Error: Inconsistent conditional result types

Fix terraform "Error: Inconsistent conditional result types"

TL;DR: The two sides of your condition ? a : b have different types. Terraform unifies both branches, so they must match structurally. Make both sides the same type: convert with tostring()/tonumber(), give both object branches the same attribute set, or restructure so the condition lives inside a for filter instead of wrapping two different shapes.

The error

Error: Inconsistent conditional result types

  on main.tf line 20, in locals:
  20:   widgets = var.preview ? local.filtered : local.unfiltered

The true and false result expressions must have consistent types. The 'true'
value is a tuple with 11 elements, but the 'false' value is a tuple with 12
elements.

Steps

1. Read which side differs. The error spells it out: different tuple lengths, or The 'true' value includes object attribute "X", which is absent in the 'false' value.

2. For object branches with mismatched attributes, align the attribute sets. Add the missing attribute (even as null) to the smaller side:

   # wrong: var.x != null ? { a = 1, b = 2 } : { a = 1 }
   # right: var.x != null ? { a = 1, b = 2 } : { a = 1, b = null }

3. For list-length mismatches, do not branch on two prebuilt lists. Build one list with a filtered comprehension:

   widgets = [for w in local.all_widgets : w if var.preview ? w.preview : true]

4. For jsondecode() results, remember it returns an object, not a map. Wrap first: { for k, v in jsondecode(...) : k => v }.
5. Run terraform validate. Expected: Success! The configuration is valid.

When this applies

• validate fails with Inconsistent conditional result types on a ternary.
• One branch returns {} and the other returns a populated object, or branches differ in attribute sets or tuple lengths.

When it does NOT apply

• Inconsistent result after apply. That is a provider bug at apply time, not a type error in your expression.
• Errors inside dynamic blocks about each.value. That is an iterator naming issue, not a conditional.

Tool and version compatibility

• Terraform CLI 0.12+ through 1.x. Type unification rules unchanged.

Why it happens

Terraform must know the expression's type without evaluating the condition, so it unifies both branches into one type. Objects unify only when their attribute sets match; tuples unify only at equal length. A mismatch is a static type error, not a runtime one, so it fails at validate.

Edge cases and pitfalls

• The error fires in both variable values, because unification ignores which branch would be taken. You cannot hide a bad branch behind a condition that is currently false.
• {} vs populated object is the most common trigger. Decide the full attribute set up front and use nulls for the absent ones.
• try() does not help here; this is a type error, not an evaluation error.