diff --git a/SwiftCompilerSources/Sources/Optimizer/InstructionSimplification/SimplifyRefCasts.swift b/SwiftCompilerSources/Sources/Optimizer/InstructionSimplification/SimplifyRefCasts.swift
index dd23c8919b1cb..529b4484eff59 100644
--- a/SwiftCompilerSources/Sources/Optimizer/InstructionSimplification/SimplifyRefCasts.swift
+++ b/SwiftCompilerSources/Sources/Optimizer/InstructionSimplification/SimplifyRefCasts.swift
@@ -86,6 +86,13 @@ private extension UnaryInstruction {
       operand.set(to: replacement, context)
     }
 
+    if let ccb = self as? CheckedCastBranchInst {
+        // Make sure that updating the formal type with the operand type is
+        // legal.
+        if operand.value.type.isLegalFormalType {
+            ccb.updateSourceFormalTypeFromOperandLoweredType()
+        }
+    }
     if canEraseInst {
       context.erase(instructionIncludingDebugUses: inst)
     }
diff --git a/SwiftCompilerSources/Sources/SIL/Instruction.swift b/SwiftCompilerSources/Sources/SIL/Instruction.swift
index 27ee0fe53ba9d..857d4f63912fa 100644
--- a/SwiftCompilerSources/Sources/SIL/Instruction.swift
+++ b/SwiftCompilerSources/Sources/SIL/Instruction.swift
@@ -1696,6 +1696,10 @@ final public class CheckedCastBranchInst : TermInst, UnaryInstruction {
   public var source: Value { operand.value }
   public var successBlock: BasicBlock { bridged.CheckedCastBranch_getSuccessBlock().block }
   public var failureBlock: BasicBlock { bridged.CheckedCastBranch_getFailureBlock().block }
+
+  public func updateSourceFormalTypeFromOperandLoweredType() {
+    bridged.CheckedCastBranch_updateSourceFormalTypeFromOperandLoweredType()
+  }
 }
 
 final public class CheckedCastAddrBranchInst : TermInst {
diff --git a/SwiftCompilerSources/Sources/SIL/Type.swift b/SwiftCompilerSources/Sources/SIL/Type.swift
index 09173a5051806..0576e5087e4b1 100644
--- a/SwiftCompilerSources/Sources/SIL/Type.swift
+++ b/SwiftCompilerSources/Sources/SIL/Type.swift
@@ -104,6 +104,8 @@ public struct Type : CustomStringConvertible, NoReflectionChildren {
   public var isBuiltinVector: Bool { bridged.isBuiltinVector() }
   public var builtinVectorElementType: Type { bridged.getBuiltinVectorElementType().type }
 
+  public var isLegalFormalType: Bool { bridged.isLegalFormalType() }
+
   public func isBuiltinInteger(withFixedWidth width: Int) -> Bool {
     bridged.isBuiltinFixedWidthInteger(width)
   }
diff --git a/include/swift/SIL/SILBridging.h b/include/swift/SIL/SILBridging.h
index 4dfe6e9b3694f..2b4e46824335e 100644
--- a/include/swift/SIL/SILBridging.h
+++ b/include/swift/SIL/SILBridging.h
@@ -277,6 +277,7 @@ struct BridgedType {
   BRIDGED_INLINE bool isBuiltinInteger() const;
   BRIDGED_INLINE bool isBuiltinFloat() const;
   BRIDGED_INLINE bool isBuiltinVector() const;
+  BRIDGED_INLINE bool isLegalFormalType() const;
   SWIFT_IMPORT_UNSAFE BRIDGED_INLINE BridgedType getBuiltinVectorElementType() const;
   BRIDGED_INLINE bool isBuiltinFixedWidthInteger(SwiftInt width) const;
   BRIDGED_INLINE bool isExactSuperclassOf(BridgedType t) const;
@@ -810,6 +811,7 @@ struct BridgedInstruction {
   SWIFT_IMPORT_UNSAFE BRIDGED_INLINE BridgedBasicBlock CheckedCastBranch_getFailureBlock() const;
   SWIFT_IMPORT_UNSAFE BRIDGED_INLINE BridgedBasicBlock CheckedCastAddrBranch_getSuccessBlock() const;
   SWIFT_IMPORT_UNSAFE BRIDGED_INLINE BridgedBasicBlock CheckedCastAddrBranch_getFailureBlock() const;
+  BRIDGED_INLINE void CheckedCastBranch_updateSourceFormalTypeFromOperandLoweredType() const;
   BRIDGED_INLINE CastConsumptionKind CheckedCastAddrBranch_getConsumptionKind() const;
   SWIFT_IMPORT_UNSAFE BRIDGED_INLINE BridgedSubstitutionMap ApplySite_getSubstitutionMap() const;
   SWIFT_IMPORT_UNSAFE BRIDGED_INLINE BridgedCanType ApplySite_getSubstitutedCalleeType() const;
diff --git a/include/swift/SIL/SILBridgingImpl.h b/include/swift/SIL/SILBridgingImpl.h
index cf45905555d9e..890034333f3d3 100644
--- a/include/swift/SIL/SILBridgingImpl.h
+++ b/include/swift/SIL/SILBridgingImpl.h
@@ -445,6 +445,10 @@ bool BridgedType::isBuiltinVector() const {
   return unbridged().isBuiltinVector();
 }
 
+bool BridgedType::isLegalFormalType() const {
+  return unbridged().getASTType()->isLegalFormalType();
+}
+
 BridgedType BridgedType::getBuiltinVectorElementType() const {
   return unbridged().getBuiltinVectorElementType();
 }
@@ -1504,6 +1508,10 @@ void BridgedInstruction::LoadInst_setOwnership(SwiftInt ownership) const {
   getAs<swift::LoadInst>()->setOwnershipQualifier((swift::LoadOwnershipQualifier)ownership);
 }
 
+void BridgedInstruction::CheckedCastBranch_updateSourceFormalTypeFromOperandLoweredType() const {
+  getAs<swift::CheckedCastBranchInst>()->updateSourceFormalTypeFromOperandLoweredType();
+}
+
 BridgedBasicBlock BridgedInstruction::CheckedCastBranch_getSuccessBlock() const {
   return {getAs<swift::CheckedCastBranchInst>()->getSuccessBB()};
 }
diff --git a/include/swift/SIL/SILInstruction.h b/include/swift/SIL/SILInstruction.h
index bddf29c364698..3d177ad9aa643 100644
--- a/include/swift/SIL/SILInstruction.h
+++ b/include/swift/SIL/SILInstruction.h
@@ -11032,6 +11032,10 @@ class CheckedCastBranchInst final
   SILType getSourceLoweredType() const { return getOperand()->getType(); }
   CanType getSourceFormalType() const { return SrcFormalTy; }
 
+  void updateSourceFormalTypeFromOperandLoweredType() {
+    SrcFormalTy = getSourceLoweredType().getASTType();
+  }
+
   SILType getTargetLoweredType() const { return DestLoweredTy; }
   CanType getTargetFormalType() const { return DestFormalTy; }
 };
diff --git a/test/DebugInfo/simplify_checked_cast_br.sil b/test/DebugInfo/simplify_checked_cast_br.sil
index 2fa1ba9c87636..89cd3ad2dfc02 100644
--- a/test/DebugInfo/simplify_checked_cast_br.sil
+++ b/test/DebugInfo/simplify_checked_cast_br.sil
@@ -11,7 +11,7 @@ class X : B {}
 
 // CHECK-LABEL: sil [ossa] @test_ossa :
 // CHECK:         %0 = alloc_ref
-// CHECK-NEXT:  checked_cast_br AnyObject in %0 : $X
+// CHECK-NEXT:  checked_cast_br X in %0 : $X
 // CHECK:     bb2([[A:%.*]] : @owned $X):
 // CHECK-NEXT:  [[U:%.*]] = upcast [[A]] : $X to $B, loc "takethis":1:1
 // CHECK-NEXT:  [[E:%.*]] = init_existential_ref [[U]] {{.+}}, loc "takethis":1:1
diff --git a/test/SILOptimizer/devirt_covariant_return.swift b/test/SILOptimizer/devirt_covariant_return.swift
index 18374cde03569..1deee904801c3 100644
--- a/test/SILOptimizer/devirt_covariant_return.swift
+++ b/test/SILOptimizer/devirt_covariant_return.swift
@@ -235,7 +235,7 @@ public class D2: D1 {
 // that D2.foo() is inlined thanks to this.
 // CHECK-LABEL: sil hidden [noinline] @$s23devirt_covariant_return7driver2ys5Int32VAA2D2CF
 // CHECK-NOT: class_method
-// CHECK: checked_cast_br [exact] D1 in
+// CHECK: checked_cast_br [exact] D2 in
 // CHECK: bb2
 // CHECK: global_addr
 // CHECK: load
diff --git a/test/SILOptimizer/simplify_checked_cast_br.sil b/test/SILOptimizer/simplify_checked_cast_br.sil
index 71f29ef11fe31..5b36431d1ef26 100644
--- a/test/SILOptimizer/simplify_checked_cast_br.sil
+++ b/test/SILOptimizer/simplify_checked_cast_br.sil
@@ -12,7 +12,7 @@ class X : B {}
 
 // CHECK-LABEL: sil @test_non_ossa :
 // CHECK:         %0 = alloc_ref
-// CHECK:         checked_cast_br AnyObject in %0 : $X
+// CHECK:         checked_cast_br X in %0 : $X
 // CHECK:       } // end sil function 'test_non_ossa'
 sil @test_non_ossa : $@convention(thin) () -> AnyObject {
 bb0:
@@ -31,7 +31,7 @@ bb2:
 
 // CHECK-LABEL: sil [ossa] @test_ossa :
 // CHECK:         %0 = alloc_ref
-// CHECK-O-NEXT:  checked_cast_br AnyObject in %0 : $X
+// CHECK-O-NEXT:  checked_cast_br X in %0 : $X
 // CHECK-ONONE:   checked_cast_br AnyObject in %2 : $AnyObject
 // CHECK-O:     bb2([[A:%.*]] : @owned $X):
 // CHECK-O-NEXT:  [[U:%.*]] = upcast [[A]] : $X to $B
@@ -75,7 +75,7 @@ bb2(%7 : @owned $B):
 
 // CHECK-LABEL: sil [ossa] @test_borrow :
 // CHECK:         %1 = begin_borrow
-// CHECK-NEXT:    checked_cast_br any P in %1 : $X
+// CHECK-NEXT:    checked_cast_br X in %1 : $X
 // CHECK:       bb2([[A:%.*]] : @guaranteed $X):
 // CHECK-NEXT:    [[U:%.*]] = upcast [[A]] : $X to $B
 // CHECK-NEXT:    [[E:%.*]] = init_existential_ref [[U]] : $B