unknown upgraded splat values may have no elems

When upgrading an unknown splat value, the resulting collection may have
0 elements if the value ends up being `null`. This means the type must
either be dynamic or a list.

hclsyntax/expression.go

@@ -1513,9 +1513,24 @@ func (e *SplatExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostics) {
 	e.Item.clearValue(ctx) // clean up our temporary value
 
 	if !isKnown || upgradedUnknown {
-		// We'll ingore the resultTy diagnostics in this case since they
-		// will just be the same errors we saw while iterating above.
-		ty, _ := resultTy()
+		// Only check the type diags for errors to return a DynamicVal when
+		// upgrading instead of a tuple, but return the original diagnostics
+		// which already contain the relevant errors.
+		ty, tyDiags := resultTy()
+		if tyDiags.HasErrors() {
+			ty = cty.DynamicPseudoType
+		}
+
+		if upgradedUnknown {
+			// Because of the way null values are upgraded we may have 0 or 1
+			// elements, so we must convert the type to an unknown list as long
+			// as we have a single element type.
+			if ty.IsTupleType() && ty.Length() == 1 {
+				// All upgraded should
+				ty = cty.List(ty.TupleElementType(0))
+			}
+		}
+
 		return cty.UnknownVal(ty), diags
 	}
 

hclsyntax/expression_test.go

@@ -1142,7 +1142,7 @@ upper(
 					"unkstr": cty.UnknownVal(cty.String),
 				},
 			},
-			cty.UnknownVal(cty.Tuple([]cty.Type{cty.String})),
+			cty.UnknownVal(cty.List(cty.String)),
 			0,
 		},
 		{
@@ -1152,7 +1152,7 @@ upper(
 					"unkstr": cty.UnknownVal(cty.String),
 				},
 			},
-			cty.UnknownVal(cty.Tuple([]cty.Type{cty.DynamicPseudoType})),
+			cty.DynamicVal,
 			1, // a string has no attribute "name"
 		},
 		{
@@ -1174,7 +1174,19 @@ upper(
 					})),
 				},
 			},
-			cty.UnknownVal(cty.Tuple([]cty.Type{cty.String})),
+			cty.UnknownVal(cty.List(cty.String)),
+			0,
+		},
+		{
+			`unkobj.*.names`,
+			&hcl.EvalContext{
+				Variables: map[string]cty.Value{
+					"unkobj": cty.UnknownVal(cty.Object(map[string]cty.Type{
+						"names": cty.List(cty.String),
+					})),
+				},
+			},
+			cty.UnknownVal(cty.List(cty.List(cty.String))),
 			0,
 		},
 		{