Fix a crash when casting _Complex and ignoring the results

[aaron.ballman]

Summary

Performing a cast where the result is ignored causes 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.

[rjmccall]

LGTM

[aaron.ballman]

Committed in 554791928088d6139e0fb3480d79cd76ea59198f

clang/lib/CodeGen/CGExprComplex.cpp

ComplexPairTy ComplexExprEmitter::EmitComplexToComplexCast(ComplexPairTy Val,
                                                            SourceLocation Loc) {
   // Get the src/dest element type.
   SrcType = SrcType->castAs<ComplexType>()->getElementType();
   DestType = DestType->castAs<ComplexType>()->getElementType();
 
   // 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.
+  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;
 }
 
 ComplexPairTy ComplexExprEmitter::EmitScalarToComplexCast(llvm::Value *Val,
                                                           QualType SrcType,
                                                           QualType DestType,
                                                           SourceLocation Loc) {
   // Convert the input element to the element type of the complex.

clang/test/CodeGen/complex-convert.c

void foo(signed char sc, unsigned char uc, signed long long sll,
   // CHECK-NEXT: %[[VAR440:[A-Za-z0-9.]+]] = add i[[LLSIZE]] %[[VAR436]], %[[VAR439]]
   // CHECK-NEXT: %[[VAR441:[A-Za-z0-9.]+]] = add i[[LLSIZE]] %[[VAR438]], 0
   // CHECK-NEXT: %[[VAR442:[A-Za-z0-9.]+]] = getelementptr inbounds { i[[LLSIZE]], i[[LLSIZE]] }, { i[[LLSIZE]], i[[LLSIZE]] }* %[[CULL1]], i{{[0-9]+}} 0, i{{[0-9]+}} 0
   // CHECK-NEXT: %[[VAR443:[A-Za-z0-9.]+]] = getelementptr inbounds { i[[LLSIZE]], i[[LLSIZE]] }, { i[[LLSIZE]], i[[LLSIZE]] }* %[[CULL1]], i{{[0-9]+}} 0, i{{[0-9]+}} 1
   // CHECK-NEXT: store i[[LLSIZE]] %[[VAR440]], i[[LLSIZE]]* %[[VAR442]]
   // 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;
+}