[clang][Sema] Fix PR47676: Handle dependent AltiVec C-style cast

[tstellar]

Summary

Fix premature decision in the presence of type-dependent expression
operands on whether AltiVec vector initializations from single
expressions are "splat" operations.

Verify that the instantiation is able to determine the correct cast
semantics for both the scalar type and the vector type case.

Note that, because the change only affects the single-expression
case (and the target type is an AltiVec-style vector type), the
replacement of a parenthesized list with a parenthesized expression
does not change the semantics of the program in a program-observable
manner.

clang/lib/Sema/SemaExpr.cpp

Sema::ActOnCastExpr(Scope *S, SourceLocation LParenLoc,
   if ((getLangOpts().AltiVec || getLangOpts().ZVector || getLangOpts().OpenCL)
        && castType->isVectorType() && (PE || PLE)) {
     if (PLE && PLE->getNumExprs() == 0) {
       Diag(PLE->getExprLoc(), diag::err_altivec_empty_initializer);
       return ExprError();
     }
     if (PE || PLE->getNumExprs() == 1) {
       Expr *E = (PE ? PE->getSubExpr() : PLE->getExpr(0));
-      if (!E->getType()->isVectorType())
+      if (!E->isTypeDependent() && !E->getType()->isVectorType())
         isVectorLiteral = true;
     }
     else
       isVectorLiteral = true;
   }
 
   // If this is a vector initializer, '(' type ')' '(' init, ..., init ')'
   // then handle it as such.

clang/test/SemaTemplate/pr47676.cpp

+// RUN: %clang_cc1 -triple=powerpc64le-unknown-linux-gnu \
+// RUN:            -target-feature +altivec -fsyntax-only -ast-dump \
+// RUN:            -xc++ < %s 2>&1 \
+// RUN:   | FileCheck %s
+
+// Ensures that casts to AltiVec type with a dependent expression operand does
+// not hit the assertion failure reported in PR47676. Further checks that casts
+// to AltiVec type with a dependent expression operand is, on instantiation,
+// able to correctly differentiate between a splat case and a bitcast case.
+template <typename T> void f(T *tp) {
+  extern void g(int, ...);
+  g(0, (__vector int)(*tp));
+  g(0, (__vector int)*tp);
+}
+
+void g(void) {
+  f<__vector float>(nullptr);
+//      CHECK: | |-FunctionDecl {{.*}} f 'void (__vector float *)'
+
+//      CHECK: |   | `-CStyleCastExpr {{.*}} '__vector int' <NoOp>
+// CHECK-NEXT: |   |   `-ImplicitCastExpr {{.*}} '__vector int' <BitCast>
+// CHECK-NEXT: |   |     `-ImplicitCastExpr {{.*}}'__vector float' <LValueToRValue>
+
+//      CHECK: |     `-CStyleCastExpr {{.*}} '__vector int' <NoOp>
+// CHECK-NEXT: |       `-ImplicitCastExpr {{.*}} '__vector int' <BitCast>
+// CHECK-NEXT: |         `-ImplicitCastExpr {{.*}}'__vector float' <LValueToRValue>
+
+  f<double>(nullptr);
+//      CHECK: | `-FunctionDecl {{.*}} f 'void (double *)'
+
+//      CHECK: |     | `-CStyleCastExpr {{.*}} '__vector int' <VectorSplat>
+// CHECK-NEXT: |     |   `-ImplicitCastExpr {{.*}} 'int' <FloatingToIntegral>
+// CHECK-NEXT: |     |     `-ImplicitCastExpr {{.*}}'double' <LValueToRValue>
+
+//      CHECK: |       `-CStyleCastExpr {{.*}} '__vector int' <VectorSplat>
+// CHECK-NEXT: |         `-ImplicitCastExpr {{.*}} 'int' <FloatingToIntegral>
+// CHECK-NEXT: |           `-ImplicitCastExpr {{.*}}:'double' <LValueToRValue>
+}