convert: Fix MismatchMessage for object attributes

When determining which object attributes cause a type mismatch, we must
disregard equal attribute types. Previously we would attempt to find a
conversion between them, deciding that the attribute type was a mismatch
if no conversion exists. However, this is invalid if the two types are
equal, because GetConversion returns nil for equal primitive types.

Because of non-deterministic map iteration order, the test case added in
this commit failed approximately 2/3 of the time before the change. It
now passes consistently.

cty/convert/mismatch_msg.go

@@ -84,6 +84,10 @@ func mismatchMessageObjects(got, want cty.Type) string {
 			continue
 		}
 
+		if gotAty.Equals(wantAty) {
+			continue // exact match, so no problem
+		}
+
 		// We'll now try to convert these attributes in isolation and
 		// see if we have a nested conversion error to report.
 		// We'll try an unsafe conversion first, and then fall back on

cty/convert/mismatch_msg_test.go

@@ -104,6 +104,24 @@ func TestMismatchMessage(t *testing.T) {
 			cty.List(cty.DynamicPseudoType),
 			`all list elements must have the same type`,
 		},
+		{
+			cty.Object(map[string]cty.Type{
+				"foo": cty.Bool,
+				"bar": cty.String,
+				"baz": cty.Object(map[string]cty.Type{
+					"boop": cty.Number,
+				}),
+			}),
+			cty.Object(map[string]cty.Type{
+				"foo": cty.Bool,
+				"bar": cty.String,
+				"baz": cty.Object(map[string]cty.Type{
+					"boop": cty.Number,
+					"beep": cty.Bool,
+				}),
+			}),
+			`attribute "baz": attribute "beep" is required`,
+		},
 	}
 
 	for _, test := range tests {