Generic arguments are not disambiguated on error propagation expressions

[CohenArthur]

I tried this code:

#[lang = "sized"]
trait Sized {}

enum Result<T, E> {
    Ok(T),
    Err(E),
}

pub struct Layout {
    // size of the requested block of memory, measured in bytes.
    size_: usize,
}

impl Layout {
    /// Constructs a `Layout` suitable for holding a value of type `T`.
    #[stable(feature = "alloc_layout", since = "1.28.0")]
    #[inline]
    pub const fn new<T>() -> Self {
        Layout { size_: 14 }
    }

    pub fn repeat(&self, n: usize) -> Result<(Self, usize), ()> {
        unsafe { Result::Ok((Layout { size_: 14 }, n + 12)) }
    }

    /// Creates a layout describing the record for a `[T; n]`.
    ///
    /// On arithmetic overflow, returns `LayoutErr`.
    #[stable(feature = "alloc_layout_manipulation", since = "1.44.0")]
    #[inline]
    pub fn array<T>(n: usize) -> Self {
        let (layout, offset) = Layout::new::<T>().repeat(n)?;

        layout
    }
}

fn main() {}

https://godbolt.org/z/he1WPf9G1

I expected to see this happen: no error

Instead, this happened:

crab1: internal compiler error: in lower_generic_args, at rust/hir/rust-ast-lower-base.cc:642
0x249d4f1 Rust::HIR::ASTLoweringBase::lower_generic_args(Rust::AST::GenericArgs&)
	/home/kagounard/Git/rust-gcc/gccrs/build/gcc/../../gcc/rust/hir/rust-ast-lower-base.cc:642
0x249d039 Rust::HIR::ASTLoweringBase::lower_path_expr_seg(Rust::AST::PathExprSegment&)
	/home/kagounard/Git/rust-gcc/gccrs/build/gcc/../../gcc/rust/hir/rust-ast-lower-base.cc:590
0x2496f93 Rust::HIR::ASTLowerPathInExpression::visit(Rust::AST::PathInExpression&)
	/home/kagounard/Git/rust-gcc/gccrs/build/gcc/../../gcc/rust/hir/rust-ast-lower.cc:413
0x228f4fd Rust::AST::PathInExpression::accept_vis(Rust::AST::ASTVisitor&)
	/home/kagounard/Git/rust-gcc/gccrs/build/gcc/../../gcc/rust/ast/rust-path.cc:189
0x249fd5f Rust::HIR::ASTLowerPathInExpression::translate(Rust::AST::PathInExpression&)
	../../gcc/rust/hir/rust-ast-lower-expr.h:36
0x24b67d0 Rust::HIR::ASTLoweringExpr::visit(Rust::AST::PathInExpression&)
	/home/kagounard/Git/rust-gcc/gccrs/build/gcc/../../gcc/rust/hir/rust-ast-lower-expr.cc:135
0x228f4fd Rust::AST::PathInExpression::accept_vis(Rust::AST::ASTVisitor&)
	/home/kagounard/Git/rust-gcc/gccrs/build/gcc/../../gcc/rust/ast/rust-path.cc:189
0x24b6126 Rust::HIR::ASTLoweringExpr::translate(Rust::AST::Expr&, bool*)
	/home/kagounard/Git/rust-gcc/gccrs/build/gcc/../../gcc/rust/hir/rust-ast-lower-expr.cc:40
0x24b6a5b Rust::HIR::ASTLoweringExpr::visit(Rust::AST::CallExpr&)
	/home/kagounard/Git/rust-gcc/gccrs/build/gcc/../../gcc/rust/hir/rust-ast-lower-expr.cc:165
0x21ae790 Rust::AST::CallExpr::accept_vis(Rust::AST::ASTVisitor&)
	/home/kagounard/Git/rust-gcc/gccrs/build/gcc/../../gcc/rust/ast/rust-ast.cc:4479
0x24b6126 Rust::HIR::ASTLoweringExpr::translate(Rust::AST::Expr&, bool*)
	/home/kagounard/Git/rust-gcc/gccrs/build/gcc/../../gcc/rust/hir/rust-ast-lower-expr.cc:40
0x24b6cfd Rust::HIR::ASTLoweringExpr::visit(Rust::AST::MethodCallExpr&)
	/home/kagounard/Git/rust-gcc/gccrs/build/gcc/../../gcc/rust/hir/rust-ast-lower-expr.cc:191
0x21ae7c0 Rust::AST::MethodCallExpr::accept_vis(Rust::AST::ASTVisitor&)
	/home/kagounard/Git/rust-gcc/gccrs/build/gcc/../../gcc/rust/ast/rust-ast.cc:4485
0x24b6126 Rust::HIR::ASTLoweringExpr::translate(Rust::AST::Expr&, bool*)
	/home/kagounard/Git/rust-gcc/gccrs/build/gcc/../../gcc/rust/hir/rust-ast-lower-expr.cc:40
0x24b9ca5 Rust::HIR::ASTLoweringExpr::visit(Rust::AST::ErrorPropagationExpr&)
	/home/kagounard/Git/rust-gcc/gccrs/build/gcc/../../gcc/rust/hir/rust-ast-lower-expr.cc:676
0x21ae3d0 Rust::AST::ErrorPropagationExpr::accept_vis(Rust::AST::ASTVisitor&)
	/home/kagounard/Git/rust-gcc/gccrs/build/gcc/../../gcc/rust/ast/rust-ast.cc:4359
0x24b6126 Rust::HIR::ASTLoweringExpr::translate(Rust::AST::Expr&, bool*)
	/home/kagounard/Git/rust-gcc/gccrs/build/gcc/../../gcc/rust/hir/rust-ast-lower-expr.cc:40
0x24c3e2d Rust::HIR::ASTLoweringStmt::visit(Rust::AST::LetStmt&)
	/home/kagounard/Git/rust-gcc/gccrs/build/gcc/../../gcc/rust/hir/rust-ast-lower-stmt.cc:75
0x21af460 Rust::AST::LetStmt::accept_vis(Rust::AST::ASTVisitor&)
	/home/kagounard/Git/rust-gcc/gccrs/build/gcc/../../gcc/rust/ast/rust-ast.cc:4851
0x24c3b43 Rust::HIR::ASTLoweringStmt::translate(Rust::AST::Stmt*, bool*)
	/home/kagounard/Git/rust-gcc/gccrs/build/gcc/../../gcc/rust/hir/rust-ast-lower-stmt.cc:32
Please submit a full bug report, with preprocessed source (by using -freport-bug).
Please include the complete backtrace with any bug report.
See <https://gcc.gnu.org/bugs/> for instructions.

the ICE happens because lower_generic_args sees a GenericArg::Ambiguous and calls rust_unreachable. at this point in time, all generic args should have been disambiguated to a type or a const generic - but we don't do that disambiguation in error propagation expressions (<expr> ?)