Fix a crash when casting _Complex and ignoring the results.

Performing a cast where the result is ignored caused Clang to crash when
performing codegen for the conversion:

  _Complex int a;
  void fn1() { (_Complex double) a; }

This patch addresses the crash by not trying to emit the scalar conversions,
causing it to be a noop. Fixes PR44624.

Author: AaronBallman

clang/lib/CodeGen/CGExprComplex.cpp

@@ -431,8 +431,10 @@ ComplexPairTy ComplexExprEmitter::EmitComplexToComplexCast(ComplexPairTy Val,
   // C99 6.3.1.6: When a value of complex type is converted to another
   // complex type, both the real and imaginary parts follow the conversion
   // rules for the corresponding real types.
-  Val.first = CGF.EmitScalarConversion(Val.first, SrcType, DestType, Loc);
-  Val.second = CGF.EmitScalarConversion(Val.second, SrcType, DestType, Loc);
+  if (Val.first)
+    Val.first = CGF.EmitScalarConversion(Val.first, SrcType, DestType, Loc);
+  if (Val.second)
+    Val.second = CGF.EmitScalarConversion(Val.second, SrcType, DestType, Loc);
   return Val;
 }
 

clang/test/CodeGen/complex-convert.c

@@ -722,3 +722,8 @@ void foo(signed char sc, unsigned char uc, signed long long sll,
   // CHECK-NEXT: store i[[LLSIZE]] %[[VAR441]], i[[LLSIZE]]* %[[VAR443]]
 }
 
+// This code used to cause a crash; test that it no longer does so.
+_Complex int a;
+void pr44624(void) {
+  (_Complex double) a;
+}