lang: EvalExpr only convert if wantType is not dynamic

This actually seems to be a bug in the underlying cty Convert function
since converting to cty.DynamicPseudoType should always just return the
input verbatim, but it seems like it's actually converting unknown values
of any type to be cty.DynamicVal, losing the type information.

We should eventually fix this in cty too, but having this extra check in
the Terraform layer is harmless and allows us to make progress without
context-switching.

lang/eval.go

@@ -88,16 +88,18 @@ func (s *Scope) EvalExpr(expr hcl.Expression, wantType cty.Type) (cty.Value, tfd
 	val, evalDiags := expr.Value(ctx)
 	diags = diags.Append(evalDiags)
 
-	var convErr error
-	val, convErr = convert.Convert(val, wantType)
-	if convErr != nil {
-		val = cty.UnknownVal(wantType)
-		diags = diags.Append(&hcl.Diagnostic{
-			Severity: hcl.DiagError,
-			Summary:  "Incorrect value type",
-			Detail:   fmt.Sprintf("Invalid expression value: %s.", tfdiags.FormatError(convErr)),
-			Subject:  expr.Range().Ptr(),
-		})
+	if wantType != cty.DynamicPseudoType {
+		var convErr error
+		val, convErr = convert.Convert(val, wantType)
+		if convErr != nil {
+			val = cty.UnknownVal(wantType)
+			diags = diags.Append(&hcl.Diagnostic{
+				Severity: hcl.DiagError,
+				Summary:  "Incorrect value type",
+				Detail:   fmt.Sprintf("Invalid expression value: %s.", tfdiags.FormatError(convErr)),
+				Subject:  expr.Range().Ptr(),
+			})
+		}
 	}
 
 	return val, diags