[clang]Fix warning for signed conversion on LP64

Currently clang emits warning with -Wconversion for the following code
on LP64 system e.g. x86_64-unknown-linux-gnu:

long foo(long x) {
  return 1LL<<x;
}
warning: implicit conversion changes signedness: 'long long' to 'long' [-Wsign-conversion]

return 1ll << x;
~~~~~~ ~~~~^~~~
This does not make sense since all operands are signed.

This patch fixes that to match -m32 and GCC behaviour.

Reviewed by: Fangrui Song

Differential Revision: https://reviews.llvm.org/D144011

clang/lib/Sema/SemaChecking.cpp

@@ -14316,6 +14316,12 @@ static void CheckImplicitConversion(Sema &S, Expr *E, QualType T,
     if (S.SourceMgr.isInSystemMacro(CC))
       return;
 
+    if (SourceBT && SourceBT->isInteger() && TargetBT &&
+        TargetBT->isInteger() &&
+        Source->isSignedIntegerType() == Target->isSignedIntegerType()) {
+      return;
+    }
+
     unsigned DiagID = diag::warn_impcast_integer_sign;
 
     // Traditionally, gcc has warned about this under -Wsign-compare.

clang/test/Sema/sign-conversion.c

@@ -1,8 +1,14 @@
 // RUN: %clang_cc1 -fsyntax-only -verify -Wsign-conversion %s
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -fsyntax-only -verify -Wsign-conversion %s
+// RUN: %clang_cc1 -triple x86_64-pc-windows-msvc -fsyntax-only -verify -Wsign-conversion %s
 
 // PR9345: make a subgroup of -Wconversion for signedness changes
 
 void test(int x) {
   unsigned t0 = x; // expected-warning {{implicit conversion changes signedness}}
   unsigned t1 = (t0 == 5 ? x : 0); // expected-warning {{operand of ? changes signedness}}
+
+  // Clang has special treatment for left shift of literal '1'.
+  // Make sure there is no diagnostics.
+  long t2 = 1LL << x;
 }