Go: Fix crash in getTypeParamParentLabel

Author: jketema

go/extractor/extractor.go

@@ -1657,8 +1657,30 @@ func extractType(tw *trap.Writer, tp types.Type) trap.Label {
 			// parent scope, so they are not dealt with by `extractScopes`
 			for i := 0; i < origintp.NumMethods(); i++ {
 				meth := origintp.Method(i).Origin()
-
 				extractMethod(tw, meth)
+
+				// Consider a generic struct and a generic method:
+				//
+				//   type S[P any] struct{}
+				//   func (*S[P]) m[Q any](x Q) {}
+				//
+				// If we have a variable 's' of type 'S[int]' and the expression
+				// 's.m[string]("")', then the type of the selector expression 's.m'
+				// is '	func(Q)'. The method 'm' here is an instantiation of the
+				// declaration, which has its own type with type parameter 'Q'.
+				// As we do not extract method instantiations, 'populateTypeParamParents'
+				// does not automatically get called for the type parameter 'Q'
+				// from the instantiation of 'm'. To compensate, we add the type
+				// parameters here.
+				//
+				// As a parent we use the origin method. This suffices, as the name
+				// and index of the type parameter in the instantiation will be
+				// identical to those of the uninstantiated method, and as only
+				// these two properties will be extracted for a type parameter.
+				if tp.Method(i) != meth {
+					signature := tp.Method(i).Type().(*types.Signature)
+					populateTypeParamParents(signature.TypeParams(), meth, false)
+				}
 			}
 
 			underlyingInterface, underlyingIsInterface := underlying.(*types.Interface)

go/ql/test/library-tests/semmle/go/Function/GenericFunctionInstantiationExpr.expected

@@ -7,3 +7,4 @@
 | genericFunctions.go:141:6:141:41 | generic function instantiation expression | genericFunctions.go:141:6:141:33 | GenericFunctionInAnotherFile | 0 | genericFunctions.go:141:35:141:40 | string |
 | genericFunctions.go:146:6:146:55 | generic function instantiation expression | genericFunctions.go:146:6:146:47 | selection of GenericFunctionInAnotherPackage | 0 | genericFunctions.go:146:49:146:54 | string |
 | genericMethods.go:13:2:13:23 | generic function instantiation expression | genericMethods.go:13:2:13:18 | selection of GenericMethod1 | 0 | genericMethods.go:13:20:13:22 | int |
+| genericMethods.go:14:2:14:26 | generic function instantiation expression | genericMethods.go:14:2:14:18 | selection of GenericMethod2 | 0 | genericMethods.go:14:20:14:25 | string |

go/ql/test/library-tests/semmle/go/Function/genericMethods.go

@@ -11,9 +11,9 @@ func (*StructForGenericMethod2[P3]) GenericMethod2[P4 any](x P4) {}
 func generic_methods(s1 StructForGenericMethod1, s2 StructForGenericMethod2[int]) {
 	// Call the generic method specifying the type
 	s1.GenericMethod1[int](1)
-	//s2.GenericMethod2[string]("hello")
+	s2.GenericMethod2[string]("hello")
 
 	// Call the generic method without specifying the type
 	s1.GenericMethod1("hello")
-	//s2.GenericMethod2(42)
+	s2.GenericMethod2(42)
 }