[PR36008] Avoid -Wsign-compare warning for enum constants in typeof expressions

[arphaman]

Summary

This patch looks through typeof type at the original expression when diagnosing -Wsign-compare to avoid an unfriendly diagnostic.

Fixes PR36008

[lebedev.ri]

lib/Sema/SemaChecking.cpp

8959

Please also add a comment what this is trying to solve.

test/Sema/compare.c

404

Can you also add the swapped test:

if (y == x) x = y; // no warning

[aaron.ballman]

lib/Sema/SemaChecking.cpp

8959

const auto *

test/Sema/compare.c

398

Formatting looks off here.

[arphaman]

Address review comments and fix the inverted check case.

[aaron.ballman]

LGTM, aside from a minor commenting nit.

lib/Sema/SemaChecking.cpp

8960

to the sign -> from the sign

lib/Sema/SemaChecking.cpp

if (!T->hasUnsignedIntegerRepresentation()) {
     // comparison:  we're only interested in integral comparisons, and
     // signed comparisons only happen in cases we don't care to warn about.
     return AnalyzeImpConvsInComparison(S, E);
   }
 
   LHS = LHS->IgnoreParenImpCasts();
   RHS = RHS->IgnoreParenImpCasts();
 
+  if (!S.getLangOpts().CPlusPlus) {
+    // Avoid warning about comparison of integers with different signs when
+    // RHS/LHS has a `typeof(E)` type whose sign is different to the sign of the
+    // type of `E`.
+    if (const auto *TET = dyn_cast<TypeOfExprType>(LHS->getType()))
+      LHS = TET->getUnderlyingExpr()->IgnoreParenImpCasts();
+    if (const auto *TET = dyn_cast<TypeOfExprType>(RHS->getType()))
+      RHS = TET->getUnderlyingExpr()->IgnoreParenImpCasts();
+  }
+
   // Check to see if one of the (unmodified) operands is of different
   // signedness.
   Expr *signedOperand, *unsignedOperand;
   if (LHS->getType()->hasSignedIntegerRepresentation()) {
     assert(!RHS->getType()->hasSignedIntegerRepresentation() &&
            "unsigned comparison between two signed integer expressions?");
     signedOperand = LHS;
     unsignedOperand = RHS;

test/Sema/compare.c

 }
 
 typedef char one_char[1];
 typedef char two_chars[2];
 
 void test12(unsigned a) {
   if (0 && -1 > a) { }
 }
+
+// PR36008
+
+enum PR36008EnumTest {
+  kPR36008Value = 0,
+};
+
+void pr36008(enum PR36008EnumTest lhs) {
+  __typeof__(lhs) x = lhs;
+  __typeof__(kPR36008Value) y = (kPR36008Value);
+  if (x == y) x = y; // no warning
+  if (y == x) y = x; // no warning
+}