Fix panic when the single interpolation result is null

Author: dsa0x

hclsyntax/expression_template.go

@@ -250,7 +250,21 @@ func (e *TemplateWrapExpr) walkChildNodes(w internalWalkFunc) {
 }
 
 func (e *TemplateWrapExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostics) {
-	return e.Wrapped.Value(ctx)
+	val, diags := e.Wrapped.Value(ctx)
+
+	// If the single interpolation result is null, we need to return the same
+	// error as we would for a TemplateExpr with a null part.
+	if val.IsNull() {
+		diags = append(diags, &hcl.Diagnostic{
+			Severity:    hcl.DiagError,
+			Summary:     "Invalid template interpolation value",
+			Detail:      "The expression result is null. Cannot include a null value in a string template.",
+			Subject:     e.Range().Ptr(),
+			Expression:  e.Wrapped,
+			EvalContext: ctx,
+		})
+	}
+	return val, diags
 }
 
 func (e *TemplateWrapExpr) Range() hcl.Range {

hclsyntax/expression_template_test.go

@@ -403,6 +403,26 @@ trim`,
 			cty.UnknownVal(cty.String).Mark("sensitive").Refine().NotNull().StringPrefixFull("test_").NewValue(),
 			0,
 		},
+		{ // template with a single wrapped expr that is null
+			`${hello}`,
+			&hcl.EvalContext{
+				Variables: map[string]cty.Value{
+					"hello": cty.NullVal(cty.String),
+				},
+			},
+			cty.NullVal(cty.String),
+			1, // error about null value
+		},
+		{
+			`foo ${hello}`,
+			&hcl.EvalContext{
+				Variables: map[string]cty.Value{
+					"hello": cty.NullVal(cty.String),
+				},
+			},
+			cty.StringVal("foo "),
+			1, // error about null value
+		},
 	}
 
 	for _, test := range tests {