splat expression on unknown should be unknown

When a splat expression is used to upgrade an unknown value to a list
with a single value, the result must be entirely unknown. For example,
if `ukstr` is an unknown string value, then `unkstr.*` could either
result in a list of 1 string value, or an empty list, depending on
whether the result is a string or null.

hclsyntax/expression.go

@@ -1432,9 +1432,22 @@ func (e *SplatExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostics) {
 		return cty.DynamicVal, diags
 	}
 
+	upgradedUnknown := false
 	if autoUpgrade {
+		// If we're upgrading an unknown value to a tuple/list, the result
+		// cannot be known. Otherwise a tuple containing an unknown value will
+		// upgrade to a different number of elements depending on whether
+		// sourceVal becomes null or not.
+		// We record this condition here so we can process any remaining
+		// expression after the * to derive the correct type. For example, it
+		// is valid to use a splat on a single object to retrieve a list of a
+		// single attribute, which means the final expression type still needs
+		// to be determined.
+		upgradedUnknown = !sourceVal.IsKnown()
+
 		sourceVal = cty.TupleVal([]cty.Value{sourceVal})
 		sourceTy = sourceVal.Type()
+
 	}
 
 	// We'll compute our result type lazily if we need it. In the normal case
@@ -1499,7 +1512,7 @@ func (e *SplatExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostics) {
 	}
 	e.Item.clearValue(ctx) // clean up our temporary value
 
-	if !isKnown {
+	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()

hclsyntax/expression_test.go

@@ -1135,6 +1135,16 @@ upper(
 			}),
 			0,
 		},
+		{
+			`unkstr[*]`,
+			&hcl.EvalContext{
+				Variables: map[string]cty.Value{
+					"unkstr": cty.UnknownVal(cty.String),
+				},
+			},
+			cty.UnknownVal(cty.Tuple([]cty.Type{cty.String})),
+			0,
+		},
 		{
 			`unkstr.*.name`,
 			&hcl.EvalContext{
@@ -1164,9 +1174,7 @@ upper(
 					})),
 				},
 			},
-			cty.TupleVal([]cty.Value{
-				cty.UnknownVal(cty.String),
-			}),
+			cty.UnknownVal(cty.Tuple([]cty.Type{cty.String})),
 			0,
 		},
 		{