convert: Don't modify the captured element type in a collection conve…

…rsion

The conversion constructor functions capture their arguments into a
closure to return, but some of them were then incorrectly modifying that
shared variable, which meant that the result of modifications made by one
call would be seen by subsequent calls, causing different (and possibly
totally incorrect) behavior.

We'll now directly return a replacement result type rather than modifying
the variable containing the requested type.

cty/convert/conversion_collection.go

@@ -45,8 +45,10 @@ func conversionCollectionToList(ety cty.Type, conv conversion) conversion {
 		}
 
 		if len(elems) == 0 {
+			// Prefer a concrete type over a dynamic type when returning an
+			// empty list
 			if ety == cty.DynamicPseudoType {
-				ety = val.Type().ElementType()
+				return cty.ListValEmpty(val.Type().ElementType()), nil
 			}
 			return cty.ListValEmpty(ety), nil
 		}
@@ -95,7 +97,7 @@ func conversionCollectionToSet(ety cty.Type, conv conversion) conversion {
 			// Prefer a concrete type over a dynamic type when returning an
 			// empty set
 			if ety == cty.DynamicPseudoType {
-				ety = val.Type().ElementType()
+				return cty.SetValEmpty(val.Type().ElementType()), nil
 			}
 			return cty.SetValEmpty(ety), nil
 		}
@@ -148,7 +150,7 @@ func conversionCollectionToMap(ety cty.Type, conv conversion) conversion {
 			// Prefer a concrete type over a dynamic type when returning an
 			// empty map
 			if ety == cty.DynamicPseudoType {
-				ety = val.Type().ElementType()
+				return cty.MapValEmpty(val.Type().ElementType()), nil
 			}
 			return cty.MapValEmpty(ety), nil
 		}