Terraform crash on plan after upgrading to 1.6.1 (value is unknown)

[adebande]

Terraform Version

Terraform v1.6.1
on darwin_arm64

Terraform Configuration Files

terraform {
  required_providers {
    aws = {
      source  = "hashicorp/aws"
      version = "~> 5.0"
    }
  }

  backend "s3" {
    region         = "xxxxx"
    bucket         = "xxxxx"
    key            = "xxxxx"
    dynamodb_table = "xxxxx"
    profile        = "xxxxx"
  }
}
...

Debug Output

!!!!!!!!!!!!!!!!!!!!!!!!!!! TERRAFORM CRASH !!!!!!!!!!!!!!!!!!!!!!!!!!!!

value is unknown
goroutine 621 [running]:
runtime/debug.Stack()
        /Users/runner/hostedtoolcache/go/1.21.1/x64/src/runtime/debug/stack.go:24 +0x64
runtime/debug.PrintStack()
        /Users/runner/hostedtoolcache/go/1.21.1/x64/src/runtime/debug/stack.go:16 +0x1c
github.com/hashicorp/terraform/internal/logging.PanicHandler()
        /Users/runner/work/terraform/terraform/internal/logging/panic.go:58 +0x164
panic({0x10738a8a0?, 0x10797a820?})
        /Users/runner/hostedtoolcache/go/1.21.1/x64/src/runtime/panic.go:920 +0x26c
github.com/zclconf/go-cty/cty.Value.AsBigFloat({{{0x1079b7bc8?, 0x140001aabd8?}}, {0x107469240?, 0x140030e9700?}})
        /Users/runner/go/pkg/mod/github.com/zclconf/go-cty@v1.14.1/cty/value_ops.go:1413 +0x110
github.com/hashicorp/hcl/v2/hclsyntax.(*ConditionalExpr).Value(0x1400086d9d0, 0x14002679770)
        /Users/runner/go/pkg/mod/github.com/hashicorp/hcl/v2@v2.18.1/hclsyntax/expression.go:728 +0x868
github.com/hashicorp/hcl/v2/ext/dynblock.(*expandBody).decodeSpec(0x1400278c4e0, 0x14002f00490, 0x14002957040)
        /Users/runner/go/pkg/mod/github.com/hashicorp/hcl/v2@v2.18.1/ext/dynblock/expand_spec.go:44 +0x168
github.com/hashicorp/hcl/v2/ext/dynblock.(*expandBody).expandBlocks(0x1400278c4e0, 0x1400278d200, {0x1400267fe80, 0x6, 0x0?}, 0x1)
        /Users/runner/go/pkg/mod/github.com/hashicorp/hcl/v2@v2.18.1/ext/dynblock/expand_body.go:186 +0x4b8
github.com/hashicorp/hcl/v2/ext/dynblock.(*expandBody).PartialContent(0x1400278c4e0, 0x1400278d200)
        /Users/runner/go/pkg/mod/github.com/hashicorp/hcl/v2@v2.18.1/ext/dynblock/expand_body.go:55 +0x7c
github.com/hashicorp/hcl/v2/ext/dynblock.WalkVariablesNode.Visit({{0x1079b89d0?, 0x1400278c4e0?}, 0x0?, 0x68?}, 0x1400278cf00)
        /Users/runner/go/pkg/mod/github.com/hashicorp/hcl/v2@v2.18.1/ext/dynblock/variables.go:75 +0x1dc
github.com/hashicorp/terraform/internal/lang/blocktoattr.walkVariables({{0x1079b89d0?, 0x1400278c4e0?}, 0x0?, 0x0?}, {0x1079b89d0, 0x1400278c4e0}, 0x14005aab2f0)
        /Users/runner/work/terraform/terraform/internal/lang/blocktoattr/variables.go:34 +0x90
github.com/hashicorp/terraform/internal/lang/blocktoattr.ExpandedVariables(...)
        /Users/runner/work/terraform/terraform/internal/lang/blocktoattr/variables.go:27
github.com/hashicorp/terraform/internal/lang.ReferencesInBlock(0x14005aab2f0?, {0x1079b89d0?, 0x1400278c4e0?}, 0x1051c5448?)
        /Users/runner/work/terraform/terraform/internal/lang/references.go:72 +0x4c
github.com/hashicorp/terraform/internal/lang.(*Scope).EvalBlock(0x1400267c240, {0x1079b89d0, 0x1400278c4e0}, 0x14002f01168?)
        /Users/runner/work/terraform/terraform/internal/lang/eval.go:53 +0x58
github.com/hashicorp/terraform/internal/terraform.(*BuiltinEvalContext).EvaluateBlock(0x0?, {0x1079b81b8, 0x14000e856b0}, 0x107699640?, {0x0?, 0x0?}, {{{{0x0, 0x0}}, {0x0, 0x0}}, ...})
        /Users/runner/work/terraform/terraform/internal/terraform/eval_context_builtin.go:282 +0xe0
github.com/hashicorp/terraform/internal/terraform.(*NodeValidatableResource).validateResource(0x14000b8a630, {0x1079d2490, 0x140015208c0})
        /Users/runner/work/terraform/terraform/internal/terraform/node_resource_validate.go:372 +0x4bc
github.com/hashicorp/terraform/internal/terraform.(*NodeValidatableResource).Execute(0x14000b8a630, {0x1079d2490, 0x140015208c0}, 0x80?)
        /Users/runner/work/terraform/terraform/internal/terraform/node_resource_validate.go:52 +0x40
github.com/hashicorp/terraform/internal/terraform.(*ContextGraphWalker).Execute(0x14006c96b40, {0x1079d2490, 0x140015208c0}, {0x150b0a098, 0x14000b8a630})
        /Users/runner/work/terraform/terraform/internal/terraform/graph_walk_context.go:143 +0xa0
github.com/hashicorp/terraform/internal/terraform.(*Graph).walk.func1({0x1078d35e0, 0x14000b8a630})
        /Users/runner/work/terraform/terraform/internal/terraform/graph.go:78 +0x280
github.com/hashicorp/terraform/internal/dag.(*Walker).walkVertex(0x140010d28a0, {0x1078d35e0, 0x14000b8a630}, 0x14002a49c00)
        /Users/runner/work/terraform/terraform/internal/dag/walk.go:384 +0x2a8
created by github.com/hashicorp/terraform/internal/dag.(*Walker).Update in goroutine 28
        /Users/runner/work/terraform/terraform/internal/dag/walk.go:307 +0xb0c

Expected Behavior

Successful terraform plan

Actual Behavior

terraform plan results in a crash

Steps to Reproduce

1. terraform init -reconfigure
2. terraform plan

Additional Context

This issue appears after upgrading to Terraform 1.6 (existing project, works fine on 1.5.7).
Big project with a lot of conditional expressions and dynamic blocks, backend on S3 with DynamoDB lock and multiple AWS provider aliases.

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