configs: Simplify required_providers blocks

We now permit at most one `required_providers` block per module (except
for overrides). This prevents users (and Terraform) from struggling to
understand how to merge multiple `required_providers` configurations,
with `version` and `source` attributes split across multiple blocks.

Because only one `required_providers` block is permitted, there is no
need to concatenate version constraints and resolve them. This allows us
to simplify the structs used to represent provider requirements,
aligning more closely with other structs in this package.

This commit also fixes a semantic use-before-initialize bug, where
resources defined before a `required_providers` block would be unable to
use its source attribute. We achieve this by processing the module's
`required_providers` configuration (and overrides) before resources.

Overrides for `required_providers` work as before, replacing the entire
block per provider.

configs/config.go

@@ -185,24 +185,22 @@ func (c *Config) addProviderRequirements(reqs getproviders.Requirements) hcl.Dia
 	var diags hcl.Diagnostics
 
 	// First we'll deal with the requirements directly in _our_ module...
-	for _, providerReqs := range c.Module.ProviderRequirements {
+	for _, providerReqs := range c.Module.ProviderRequirements.RequiredProviders {
 		fqn := providerReqs.Type
 		if _, ok := reqs[fqn]; !ok {
 			// We'll at least have an unconstrained dependency then, but might
 			// add to this in the loop below.
 			reqs[fqn] = nil
 		}
-		for _, constraintsSrc := range providerReqs.VersionConstraints {
 		// The model of version constraints in this package is still the
 		// old one using a different upstream module to represent versions,
 		// so we'll need to shim that out here for now. We assume this
 		// will always succeed because these constraints already succeeded
 		// parsing with the other constraint parser, which uses the same
 		// syntax.
-			constraints := getproviders.MustParseVersionConstraints(constraintsSrc.Required.String())
+		constraints := getproviders.MustParseVersionConstraints(providerReqs.Requirement.Required.String())
 		reqs[fqn] = append(reqs[fqn], constraints...)
 	}
-	}
 	// Each resource in the configuration creates an *implicit* provider
 	// dependency, though we'll only record it if there isn't already
 	// an explicit dependency on the same provider.
@@ -321,7 +319,7 @@ func (c *Config) ResolveAbsProviderAddr(addr addrs.ProviderConfig, inModule addr
 		}
 
 		var provider addrs.Provider
-		if providerReq, exists := c.Module.ProviderRequirements[addr.LocalName]; exists {
+		if providerReq, exists := c.Module.ProviderRequirements.RequiredProviders[addr.LocalName]; exists {
 			provider = providerReq.Type
 		} else {
 			provider = addrs.ImpliedProviderForUnqualifiedType(addr.LocalName)
@@ -343,7 +341,7 @@ func (c *Config) ResolveAbsProviderAddr(addr addrs.ProviderConfig, inModule addr
 // by checking for the provider in module.ProviderRequirements and falling
 // back to addrs.NewDefaultProvider if it is not found.
 func (c *Config) ProviderForConfigAddr(addr addrs.LocalProviderConfig) addrs.Provider {
-	if provider, exists := c.Module.ProviderRequirements[addr.LocalName]; exists {
+	if provider, exists := c.Module.ProviderRequirements.RequiredProviders[addr.LocalName]; exists {
 		return provider.Type
 	}
 	return c.ResolveAbsProviderAddr(addr, addrs.RootModule).Provider

configs/module.go

@@ -7,7 +7,6 @@ import (
 
 	"github.com/hashicorp/terraform/addrs"
 	"github.com/hashicorp/terraform/experiments"
-	"github.com/hashicorp/terraform/tfdiags"
 )
 
 // Module is a container for a set of configuration constructs that are
@@ -31,7 +30,7 @@ type Module struct {
 
 	Backend              *Backend
 	ProviderConfigs      map[string]*Provider
-	ProviderRequirements map[string]ProviderRequirements
+	ProviderRequirements *RequiredProviders
 	ProviderLocalNames   map[addrs.Provider]string
 	ProviderMetas        map[addrs.Provider]*ProviderMeta
 
@@ -64,7 +63,7 @@ type File struct {
 	Backends          []*Backend
 	ProviderConfigs   []*Provider
 	ProviderMetas     []*ProviderMeta
-	RequiredProviders []*RequiredProvider
+	RequiredProviders []*RequiredProviders
 
 	Variables []*Variable
 	Locals    []*Local
@@ -88,7 +87,6 @@ func NewModule(primaryFiles, overrideFiles []*File) (*Module, hcl.Diagnostics) {
 	var diags hcl.Diagnostics
 	mod := &Module{
 		ProviderConfigs:    map[string]*Provider{},
-		ProviderRequirements: map[string]ProviderRequirements{},
 		ProviderLocalNames: map[addrs.Provider]string{},
 		Variables:          map[string]*Variable{},
 		Locals:             map[string]*Local{},
@@ -99,6 +97,41 @@ func NewModule(primaryFiles, overrideFiles []*File) (*Module, hcl.Diagnostics) {
 		ProviderMetas:      map[addrs.Provider]*ProviderMeta{},
 	}
 
+	// Process the required_providers blocks first, to ensure that all
+	// resources have access to the correct provider FQNs
+	for _, file := range primaryFiles {
+		for _, r := range file.RequiredProviders {
+			if mod.ProviderRequirements != nil {
+				diags = append(diags, &hcl.Diagnostic{
+					Severity: hcl.DiagError,
+					Summary:  "Duplicate required providers configuration",
+					Detail:   fmt.Sprintf("A module may have only one required providers configuration. The required providers were previously configured at %s.", mod.ProviderRequirements.DeclRange),
+					Subject:  &r.DeclRange,
+				})
+				continue
+			}
+			mod.ProviderRequirements = r
+		}
+	}
+
+	// If no required_providers block is configured, create a useful empty
+	// state to reduce nil checks elsewhere
+	if mod.ProviderRequirements == nil {
+		mod.ProviderRequirements = &RequiredProviders{
+			RequiredProviders: make(map[string]*RequiredProvider),
+		}
+	}
+
+	// Any required_providers blocks in override files replace the entire
+	// block for each provider
+	for _, file := range overrideFiles {
+		for _, override := range file.RequiredProviders {
+			for name, rp := range override.RequiredProviders {
+				mod.ProviderRequirements.RequiredProviders[name] = rp
+			}
+		}
+	}
+
 	for _, file := range primaryFiles {
 		fileDiags := mod.appendFile(file)
 		diags = append(diags, fileDiags...)
@@ -178,35 +211,6 @@ func (m *Module) appendFile(file *File) hcl.Diagnostics {
 		m.ProviderConfigs[key] = pc
 	}
 
-	for _, reqd := range file.RequiredProviders {
-		var fqn addrs.Provider
-		if reqd.Source.SourceStr != "" {
-			var sourceDiags tfdiags.Diagnostics
-			fqn, sourceDiags = addrs.ParseProviderSourceString(reqd.Source.SourceStr)
-			hclDiags := sourceDiags.ToHCL()
-			// The diagnostics from ParseProviderSourceString don't contain
-			// source location information because it has no context to compute
-			// them from, and so we'll add those in quickly here before we
-			// return.
-			for _, diag := range hclDiags {
-				if diag.Subject == nil {
-					diag.Subject = reqd.Source.DeclRange.Ptr()
-				}
-			}
-			diags = append(diags, hclDiags...)
-		} else {
-			fqn = addrs.ImpliedProviderForUnqualifiedType(reqd.Name)
-		}
-		if existing, exists := m.ProviderRequirements[reqd.Name]; exists {
-			if existing.Type != fqn {
-				panic("provider fqn mismatch")
-			}
-			existing.VersionConstraints = append(existing.VersionConstraints, reqd.Requirement)
-		} else {
-			m.ProviderRequirements[reqd.Name] = ProviderRequirements{Type: fqn, VersionConstraints: []VersionConstraint{reqd.Requirement}}
-		}
-	}
-
 	for _, pm := range file.ProviderMetas {
 		provider := m.ProviderForLocalConfig(addrs.LocalProviderConfig{LocalName: pm.Provider})
 		if existing, exists := m.ProviderMetas[provider]; exists {
@@ -283,7 +287,7 @@ func (m *Module) appendFile(file *File) hcl.Diagnostics {
 
 		// set the provider FQN for the resource
 		if r.ProviderConfigRef != nil {
-			if existing, exists := m.ProviderRequirements[r.ProviderConfigAddr().LocalName]; exists {
+			if existing, exists := m.ProviderRequirements.RequiredProviders[r.ProviderConfigAddr().LocalName]; exists {
 				r.Provider = existing.Type
 			} else {
 				r.Provider = addrs.ImpliedProviderForUnqualifiedType(r.ProviderConfigAddr().LocalName)
@@ -308,7 +312,7 @@ func (m *Module) appendFile(file *File) hcl.Diagnostics {
 
 		// set the provider FQN for the resource
 		if r.ProviderConfigRef != nil {
-			if existing, exists := m.ProviderRequirements[r.ProviderConfigAddr().LocalName]; exists {
+			if existing, exists := m.ProviderRequirements.RequiredProviders[r.ProviderConfigAddr().LocalName]; exists {
 				r.Provider = existing.Type
 			} else {
 				r.Provider = addrs.ImpliedProviderForUnqualifiedType(r.ProviderConfigAddr().LocalName)
@@ -382,10 +386,6 @@ func (m *Module) mergeFile(file *File) hcl.Diagnostics {
 		}
 	}
 
-	if len(file.RequiredProviders) != 0 {
-		mergeProviderVersionConstraints(m.ProviderRequirements, file.RequiredProviders)
-	}
-
 	for _, v := range file.Variables {
 		existing, exists := m.Variables[v.Name]
 		if !exists {
@@ -458,7 +458,7 @@ func (m *Module) mergeFile(file *File) hcl.Diagnostics {
 			})
 			continue
 		}
-		mergeDiags := existing.merge(r, m.ProviderRequirements)
+		mergeDiags := existing.merge(r, m.ProviderRequirements.RequiredProviders)
 		diags = append(diags, mergeDiags...)
 	}
 
@@ -474,7 +474,7 @@ func (m *Module) mergeFile(file *File) hcl.Diagnostics {
 			})
 			continue
 		}
-		mergeDiags := existing.merge(r, m.ProviderRequirements)
+		mergeDiags := existing.merge(r, m.ProviderRequirements.RequiredProviders)
 		diags = append(diags, mergeDiags...)
 	}
 
@@ -487,7 +487,7 @@ func (m *Module) mergeFile(file *File) hcl.Diagnostics {
 // only be populated after the module has been parsed.
 func (m *Module) gatherProviderLocalNames() {
 	providers := make(map[addrs.Provider]string)
-	for k, v := range m.ProviderRequirements {
+	for k, v := range m.ProviderRequirements.RequiredProviders {
 		providers[v.Type] = k
 	}
 	m.ProviderLocalNames = providers
@@ -507,7 +507,7 @@ func (m *Module) LocalNameForProvider(p addrs.Provider) string {
 
 // ProviderForLocalConfig returns the provider FQN for a given LocalProviderConfig
 func (m *Module) ProviderForLocalConfig(pc addrs.LocalProviderConfig) addrs.Provider {
-	if provider, exists := m.ProviderRequirements[pc.LocalName]; exists {
+	if provider, exists := m.ProviderRequirements.RequiredProviders[pc.LocalName]; exists {
 		return provider.Type
 	}
 	return addrs.ImpliedProviderForUnqualifiedType(pc.LocalName)

configs/module_merge.go

@@ -35,25 +35,6 @@ func (p *Provider) merge(op *Provider) hcl.Diagnostics {
 	return diags
 }
 
-func mergeProviderVersionConstraints(recv map[string]ProviderRequirements, ovrd []*RequiredProvider) {
-	// Any provider name that's mentioned in the override gets nilled out in
-	// our map so that we'll rebuild it below. Any provider not mentioned is
-	// left unchanged.
-	for _, reqd := range ovrd {
-		delete(recv, reqd.Name)
-	}
-	for _, reqd := range ovrd {
-		var fqn addrs.Provider
-		if reqd.Source.SourceStr != "" {
-			// any errors parsing the source string will have already been captured.
-			fqn, _ = addrs.ParseProviderSourceString(reqd.Source.SourceStr)
-		} else {
-			fqn = addrs.ImpliedProviderForUnqualifiedType(reqd.Name)
-		}
-		recv[reqd.Name] = ProviderRequirements{Type: fqn, VersionConstraints: []VersionConstraint{reqd.Requirement}}
-	}
-}
-
 func (v *Variable) merge(ov *Variable) hcl.Diagnostics {
 	var diags hcl.Diagnostics
 
@@ -197,7 +178,7 @@ func (mc *ModuleCall) merge(omc *ModuleCall) hcl.Diagnostics {
 	return diags
 }
 
-func (r *Resource) merge(or *Resource, prs map[string]ProviderRequirements) hcl.Diagnostics {
+func (r *Resource) merge(or *Resource, rps map[string]*RequiredProvider) hcl.Diagnostics {
 	var diags hcl.Diagnostics
 
 	if r.Mode != or.Mode {
@@ -215,7 +196,7 @@ func (r *Resource) merge(or *Resource, prs map[string]ProviderRequirements) hcl.
 
 	if or.ProviderConfigRef != nil {
 		r.ProviderConfigRef = or.ProviderConfigRef
-		if existing, exists := prs[or.ProviderConfigRef.Name]; exists {
+		if existing, exists := rps[or.ProviderConfigRef.Name]; exists {
 			r.Provider = existing.Type
 		} else {
 			r.Provider = addrs.ImpliedProviderForUnqualifiedType(r.ProviderConfigRef.Name)

configs/module_merge_test.go

@@ -3,7 +3,6 @@ package configs
 import (
 	"testing"
 
-	version "github.com/hashicorp/go-version"
 	"github.com/hashicorp/hcl/v2"
 	"github.com/hashicorp/hcl/v2/gohcl"
 	"github.com/hashicorp/terraform/addrs"
@@ -232,98 +231,3 @@ func TestModuleOverrideResourceFQNs(t *testing.T) {
 		t.Fatalf("wrong result: found provider config ref %s, expected nil", got.ProviderConfigRef)
 	}
 }
-
-func TestMergeProviderVersionConstraints(t *testing.T) {
-	v1, _ := version.NewConstraint("1.0.0")
-	vc1 := VersionConstraint{
-		Required: v1,
-	}
-	v2, _ := version.NewConstraint("2.0.0")
-	vc2 := VersionConstraint{
-		Required: v2,
-	}
-
-	tests := map[string]struct {
-		Input    map[string]ProviderRequirements
-		Override []*RequiredProvider
-		Want     map[string]ProviderRequirements
-	}{
-		"basic merge": {
-			map[string]ProviderRequirements{
-				"random": ProviderRequirements{
-					Type:               addrs.Provider{Type: "random"},
-					VersionConstraints: []VersionConstraint{},
-				},
-			},
-			[]*RequiredProvider{
-				&RequiredProvider{
-					Name:        "null",
-					Requirement: VersionConstraint{},
-				},
-			},
-			map[string]ProviderRequirements{
-				"random": ProviderRequirements{
-					Type:               addrs.Provider{Type: "random"},
-					VersionConstraints: []VersionConstraint{},
-				},
-				"null": ProviderRequirements{
-					Type: addrs.NewDefaultProvider("null"),
-					VersionConstraints: []VersionConstraint{
-						VersionConstraint{
-							Required:  version.Constraints(nil),
-							DeclRange: hcl.Range{},
-						},
-					},
-				},
-			},
-		},
-		"override version constraint": {
-			map[string]ProviderRequirements{
-				"random": ProviderRequirements{
-					Type:               addrs.Provider{Type: "random"},
-					VersionConstraints: []VersionConstraint{vc1},
-				},
-			},
-			[]*RequiredProvider{
-				&RequiredProvider{
-					Name:        "random",
-					Requirement: vc2,
-				},
-			},
-			map[string]ProviderRequirements{
-				"random": ProviderRequirements{
-					Type:               addrs.NewDefaultProvider("random"),
-					VersionConstraints: []VersionConstraint{vc2},
-				},
-			},
-		},
-		"merge with source constraint": {
-			map[string]ProviderRequirements{
-				"random": ProviderRequirements{
-					Type:               addrs.Provider{Type: "random"},
-					VersionConstraints: []VersionConstraint{vc1},
-				},
-			},
-			[]*RequiredProvider{
-				&RequiredProvider{
-					Name:        "random",
-					Source:      Source{SourceStr: "hashicorp/random"},
-					Requirement: vc2,
-				},
-			},
-			map[string]ProviderRequirements{
-				"random": ProviderRequirements{
-					Type:               addrs.NewDefaultProvider("random"),
-					VersionConstraints: []VersionConstraint{vc2},
-				},
-			},
-		},
-	}
-
-	for name, test := range tests {
-		t.Run(name, func(t *testing.T) {
-			mergeProviderVersionConstraints(test.Input, test.Override)
-			assertResultDeepEqual(t, test.Input, test.Want)
-		})
-	}
-}

configs/module_test.go

@@ -1,6 +1,7 @@
 package configs
 
 import (
+	"strings"
 	"testing"
 
 	"github.com/hashicorp/terraform/addrs"
@@ -111,3 +112,94 @@ func TestProviderForLocalConfig(t *testing.T) {
 		t.Fatalf("wrong result! got %#v, want %#v\n", got, want)
 	}
 }
+
+// At most one required_providers block per module is permitted.
+func TestModule_required_providers_multiple(t *testing.T) {
+	_, diags := testModuleFromDir("testdata/invalid-modules/multiple-required-providers")
+	if !diags.HasErrors() {
+		t.Fatal("module should have error diags, but does not")
+	}
+
+	want := `Duplicate required providers configuration`
+	if got := diags.Error(); !strings.Contains(got, want) {
+		t.Fatalf("expected error to contain %q\nerror was:\n%s", want, got)
+	}
+}
+
+// A module may have required_providers configured in files loaded later than
+// resources. These provider settings should still be reflected in the
+// resources' configuration.
+func TestModule_required_providers_after_resource(t *testing.T) {
+	mod, diags := testModuleFromDir("testdata/valid-modules/required-providers-after-resource")
+	if diags.HasErrors() {
+		t.Fatal(diags.Error())
+	}
+
+	want := addrs.NewProvider(addrs.DefaultRegistryHost, "foo", "test")
+
+	req, exists := mod.ProviderRequirements.RequiredProviders["test"]
+	if !exists {
+		t.Fatal("no provider requirements found for \"test\"")
+	}
+	if req.Type != want {
+		t.Errorf("wrong provider addr for \"test\"\ngot:  %s\nwant: %s",
+			req.Type, want,
+		)
+	}
+
+	if got := mod.ManagedResources["test_instance.my-instance"].Provider; !got.Equals(want) {
+		t.Errorf("wrong provider addr for \"test_instance.my-instance\"\ngot:  %s\nwant: %s",
+			got, want,
+		)
+	}
+}
+
+// We support overrides for required_providers blocks, which should replace the
+// entire block for each provider localname, leaving other blocks unaffected.
+// This should also be reflected in any resources in the module using this
+// provider.
+func TestModule_required_provider_overrides(t *testing.T) {
+	mod, diags := testModuleFromDir("testdata/valid-modules/required-providers-overrides")
+	if diags.HasErrors() {
+		t.Fatal(diags.Error())
+	}
+
+	// The foo provider and resource should be unaffected
+	want := addrs.NewProvider(addrs.DefaultRegistryHost, "acme", "foo")
+	req, exists := mod.ProviderRequirements.RequiredProviders["foo"]
+	if !exists {
+		t.Fatal("no provider requirements found for \"foo\"")
+	}
+	if req.Type != want {
+		t.Errorf("wrong provider addr for \"foo\"\ngot:  %s\nwant: %s",
+			req.Type, want,
+		)
+	}
+	if got := mod.ManagedResources["foo_thing.ft"].Provider; !got.Equals(want) {
+		t.Errorf("wrong provider addr for \"foo_thing.ft\"\ngot:  %s\nwant: %s",
+			got, want,
+		)
+	}
+
+	// The bar provider and resource should be using the override config
+	want = addrs.NewProvider(addrs.DefaultRegistryHost, "blorp", "bar")
+	req, exists = mod.ProviderRequirements.RequiredProviders["bar"]
+	if !exists {
+		t.Fatal("no provider requirements found for \"bar\"")
+	}
+	if req.Type != want {
+		t.Errorf("wrong provider addr for \"bar\"\ngot:  %s\nwant: %s",
+			req.Type, want,
+		)
+	}
+	if gotVer, wantVer := req.Requirement.Required.String(), "~>2.0.0"; gotVer != wantVer {
+		t.Errorf("wrong provider version constraint for \"bar\"\ngot:  %s\nwant: %s",
+			gotVer, wantVer,
+		)
+	}
+	if got := mod.ManagedResources["bar_thing.bt"].Provider; !got.Equals(want) {
+		t.Errorf("wrong provider addr for \"bar_thing.bt\"\ngot:  %s\nwant: %s",
+			got, want,
+		)
+	}
+}

configs/parser_config.go

@@ -75,7 +75,7 @@ func (p *Parser) loadConfigFile(path string, override bool) (*File, hcl.Diagnost
 				case "required_providers":
 					reqs, reqsDiags := decodeRequiredProvidersBlock(innerBlock)
 					diags = append(diags, reqsDiags...)
-					file.RequiredProviders = append(file.RequiredProviders, reqs...)
+					file.RequiredProviders = append(file.RequiredProviders, reqs)
 
 				case "provider_meta":
 					providerCfg, cfgDiags := decodeProviderMetaBlock(innerBlock)

configs/provider_requirements.go

@@ -12,41 +12,40 @@ import (
 // parent.
 type RequiredProvider struct {
 	Name        string
-	Source      Source
+	Type        addrs.Provider
 	Requirement VersionConstraint
-}
-
-type Source struct {
-	SourceStr string
 	DeclRange   hcl.Range
 }
 
-// ProviderRequirements represents provider version constraints from
-// required_providers blocks.
-type ProviderRequirements struct {
-	Type               addrs.Provider
-	VersionConstraints []VersionConstraint
+type RequiredProviders struct {
+	RequiredProviders map[string]*RequiredProvider
+	DeclRange         hcl.Range
 }
 
-func decodeRequiredProvidersBlock(block *hcl.Block) ([]*RequiredProvider, hcl.Diagnostics) {
+func decodeRequiredProvidersBlock(block *hcl.Block) (*RequiredProviders, hcl.Diagnostics) {
 	attrs, diags := block.Body.JustAttributes()
-	var reqs []*RequiredProvider
+	ret := &RequiredProviders{
+		RequiredProviders: make(map[string]*RequiredProvider),
+		DeclRange:         block.DefRange,
+	}
 	for name, attr := range attrs {
 		expr, err := attr.Expr.Value(nil)
 		if err != nil {
 			diags = append(diags, err...)
 		}
 
+		rp := &RequiredProvider{
+			Name:      name,
+			DeclRange: attr.Expr.Range(),
+		}
+
 		switch {
 		case expr.Type().IsPrimitiveType():
 			vc, reqDiags := decodeVersionConstraint(attr)
 			diags = append(diags, reqDiags...)
-			reqs = append(reqs, &RequiredProvider{
-				Name:        name,
-				Requirement: vc,
-			})
+			rp.Requirement = vc
+
 		case expr.Type().IsObjectType():
-			ret := &RequiredProvider{Name: name}
 			if expr.Type().HasAttribute("version") {
 				vc := VersionConstraint{
 					DeclRange: attr.Range,
@@ -64,25 +63,55 @@ func decodeRequiredProvidersBlock(block *hcl.Block) ([]*RequiredProvider, hcl.Di
 					})
 				} else {
 					vc.Required = constraints
-					ret.Requirement = vc
+					rp.Requirement = vc
 				}
 			}
 			if expr.Type().HasAttribute("source") {
-				ret.Source.SourceStr = expr.GetAttr("source").AsString()
-				ret.Source.DeclRange = attr.Range
+				fqn, sourceDiags := addrs.ParseProviderSourceString(expr.GetAttr("source").AsString())
+
+				if sourceDiags.HasErrors() {
+					hclDiags := sourceDiags.ToHCL()
+					// The diagnostics from ParseProviderSourceString don't contain
+					// source location information because it has no context to compute
+					// them from, and so we'll add those in quickly here before we
+					// return.
+					for _, diag := range hclDiags {
+						if diag.Subject == nil {
+							diag.Subject = attr.Expr.Range().Ptr()
 						}
-			reqs = append(reqs, ret)
+					}
+					diags = append(diags, hclDiags...)
+				} else {
+					rp.Type = fqn
+				}
+			}
+
 		default:
 			// should not happen
 			diags = append(diags, &hcl.Diagnostic{
 				Severity: hcl.DiagError,
-				Summary:  "Invalid provider_requirements syntax",
-				Detail:   "provider_requirements entries must be strings or objects.",
+				Summary:  "Invalid required_providers syntax",
+				Detail:   "required_providers entries must be strings or objects.",
 				Subject:  attr.Expr.Range().Ptr(),
 			})
-			reqs = append(reqs, &RequiredProvider{Name: name})
-			return reqs, diags
+		}
+
+		if rp.Type.IsZero() {
+			pType, err := addrs.ParseProviderPart(rp.Name)
+			if err != nil {
+				diags = append(diags, &hcl.Diagnostic{
+					Severity: hcl.DiagError,
+					Summary:  "Invalid provider name",
+					Detail:   err.Error(),
+					Subject:  attr.Expr.Range().Ptr(),
+				})
+			} else {
+				rp.Type = addrs.ImpliedProviderForUnqualifiedType(pType)
 			}
 		}
-	return reqs, diags
+
+		ret.RequiredProviders[rp.Name] = rp
+	}
+
+	return ret, diags
 }

configs/provider_requirements_test.go

@@ -1,8 +1,6 @@
 package configs
 
 import (
-	"fmt"
-	"sort"
 	"testing"
 
 	"github.com/google/go-cmp/cmp"
@@ -10,6 +8,7 @@ import (
 	version "github.com/hashicorp/go-version"
 	"github.com/hashicorp/hcl/v2"
 	"github.com/hashicorp/hcl/v2/hcltest"
+	"github.com/hashicorp/terraform/addrs"
 	"github.com/zclconf/go-cty/cty"
 )
 
@@ -19,18 +18,35 @@ var (
 		if x.Name != y.Name {
 			return false
 		}
-		if x.Source != y.Source {
+		if x.Type != y.Type {
 			return false
 		}
 		if x.Requirement.Required.String() != y.Requirement.Required.String() {
 			return false
 		}
+		if x.DeclRange != y.DeclRange {
+			return false
+		}
 		return true
 	})
+	blockRange = hcl.Range{
+		Filename: "mock.tf",
+		Start:    hcl.Pos{Line: 3, Column: 12, Byte: 27},
+		End:      hcl.Pos{Line: 3, Column: 19, Byte: 34},
+	}
+	mockRange = hcl.Range{
+		Filename: "MockExprLiteral",
+	}
 )
 
-func TestDecodeRequiredProvidersBlock_legacy(t *testing.T) {
-	block := &hcl.Block{
+func TestDecodeRequiredProvidersBlock(t *testing.T) {
+	tests := map[string]struct {
+		Block *hcl.Block
+		Want  *RequiredProviders
+		Error string
+	}{
+		"legacy": {
+			Block: &hcl.Block{
 				Type: "required_providers",
 				Body: hcltest.MockBody(&hcl.BodyContent{
 					Attributes: hcl.Attributes{
@@ -40,33 +56,22 @@ func TestDecodeRequiredProvidersBlock_legacy(t *testing.T) {
 						},
 					},
 				}),
-	}
-
-	want := &RequiredProvider{
+				DefRange: blockRange,
+			},
+			Want: &RequiredProviders{
+				RequiredProviders: map[string]*RequiredProvider{
+					"default": {
 						Name:        "default",
+						Type:        addrs.NewDefaultProvider("default"),
 						Requirement: testVC("1.0.0"),
-	}
-
-	got, diags := decodeRequiredProvidersBlock(block)
-	if diags.HasErrors() {
-		t.Fatalf("unexpected error")
-	}
-	if len(got) != 1 {
-		t.Fatalf("wrong number of results, got %d, wanted 1", len(got))
-	}
-	if !cmp.Equal(got[0], want, ignoreUnexported, comparer) {
-		t.Fatalf("wrong result:\n %s", cmp.Diff(got[0], want, ignoreUnexported, comparer))
-	}
-}
-
-func TestDecodeRequiredProvidersBlock_provider_source(t *testing.T) {
-	mockRange := hcl.Range{
-		Filename: "mock.tf",
-		Start:    hcl.Pos{Line: 3, Column: 12, Byte: 27},
-		End:      hcl.Pos{Line: 3, Column: 19, Byte: 34},
-	}
-
-	block := &hcl.Block{
+						DeclRange:   mockRange,
+					},
+				},
+				DeclRange: blockRange,
+			},
+		},
+		"provider source": {
+			Block: &hcl.Block{
 				Type: "required_providers",
 				Body: hcltest.MockBody(&hcl.BodyContent{
 					Attributes: hcl.Attributes{
@@ -76,31 +81,25 @@ func TestDecodeRequiredProvidersBlock_provider_source(t *testing.T) {
 								"source":  cty.StringVal("mycloud/test"),
 								"version": cty.StringVal("2.0.0"),
 							})),
-					Range: mockRange,
 						},
 					},
 				}),
-	}
-
-	want := &RequiredProvider{
+				DefRange: blockRange,
+			},
+			Want: &RequiredProviders{
+				RequiredProviders: map[string]*RequiredProvider{
+					"my_test": {
 						Name:        "my_test",
-		Source:      Source{SourceStr: "mycloud/test", DeclRange: mockRange},
+						Type:        addrs.NewProvider(addrs.DefaultRegistryHost, "mycloud", "test"),
 						Requirement: testVC("2.0.0"),
-	}
-	got, diags := decodeRequiredProvidersBlock(block)
-	if diags.HasErrors() {
-		t.Fatalf("unexpected error")
-	}
-	if len(got) != 1 {
-		t.Fatalf("wrong number of results, got %d, wanted 1", len(got))
-	}
-	if !cmp.Equal(got[0], want, ignoreUnexported, comparer) {
-		t.Fatalf("wrong result:\n %s", cmp.Diff(got[0], want, ignoreUnexported, comparer))
-	}
-}
-
-func TestDecodeRequiredProvidersBlock_mixed(t *testing.T) {
-	block := &hcl.Block{
+						DeclRange:   mockRange,
+					},
+				},
+				DeclRange: blockRange,
+			},
+		},
+		"mixed": {
+			Block: &hcl.Block{
 				Type: "required_providers",
 				Body: hcltest.MockBody(&hcl.BodyContent{
 					Attributes: hcl.Attributes{
@@ -117,41 +116,112 @@ func TestDecodeRequiredProvidersBlock_mixed(t *testing.T) {
 						},
 					},
 				}),
-	}
-
-	want := []*RequiredProvider{
-		{
+				DefRange: blockRange,
+			},
+			Want: &RequiredProviders{
+				RequiredProviders: map[string]*RequiredProvider{
+					"legacy": {
 						Name:        "legacy",
+						Type:        addrs.NewDefaultProvider("legacy"),
 						Requirement: testVC("1.0.0"),
+						DeclRange:   mockRange,
 					},
-		{
+					"my_test": {
 						Name:        "my_test",
-			Source:      Source{SourceStr: "mycloud/test", DeclRange: hcl.Range{}},
+						Type:        addrs.NewProvider(addrs.DefaultRegistryHost, "mycloud", "test"),
 						Requirement: testVC("2.0.0"),
+						DeclRange:   mockRange,
 					},
-	}
-
-	got, diags := decodeRequiredProvidersBlock(block)
-
-	sort.SliceStable(got, func(i, j int) bool {
-		return got[i].Name < got[j].Name
-	})
-
-	if diags.HasErrors() {
-		t.Fatalf("unexpected error")
-	}
-	if len(got) != 2 {
-		t.Fatalf("wrong number of results, got %d, wanted 2", len(got))
-	}
-	for i, rp := range want {
-		if !cmp.Equal(got[i], rp, ignoreUnexported, comparer) {
-			t.Fatalf("wrong result:\n %s", cmp.Diff(got[0], rp, ignoreUnexported, comparer))
-		}
-	}
-}
-
-func TestDecodeRequiredProvidersBlock_version_error(t *testing.T) {
-	block := &hcl.Block{
+				},
+				DeclRange: blockRange,
+			},
+		},
+		"version-only block": {
+			Block: &hcl.Block{
+				Type: "required_providers",
+				Body: hcltest.MockBody(&hcl.BodyContent{
+					Attributes: hcl.Attributes{
+						"test": {
+							Name: "test",
+							Expr: hcltest.MockExprLiteral(cty.ObjectVal(map[string]cty.Value{
+								"version": cty.StringVal("~>2.0.0"),
+							})),
+						},
+					},
+				}),
+				DefRange: blockRange,
+			},
+			Want: &RequiredProviders{
+				RequiredProviders: map[string]*RequiredProvider{
+					"test": {
+						Name:        "test",
+						Type:        addrs.NewDefaultProvider("test"),
+						Requirement: testVC("~>2.0.0"),
+						DeclRange:   mockRange,
+					},
+				},
+				DeclRange: blockRange,
+			},
+		},
+		"invalid source": {
+			Block: &hcl.Block{
+				Type: "required_providers",
+				Body: hcltest.MockBody(&hcl.BodyContent{
+					Attributes: hcl.Attributes{
+						"my_test": {
+							Name: "my_test",
+							Expr: hcltest.MockExprLiteral(cty.ObjectVal(map[string]cty.Value{
+								"source":  cty.StringVal("some/invalid/provider/source/test"),
+								"version": cty.StringVal("~>2.0.0"),
+							})),
+						},
+					},
+				}),
+				DefRange: blockRange,
+			},
+			Want: &RequiredProviders{
+				RequiredProviders: map[string]*RequiredProvider{
+					"my_test": {
+						Name:        "my_test",
+						Type:        addrs.Provider{},
+						Requirement: testVC("~>2.0.0"),
+						DeclRange:   mockRange,
+					},
+				},
+				DeclRange: blockRange,
+			},
+			Error: "Invalid provider source string",
+		},
+		"localname is invalid provider name": {
+			Block: &hcl.Block{
+				Type: "required_providers",
+				Body: hcltest.MockBody(&hcl.BodyContent{
+					Attributes: hcl.Attributes{
+						"my_test": {
+							Name: "my_test",
+							Expr: hcltest.MockExprLiteral(cty.ObjectVal(map[string]cty.Value{
+								"version": cty.StringVal("~>2.0.0"),
+							})),
+						},
+					},
+				}),
+				DefRange: blockRange,
+			},
+			Want: &RequiredProviders{
+				RequiredProviders: map[string]*RequiredProvider{
+					"my_test": {
+						Name:        "my_test",
+						Type:        addrs.Provider{},
+						Requirement: testVC("~>2.0.0"),
+						DeclRange:   mockRange,
+					},
+				},
+				DeclRange: blockRange,
+			},
+			Error: "Invalid provider name",
+		},
+		"version constraint error": {
+			Block: &hcl.Block{
 				Type: "required_providers",
 				Body: hcltest.MockBody(&hcl.BodyContent{
 					Attributes: hcl.Attributes{
@@ -164,28 +234,65 @@ func TestDecodeRequiredProvidersBlock_version_error(t *testing.T) {
 						},
 					},
 				}),
-	}
-
-	want := []*RequiredProvider{
-		{
+				DefRange: blockRange,
+			},
+			Want: &RequiredProviders{
+				RequiredProviders: map[string]*RequiredProvider{
+					"my_test": {
 						Name:      "my_test",
-			Source: Source{SourceStr: "mycloud/test", DeclRange: hcl.Range{}},
+						Type:      addrs.NewProvider(addrs.DefaultRegistryHost, "mycloud", "test"),
+						DeclRange: mockRange,
+					},
+				},
+				DeclRange: blockRange,
+			},
+			Error: "Invalid version constraint",
+		},
+		"invalid required_providers attribute value": {
+			Block: &hcl.Block{
+				Type: "required_providers",
+				Body: hcltest.MockBody(&hcl.BodyContent{
+					Attributes: hcl.Attributes{
+						"test": {
+							Name: "test",
+							Expr: hcltest.MockExprLiteral(cty.ListVal([]cty.Value{cty.StringVal("2.0.0")})),
+						},
+					},
+				}),
+				DefRange: blockRange,
+			},
+			Want: &RequiredProviders{
+				RequiredProviders: map[string]*RequiredProvider{
+					"test": {
+						Name:      "test",
+						Type:      addrs.NewDefaultProvider("test"),
+						DeclRange: mockRange,
+					},
+				},
+				DeclRange: blockRange,
+			},
+			Error: "Invalid required_providers syntax",
 		},
 	}
 
-	got, diags := decodeRequiredProvidersBlock(block)
-	if !diags.HasErrors() {
-		t.Fatalf("expected error, got success")
-	} else {
-		fmt.Printf(diags[0].Summary)
+	for name, test := range tests {
+		t.Run(name, func(t *testing.T) {
+			got, diags := decodeRequiredProvidersBlock(test.Block)
+			if diags.HasErrors() {
+				if test.Error == "" {
+					t.Fatalf("unexpected error")
 				}
-	if len(got) != 1 {
-		t.Fatalf("wrong number of results, got %d, wanted 1", len(got))
+				if gotErr := diags[0].Summary; gotErr != test.Error {
+					t.Errorf("wrong error, got %q, want %q", gotErr, test.Error)
 				}
-	for i, rp := range want {
-		if !cmp.Equal(got[i], rp, ignoreUnexported, comparer) {
-			t.Fatalf("wrong result:\n %s", cmp.Diff(got[0], rp, ignoreUnexported, comparer))
+			} else if test.Error != "" {
+				t.Fatalf("expected error")
 			}
+
+			if !cmp.Equal(got, test.Want, ignoreUnexported, comparer) {
+				t.Fatalf("wrong result:\n %s", cmp.Diff(got, test.Want, ignoreUnexported, comparer))
+			}
+		})
 	}
 }
 

configs/testdata/invalid-modules/multiple-required-providers/a.tf

@@ -0,0 +1,7 @@
+terraform {
+  required_providers {
+    bar = {
+      version = "~>1.0.0"
+    }
+  }
+}

configs/testdata/invalid-modules/multiple-required-providers/b.tf

@@ -0,0 +1,7 @@
+terraform {
+  required_providers {
+    foo = {
+      version = "~>2.0.0"
+    }
+  }
+}

configs/testdata/valid-modules/required-providers-after-resource/main.tf

@@ -0,0 +1,3 @@
+resource test_instance "my-instance" {
+  provider = test
+}

configs/testdata/valid-modules/required-providers-after-resource/providers.tf

@@ -0,0 +1,8 @@
+terraform {
+  required_providers {
+    test = {
+      source = "foo/test"
+      version = "~>1.0.0"
+    }
+  }
+}

configs/testdata/valid-modules/required-providers-overrides/bar_provider_override.tf

@@ -0,0 +1,9 @@
+terraform {
+  required_providers {
+    bar = {
+      source = "blorp/bar"
+      version = "~>2.0.0"
+    }
+  }
+}
+

configs/testdata/valid-modules/required-providers-overrides/main.tf

@@ -0,0 +1,7 @@
+resource bar_thing "bt" {
+  provider = bar
+}
+
+resource foo_thing "ft" {
+  provider = foo
+}

configs/testdata/valid-modules/required-providers-overrides/providers.tf

@@ -0,0 +1,11 @@
+terraform {
+  required_providers {
+    bar = {
+      source = "acme/bar"
+    }
+
+    foo = {
+      source = "acme/foo"
+    }
+  }
+}