[clang] fix constexpr code generation for user conversions.

[mizvekov]

Summary

When building the member call to a user conversion function during an
implicit cast, the expression was not being checked for immediate
invocation, so we were never adding the ConstantExpr node to AST.

This would cause the call to the user conversion operator to be emitted
even if it was constantexpr evaluated, and this would even trip an
assert when said user conversion was declared consteval:
Assertion failed: !cast<FunctionDecl>(GD.getDecl())->isConsteval() && "consteval function should never be emitted", file clang\lib\CodeGen\CodeGenModule.cpp, line 3530

Fixes PR48855.

Signed-off-by: Matheus Izvekov <[e-mail]>

[mizvekov]

.

clang/lib/Sema/SemaExprCXX.cpp

ExprResult Sema::BuildCXXMemberCallExpr(Expr *E, NamedDecl *FoundDecl,
   CXXMemberCallExpr *CE = CXXMemberCallExpr::Create(
       Context, ME, /*Args=*/{}, ResultType, VK, Exp.get()->getEndLoc(),
       CurFPFeatureOverrides());
 
   if (CheckFunctionCall(Method, CE,
                         Method->getType()->castAs<FunctionProtoType>()))
     return ExprError();
 
-  return CE;
+  return CheckForImmediateInvocation(MaybeBindToTemporary(CE),
+                                     CE->getMethodDecl());
 }
 
 ExprResult Sema::BuildCXXNoexceptExpr(SourceLocation KeyLoc, Expr *Operand,
                                       SourceLocation RParen) {
   // If the operand is an unresolved lookup expression, the expression is ill-
   // formed per [over.over]p1, because overloaded function names cannot be used
   // without arguments except in explicit contexts.
   ExprResult R = CheckPlaceholderExpr(Operand);

clang/test/CodeGenCXX/cxx2a-consteval.cpp

 // EVAL-FN-NEXT:    [[ADD:%.*]] = add nsw i64 [[CONV]], [[TMP3]]
 // EVAL-FN-NEXT:    ret i64 [[ADD]]
 //
 long test_AggCtor() {
   const int i = 2;
   AggCtor C(i);
   return C.a + C.b;
 }
+
+struct UserConv {
+  consteval operator int() const noexcept { return 42; }
+};
+
+// EVAL-FN-LABEL: @_Z13test_UserConvv(
+// EVAL-FN-NEXT:  entry:
+// EVAL-FN-NEXT:    ret i32 42
+//
+int test_UserConv() {
+  return UserConv();
+}