config: allow version constraints on providers, but validate them

We now accept syntactically-valid version constraints on provider blocks,
though we still don't actually do anything with them.

config/config.go

@@ -8,6 +8,7 @@ import (
 	"strconv"
 	"strings"
 
+	"github.com/blang/semver"
 	"github.com/hashicorp/go-multierror"
 	"github.com/hashicorp/hil"
 	"github.com/hashicorp/hil/ast"
@@ -350,8 +351,8 @@ func (c *Config) Validate() error {
 		}
 	}
 
-	// Check that providers aren't declared multiple times, and that versions
-	// aren't used yet since they aren't properly supported.
+	// Check that providers aren't declared multiple times and that their
+	// version constraints, where present, are syntactically valid.
 	providerSet := make(map[string]struct{})
 	for _, p := range c.ProviderConfigs {
 		name := p.FullName()
@@ -363,11 +364,14 @@ func (c *Config) Validate() error {
 		}
 
 		if p.Version != "" {
+			_, err := semver.ParseRange(p.Version)
+			if err != nil {
 				errs = append(errs, fmt.Errorf(
-				"provider.%s: version constraints are not yet supported; remove the 'version' argument from configuration",
-				name,
+					"provider.%s: invalid version constraint %q: %s",
+					name, p.Version, err,
 				))
 			}
+		}
 
 		providerSet[name] = struct{}{}
 	}

config/config_test.go

@@ -208,10 +208,16 @@ func TestConfigValidate_table(t *testing.T) {
 			"",
 		},
 		{
-			"provider with version constraint",
+			"provider with valid version constraint",
 			"provider-version",
+			false,
+			"",
+		},
+		{
+			"provider with invalid version constraint",
+			"provider-version-invalid",
 			true,
-			"version constraints are not yet supported",
+			"invalid version constraint",
 		},
 	}
 

config/test-fixtures/provider-version-invalid/main.tf

@@ -0,0 +1,5 @@
+provider "aws" {
+  version = "bananas"
+  a       = "a"
+  b       = "b"
+}