go/types, types2: explicitly check for non-nil type in LookupFieldOrMethod

Document and enforce API expectation. Add a test so we don't
inadvertently change the function behavior with respect to nil
type arguments.

Fixes #50620.

Change-Id: Ic000bff7504a03006bd248a319c7a2d49dcf09c8
Reviewed-on: https://go-review.googlesource.com/c/go/+/379374
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>

src/cmd/compile/internal/types2/api_test.go

@@ -1443,6 +1443,18 @@ var _ = a.C2
 	makePkg("main", mainSrc) // don't crash when type-checking this package
 }
 
+func TestLookupFieldOrMethodOnNil(t *testing.T) {
+	// LookupFieldOrMethod on a nil type is expected to produce a run-time panic.
+	defer func() {
+		const want = "LookupFieldOrMethod on nil type"
+		p := recover()
+		if s, ok := p.(string); !ok || s != want {
+			t.Fatalf("got %v, want %s", p, want)
+		}
+	}()
+	LookupFieldOrMethod(nil, false, nil, "")
+}
+
 func TestLookupFieldOrMethod(t *testing.T) {
 	// Test cases assume a lookup of the form a.f or x.f, where a stands for an
 	// addressable value, and x for a non-addressable value (even though a variable

src/cmd/compile/internal/types2/lookup.go

@@ -19,7 +19,7 @@ import (
 // in T and returns the corresponding *Var or *Func, an index sequence, and a
 // bool indicating if there were any pointer indirections on the path to the
 // field or method. If addressable is set, T is the type of an addressable
-// variable (only matters for method lookups).
+// variable (only matters for method lookups). T must not be nil.
 //
 // The last index entry is the field or method index in the (possibly embedded)
 // type where the entry was found, either:
@@ -42,6 +42,10 @@ import (
 //	the method's formal receiver base type, nor was the receiver addressable.
 //
 func LookupFieldOrMethod(T Type, addressable bool, pkg *Package, name string) (obj Object, index []int, indirect bool) {
+	if T == nil {
+		panic("LookupFieldOrMethod on nil type")
+	}
+
 	// Methods cannot be associated to a named pointer type.
 	// (spec: "The type denoted by T is called the receiver base type;
 	// it must not be a pointer or interface type and it must be declared

src/go/types/api_test.go

@@ -1426,6 +1426,18 @@ var _ = a.C2
 	makePkg("main", mainSrc) // don't crash when type-checking this package
 }
 
+func TestLookupFieldOrMethodOnNil(t *testing.T) {
+	// LookupFieldOrMethod on a nil type is expected to produce a run-time panic.
+	defer func() {
+		const want = "LookupFieldOrMethod on nil type"
+		p := recover()
+		if s, ok := p.(string); !ok || s != want {
+			t.Fatalf("got %v, want %s", p, want)
+		}
+	}()
+	LookupFieldOrMethod(nil, false, nil, "")
+}
+
 func TestLookupFieldOrMethod(t *testing.T) {
 	// Test cases assume a lookup of the form a.f or x.f, where a stands for an
 	// addressable value, and x for a non-addressable value (even though a variable

src/go/types/lookup.go

@@ -19,7 +19,7 @@ import (
 // in T and returns the corresponding *Var or *Func, an index sequence, and a
 // bool indicating if there were any pointer indirections on the path to the
 // field or method. If addressable is set, T is the type of an addressable
-// variable (only matters for method lookups).
+// variable (only matters for method lookups). T must not be nil.
 //
 // The last index entry is the field or method index in the (possibly embedded)
 // type where the entry was found, either:
@@ -42,6 +42,10 @@ import (
 //	the method's formal receiver base type, nor was the receiver addressable.
 //
 func LookupFieldOrMethod(T Type, addressable bool, pkg *Package, name string) (obj Object, index []int, indirect bool) {
+	if T == nil {
+		panic("LookupFieldOrMethod on nil type")
+	}
+
 	// Methods cannot be associated to a named pointer type.
 	// (spec: "The type denoted by T is called the receiver base type;
 	// it must not be a pointer or interface type and it must be declared