[clang][Interp] Don't fail on shifts greater than type bitwidth

We need to limit the shift width to the type bitwidth, then do the
shift and report success, but still diagnose what we limited the
shiftwidth.

Author: tbaederr

clang/lib/AST/Interp/Interp.h

@@ -135,7 +135,7 @@ bool CheckShift(InterpState &S, CodePtr OpPC, const LT &LHS, const RT &RHS,
     const APSInt Val = RHS.toAPSInt();
     QualType Ty = E->getType();
     S.CCEDiag(E, diag::note_constexpr_large_shift) << Val << Ty << Bits;
-    return false;
+    return true; // We will do the shift anyway but fix up the shift amount.
   }
 
   if (LHS.isSigned() && !S.getLangOpts().CPlusPlus20) {
@@ -1798,11 +1798,17 @@ inline bool Shr(InterpState &S, CodePtr OpPC) {
   if (!CheckShift(S, OpPC, LHS, RHS, Bits))
     return false;
 
+  // Limit the shift amount to Bits - 1. If this happened,
+  // it has already been diagnosed by CheckShift() above,
+  // but we still need to handle it.
   typename LT::AsUnsigned R;
-  LT::AsUnsigned::shiftRight(LT::AsUnsigned::from(LHS),
-                             LT::AsUnsigned::from(RHS), Bits, &R);
+  if (RHS > RT::from(Bits - 1, RHS.bitWidth()))
+    LT::AsUnsigned::shiftRight(LT::AsUnsigned::from(LHS),
+                               LT::AsUnsigned::from(Bits - 1), Bits, &R);
+  else
+    LT::AsUnsigned::shiftRight(LT::AsUnsigned::from(LHS),
+                               LT::AsUnsigned::from(RHS, Bits), Bits, &R);
   S.Stk.push<LT>(LT::from(R));
-
   return true;
 }
 
@@ -1817,9 +1823,17 @@ inline bool Shl(InterpState &S, CodePtr OpPC) {
   if (!CheckShift(S, OpPC, LHS, RHS, Bits))
     return false;
 
+  // Limit the shift amount to Bits - 1. If this happened,
+  // it has already been diagnosed by CheckShift() above,
+  // but we still need to handle it.
   typename LT::AsUnsigned R;
-  LT::AsUnsigned::shiftLeft(LT::AsUnsigned::from(LHS),
-                            LT::AsUnsigned::from(RHS, Bits), Bits, &R);
+  if (RHS > RT::from(Bits - 1, RHS.bitWidth()))
+    LT::AsUnsigned::shiftLeft(LT::AsUnsigned::from(LHS),
+                              LT::AsUnsigned::from(Bits - 1), Bits, &R);
+  else
+    LT::AsUnsigned::shiftLeft(LT::AsUnsigned::from(LHS),
+                              LT::AsUnsigned::from(RHS, Bits), Bits, &R);
+
   S.Stk.push<LT>(LT::from(R));
   return true;
 }

clang/test/AST/Interp/shifts.cpp

@@ -34,12 +34,13 @@ namespace shifts {
                  // ref-warning {{shift count is negative}} \
                  // ref-cxx17-warning {{shift count is negative}}
     c = 1 << (unsigned)-1; // expected-warning {{shift count >= width of type}} \
-                           // FIXME: 'implicit conversion' warning missing in the new interpreter. \
+                           // expected-warning {{implicit conversion from 'int' to 'char' changes value from -2147483648 to 0}} \
                            // cxx17-warning {{shift count >= width of type}} \
+                           // cxx17-warning {{implicit conversion from 'int' to 'char' changes value from -2147483648 to 0}} \
                            // ref-warning {{shift count >= width of type}} \
-                           // ref-warning {{implicit conversion}} \
+                           // ref-warning {{implicit conversion from 'int' to 'char' changes value from -2147483648 to 0}} \
                            // ref-cxx17-warning {{shift count >= width of type}} \
-                           // ref-cxx17-warning {{implicit conversion}}
+                           // ref-cxx17-warning {{implicit conversion from 'int' to 'char' changes value from -2147483648 to 0}}
     c = 1 >> (unsigned)-1; // expected-warning {{shift count >= width of type}} \
                            // cxx17-warning {{shift count >= width of type}} \
                            // ref-warning {{shift count >= width of type}} \