[llvm-bugs] [Bug 38166] New: AST should explain the implicitness of the casts better

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=38166

            Bug ID: 38166
           Summary: AST should explain the implicitness of the casts
                    better
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Frontend
          Assignee: unassignedclangbugs at nondot.org
          Reporter: lebedev.ri at gmail.com
                CC: llvm-bugs at lists.llvm.org

Related to #38017.
We can't actually distinguish whether the implicit cast is implicit in source
code, or is part of explicit cast.
We can't use ASTContext::getParents() (partly performance, partly semantics).
But we also sometimes don't visit the ExplicitCastExpr.

<zygoloid> CodeGenFunction::EmitMaterializeTemporaryExpr calls
skipRValueSubobjectAdjustments, which steps over the CK_NoOp cast

<zygoloid> OK, so we could fix that, or we could add some state to the CastExpr
so you can tell whether it's an "implicitly doing bad things" cast kind

<zygoloid> or maybe a CastKind; we could use CK_UncheckedIntegralCast for
"integral cast where the user explicitly asked for it" versus CK_IntegralCast
for "integral cast that was performed implicitly"

<zygoloid> i don't think "is part of explicit cast" is quite right; given
"struct X { X(char); }", X(some_int) should still be checked

<zygoloid> but perhaps "enum E : char {}; E(some_int)" is more borderline
<LebedevRI> which solution does sound best to you? the ast one, or something
involving skipRValueSubobjectAdjustments() ? /me needs to know how to title the
bug

<zygoloid> I think it makes sense to make skipRValueSubobjectAdjustments skip
the minimum amount of stuff necessary to get to the expression constructing the
temporary
<zygoloid> but I'd be OK with having a new cast kind for this, because in the
generalized Clang model (including UBSan) the cast has different semantics

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