Index: lib/Transforms/Scalar/SCCP.cpp
===================================================================
--- lib/Transforms/Scalar/SCCP.cpp
+++ lib/Transforms/Scalar/SCCP.cpp
@@ -428,6 +428,9 @@
     assert(!V->getType()->isStructTy() && "structs should use mergeInValue");
     LatticeVal &IV = ValueState[V];
     IV.markForcedConstant(C);
+    // ParamState lattice elements do not have support for forced constants,
+    // so we have to drop any more precise information.
+    ParamState.erase(V);
     LLVM_DEBUG(dbgs() << "markForcedConstant: " << *C << ": " << *V << '\n');
     pushToWorkList(IV, V);
   }
Index: test/Transforms/SCCP/ipsccp-paramstate-forcedconst.ll
===================================================================
--- /dev/null
+++ test/Transforms/SCCP/ipsccp-paramstate-forcedconst.ll
@@ -0,0 +1,24 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -S -ipsccp | FileCheck %s
+
+define void @main() {
+; CHECK-LABEL: @main(
+; CHECK-NEXT:    [[CALL:%.*]] = call i1 @patatino(i1 undef)
+; CHECK-NEXT:    ret void
+;
+  %call = call i1 @patatino(i1 undef)
+  ret void
+}
+
+define internal i1 @patatino(i1 %a) {
+; CHECK-LABEL: @patatino(
+; CHECK-NEXT:    br label [[ONFALSE:%.*]]
+; CHECK:       onfalse:
+; CHECK-NEXT:    ret i1 undef
+;
+  br i1 %a, label %ontrue, label %onfalse
+ontrue:
+  ret i1 false
+onfalse:
+  ret i1 false
+}