Index: clang/lib/AST/Interp/ByteCodeExprGen.cpp
===================================================================
--- clang/lib/AST/Interp/ByteCodeExprGen.cpp
+++ clang/lib/AST/Interp/ByteCodeExprGen.cpp
@@ -2481,10 +2481,18 @@
     return this->emitStore(*T, E);
   }
   case UO_LNot: // !x
-    if (!this->visit(SubExpr))
+    if (DiscardResult)
+      return this->discard(SubExpr);
+
+    if (!this->visitBool(SubExpr))
       return false;
-    // The Inv doesn't change anything, so skip it if we don't need the result.
-    return DiscardResult ? this->emitPop(*T, E) : this->emitInvBool(E);
+
+    if (!this->emitInvBool(E))
+      return false;
+
+    if (PrimType ET = classifyPrim(E->getType()); ET != PT_Bool)
+      return this->emitCast(PT_Bool, ET, E);
+    return true;
   case UO_Minus: // -x
     if (!this->visit(SubExpr))
       return false;
Index: clang/test/AST/Interp/c.c
===================================================================
--- clang/test/AST/Interp/c.c
+++ clang/test/AST/Interp/c.c
@@ -9,6 +9,8 @@
                                 // pedantic-expected-warning {{not an integer constant expression}}
 _Static_assert(1 && 1.0, "");
 _Static_assert( (5 > 4) + (3 > 2) == 2, "");
+_Static_assert(!!1.0, "");
+_Static_assert(!!1, "");
 
 /// FIXME: Should also be rejected in the new interpreter
 int a = (1 == 1 ? 5 : 3);