go/types, types2: move validType code into its own file

The validType check is independent of the work of declaring objects.
Move it into a separate file for better separation of concerns and
code organization.

No other changes - this is purely a code move.

Preparation for fixing issue #48962.

Change-Id: Ib08db2d009c4890882d0978b278e965ca3078851
Reviewed-on: https://go-review.googlesource.com/c/go/+/378674
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

4 files changed, 190 insertions, 180 deletions

diff --git a/src/cmd/compile/internal/types2/decl.go b/src/cmd/compile/internal/types2/decl.go
index d61d2a8b0d..22cea584d4 100644
--- a/src/cmd/compile/internal/types2/decl.go
+++ b/src/cmd/compile/internal/types2/decl.go
@@ -304,96 +304,6 @@ loop:
 	return false
 }
 
-type typeInfo uint
-
-// validType verifies that the given type does not "expand" infinitely
-// producing a cycle in the type graph. Cycles are detected by marking
-// defined types.
-// (Cycles involving alias types, as in "type A = [10]A" are detected
-// earlier, via the objDecl cycle detection mechanism.)
-func (check *Checker) validType(typ Type, path []Object) typeInfo {
-	const (
-		unknown typeInfo = iota
-		marked
-		valid
-		invalid
-	)
-
-	switch t := typ.(type) {
-	case *Array:
-		return check.validType(t.elem, path)
-
-	case *Struct:
-		for _, f := range t.fields {
-			if check.validType(f.typ, path) == invalid {
-				return invalid
-			}
-		}
-
-	case *Union:
-		for _, t := range t.terms {
-			if check.validType(t.typ, path) == invalid {
-				return invalid
-			}
-		}
-
-	case *Interface:
-		for _, etyp := range t.embeddeds {
-			if check.validType(etyp, path) == invalid {
-				return invalid
-			}
-		}
-
-	case *Named:
-		// If t is parameterized, we should be considering the instantiated (expanded)
-		// form of t, but in general we can't with this algorithm: if t is an invalid
-		// type it may be so because it infinitely expands through a type parameter.
-		// Instantiating such a type would lead to an infinite sequence of instantiations.
-		// In general, we need "type flow analysis" to recognize those cases.
-		// Example: type A[T any] struct{ x A[*T] } (issue #48951)
-		// In this algorithm we always only consider the original, uninstantiated type.
-		// This won't recognize some invalid cases with parameterized types, but it
-		// will terminate.
-		t = t.orig
-
-		// don't touch the type if it is from a different package or the Universe scope
-		// (doing so would lead to a race condition - was issue #35049)
-		if t.obj.pkg != check.pkg {
-			return valid
-		}
-
-		// don't report a 2nd error if we already know the type is invalid
-		// (e.g., if a cycle was detected earlier, via under).
-		if t.underlying == Typ[Invalid] {
-			t.info = invalid
-			return invalid
-		}
-
-		switch t.info {
-		case unknown:
-			t.info = marked
-			t.info = check.validType(t.fromRHS, append(path, t.obj)) // only types of current package added to path
-		case marked:
-			// cycle detected
-			for i, tn := range path {
-				if t.obj.pkg != check.pkg {
-					panic("type cycle via package-external type")
-				}
-				if tn == t.obj {
-					check.cycleError(path[i:])
-					t.info = invalid
-					t.underlying = Typ[Invalid]
-					return invalid
-				}
-			}
-			panic("cycle start not found")
-		}
-		return t.info
-	}
-
-	return valid
-}
-
 // cycleError reports a declaration cycle starting with
 // the object in cycle that is "first" in the source.
 func (check *Checker) cycleError(cycle []Object) {
diff --git a/src/cmd/compile/internal/types2/validtype.go b/src/cmd/compile/internal/types2/validtype.go
new file mode 100644
index 0000000000..24d65e2c24
--- /dev/null
+++ b/src/cmd/compile/internal/types2/validtype.go
@@ -0,0 +1,95 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package types2
+
+type typeInfo uint
+
+// validType verifies that the given type does not "expand" infinitely
+// producing a cycle in the type graph. Cycles are detected by marking
+// defined types.
+// (Cycles involving alias types, as in "type A = [10]A" are detected
+// earlier, via the objDecl cycle detection mechanism.)
+func (check *Checker) validType(typ Type, path []Object) typeInfo {
+	const (
+		unknown typeInfo = iota
+		marked
+		valid
+		invalid
+	)
+
+	switch t := typ.(type) {
+	case *Array:
+		return check.validType(t.elem, path)
+
+	case *Struct:
+		for _, f := range t.fields {
+			if check.validType(f.typ, path) == invalid {
+				return invalid
+			}
+		}
+
+	case *Union:
+		for _, t := range t.terms {
+			if check.validType(t.typ, path) == invalid {
+				return invalid
+			}
+		}
+
+	case *Interface:
+		for _, etyp := range t.embeddeds {
+			if check.validType(etyp, path) == invalid {
+				return invalid
+			}
+		}
+
+	case *Named:
+		// If t is parameterized, we should be considering the instantiated (expanded)
+		// form of t, but in general we can't with this algorithm: if t is an invalid
+		// type it may be so because it infinitely expands through a type parameter.
+		// Instantiating such a type would lead to an infinite sequence of instantiations.
+		// In general, we need "type flow analysis" to recognize those cases.
+		// Example: type A[T any] struct{ x A[*T] } (issue #48951)
+		// In this algorithm we always only consider the original, uninstantiated type.
+		// This won't recognize some invalid cases with parameterized types, but it
+		// will terminate.
+		t = t.orig
+
+		// don't touch the type if it is from a different package or the Universe scope
+		// (doing so would lead to a race condition - was issue #35049)
+		if t.obj.pkg != check.pkg {
+			return valid
+		}
+
+		// don't report a 2nd error if we already know the type is invalid
+		// (e.g., if a cycle was detected earlier, via under).
+		if t.underlying == Typ[Invalid] {
+			t.info = invalid
+			return invalid
+		}
+
+		switch t.info {
+		case unknown:
+			t.info = marked
+			t.info = check.validType(t.fromRHS, append(path, t.obj)) // only types of current package added to path
+		case marked:
+			// cycle detected
+			for i, tn := range path {
+				if t.obj.pkg != check.pkg {
+					panic("type cycle via package-external type")
+				}
+				if tn == t.obj {
+					check.cycleError(path[i:])
+					t.info = invalid
+					t.underlying = Typ[Invalid]
+					return invalid
+				}
+			}
+			panic("cycle start not found")
+		}
+		return t.info
+	}
+
+	return valid
+}
diff --git a/src/go/types/decl.go b/src/go/types/decl.go
index 02af0d5f3e..5b54465f18 100644
--- a/src/go/types/decl.go
+++ b/src/go/types/decl.go
@@ -303,96 +303,6 @@ loop:
 	return false
 }
 
-type typeInfo uint
-
-// validType verifies that the given type does not "expand" infinitely
-// producing a cycle in the type graph. Cycles are detected by marking
-// defined types.
-// (Cycles involving alias types, as in "type A = [10]A" are detected
-// earlier, via the objDecl cycle detection mechanism.)
-func (check *Checker) validType(typ Type, path []Object) typeInfo {
-	const (
-		unknown typeInfo = iota
-		marked
-		valid
-		invalid
-	)
-
-	switch t := typ.(type) {
-	case *Array:
-		return check.validType(t.elem, path)
-
-	case *Struct:
-		for _, f := range t.fields {
-			if check.validType(f.typ, path) == invalid {
-				return invalid
-			}
-		}
-
-	case *Union:
-		for _, t := range t.terms {
-			if check.validType(t.typ, path) == invalid {
-				return invalid
-			}
-		}
-
-	case *Interface:
-		for _, etyp := range t.embeddeds {
-			if check.validType(etyp, path) == invalid {
-				return invalid
-			}
-		}
-
-	case *Named:
-		// If t is parameterized, we should be considering the instantiated (expanded)
-		// form of t, but in general we can't with this algorithm: if t is an invalid
-		// type it may be so because it infinitely expands through a type parameter.
-		// Instantiating such a type would lead to an infinite sequence of instantiations.
-		// In general, we need "type flow analysis" to recognize those cases.
-		// Example: type A[T any] struct{ x A[*T] } (issue #48951)
-		// In this algorithm we always only consider the original, uninstantiated type.
-		// This won't recognize some invalid cases with parameterized types, but it
-		// will terminate.
-		t = t.orig
-
-		// don't touch the type if it is from a different package or the Universe scope
-		// (doing so would lead to a race condition - was issue #35049)
-		if t.obj.pkg != check.pkg {
-			return valid
-		}
-
-		// don't report a 2nd error if we already know the type is invalid
-		// (e.g., if a cycle was detected earlier, via under).
-		if t.underlying == Typ[Invalid] {
-			t.info = invalid
-			return invalid
-		}
-
-		switch t.info {
-		case unknown:
-			t.info = marked
-			t.info = check.validType(t.fromRHS, append(path, t.obj)) // only types of current package added to path
-		case marked:
-			// cycle detected
-			for i, tn := range path {
-				if t.obj.pkg != check.pkg {
-					panic("type cycle via package-external type")
-				}
-				if tn == t.obj {
-					check.cycleError(path[i:])
-					t.info = invalid
-					t.underlying = Typ[Invalid]
-					return invalid
-				}
-			}
-			panic("cycle start not found")
-		}
-		return t.info
-	}
-
-	return valid
-}
-
 // cycleError reports a declaration cycle starting with
 // the object in cycle that is "first" in the source.
 func (check *Checker) cycleError(cycle []Object) {
diff --git a/src/go/types/validtype.go b/src/go/types/validtype.go
new file mode 100644
index 0000000000..8972a7ad85
--- /dev/null
+++ b/src/go/types/validtype.go
@@ -0,0 +1,95 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package types
+
+type typeInfo uint
+
+// validType verifies that the given type does not "expand" infinitely
+// producing a cycle in the type graph. Cycles are detected by marking
+// defined types.
+// (Cycles involving alias types, as in "type A = [10]A" are detected
+// earlier, via the objDecl cycle detection mechanism.)
+func (check *Checker) validType(typ Type, path []Object) typeInfo {
+	const (
+		unknown typeInfo = iota
+		marked
+		valid
+		invalid
+	)
+
+	switch t := typ.(type) {
+	case *Array:
+		return check.validType(t.elem, path)
+
+	case *Struct:
+		for _, f := range t.fields {
+			if check.validType(f.typ, path) == invalid {
+				return invalid
+			}
+		}
+
+	case *Union:
+		for _, t := range t.terms {
+			if check.validType(t.typ, path) == invalid {
+				return invalid
+			}
+		}
+
+	case *Interface:
+		for _, etyp := range t.embeddeds {
+			if check.validType(etyp, path) == invalid {
+				return invalid
+			}
+		}
+
+	case *Named:
+		// If t is parameterized, we should be considering the instantiated (expanded)
+		// form of t, but in general we can't with this algorithm: if t is an invalid
+		// type it may be so because it infinitely expands through a type parameter.
+		// Instantiating such a type would lead to an infinite sequence of instantiations.
+		// In general, we need "type flow analysis" to recognize those cases.
+		// Example: type A[T any] struct{ x A[*T] } (issue #48951)
+		// In this algorithm we always only consider the original, uninstantiated type.
+		// This won't recognize some invalid cases with parameterized types, but it
+		// will terminate.
+		t = t.orig
+
+		// don't touch the type if it is from a different package or the Universe scope
+		// (doing so would lead to a race condition - was issue #35049)
+		if t.obj.pkg != check.pkg {
+			return valid
+		}
+
+		// don't report a 2nd error if we already know the type is invalid
+		// (e.g., if a cycle was detected earlier, via under).
+		if t.underlying == Typ[Invalid] {
+			t.info = invalid
+			return invalid
+		}
+
+		switch t.info {
+		case unknown:
+			t.info = marked
+			t.info = check.validType(t.fromRHS, append(path, t.obj)) // only types of current package added to path
+		case marked:
+			// cycle detected
+			for i, tn := range path {
+				if t.obj.pkg != check.pkg {
+					panic("type cycle via package-external type")
+				}
+				if tn == t.obj {
+					check.cycleError(path[i:])
+					t.info = invalid
+					t.underlying = Typ[Invalid]
+					return invalid
+				}
+			}
+			panic("cycle start not found")
+		}
+		return t.info
+	}
+
+	return valid
+}