rule
   /// have succeeded.
-  ActionVec Actions;
+  ActionList Actions;
 
   using DefinedInsnVariablesMap =
       std::map<const InstructionMatcher *, unsigned>;
@@ -2125,6 +2125,7 @@
   BuildMIAction(unsigned InsnID, const CodeGenInstruction *I)
       : InsnID(InsnID), I(I), Matched(nullptr) {}
 
+  unsigned getInsnID() const { return InsnID; }
   const CodeGenInstruction *getCGI() const { return I; }
 
   void chooseInsnToMutate(RuleMatcher &Rule) {
@@ -3199,7 +3200,7 @@
 
 Expected<action_iterator>
 GlobalISelEmitter::createAndImportSubInstructionRenderer(
-    action_iterator InsertPt, RuleMatcher &M, const TreePatternNode *Dst,
+    const action_iterator InsertPt, RuleMatcher &M, const TreePatternNode *Dst,
     unsigned TempRegID) {
   auto InsertPtOrError = createInstructionRenderer(InsertPt, M, Dst);
 
@@ -3207,7 +3208,6 @@
 
   if (auto Error = InsertPtOrError.takeError())
     return std::move(Error);
-  InsertPt = InsertPtOrError.get();
 
   BuildMIAction &DstMIBuilder =
       *static_cast<BuildMIAction *>(InsertPtOrError.get()->get());
@@ -3215,10 +3215,13 @@
   // Assign the result to TempReg.
   DstMIBuilder.addRenderer<TempRegRenderer>(TempRegID, true);
 
-  InsertPtOrError = importExplicitUseRenderers(InsertPt, M, DstMIBuilder, Dst);
+  InsertPtOrError =
+      importExplicitUseRenderers(InsertPtOrError.get(), M, DstMIBuilder, Dst);
   if (auto Error = InsertPtOrError.takeError())
     return std::move(Error);
 
+  M.insertAction<ConstrainOperandsToDefinitionAction>(InsertPt,
+                                                      DstMIBuilder.getInsnID());
   return InsertPtOrError.get();
 }