[lldb][ClangUserExpression] Emit workaround hint when trying to call function templates (#179916)

Before:
```
(lldb) expression some_template_func<int, long>(5)
                  ˄
                  ╰─ error: 'some_template_func' does not name a template but is followed by template arguments
note: Ran expression as 'C++14'.
note: note: non-template declaration found by name lookup
```

After:
```
(lldb) expression some_template_func<int, long>(5)
                  ˄
                  ╰─ error: 'some_template_func' does not name a template but is followed by template arguments
note: Ran expression as 'C++14'.
note: note: non-template declaration found by name lookup
note: Naming template instantiation not yet supported. Template functions can be invoked via their mangled name. E.g., expression _Z3fooIiEvi(5)
```

There isn't a great way to get to the actual function being named (and
its mangled name) since we're just dealing with raw text. So I just
print an example mangled name.

This doesn't work for all template instantiations. E.g.,:
```
(lldb) p f.method<long>(10)
           ˄          ˄
           │          ╰─ error: expected '(' for function-style cast or type construction
           ╰─ error: no member named 'method' in 'Foo<int>'
note: Ran expression as 'C++14'.
```
This is a consequence of how we construct the AST for template methods.
Once we fix that, this hint will get emitted there too.

Note this will also trigger in cases where no function is being called
(hence I used the defensive phrase "Template functions can be invoked").

rdar://135725807

3 files changed, 65 insertions, 0 deletions

diff --git a/lldb/test/API/lang/cpp/template-diagnostic-hint/Makefile b/lldb/test/API/lang/cpp/template-diagnostic-hint/Makefile
new file mode 100644
index 000000000000..99998b20bcb0
--- /dev/null
+++ b/lldb/test/API/lang/cpp/template-diagnostic-hint/Makefile
@@ -0,0 +1,3 @@
+CXX_SOURCES := main.cpp
+
+include Makefile.rules
diff --git a/lldb/test/API/lang/cpp/template-diagnostic-hint/TestTemplateDiagnosticHint.py b/lldb/test/API/lang/cpp/template-diagnostic-hint/TestTemplateDiagnosticHint.py
new file mode 100644
index 000000000000..e8287108f758
--- /dev/null
+++ b/lldb/test/API/lang/cpp/template-diagnostic-hint/TestTemplateDiagnosticHint.py
@@ -0,0 +1,48 @@
+import lldb
+from lldbsuite.test.decorators import *
+from lldbsuite.test.lldbtest import *
+from lldbsuite.test import lldbutil
+
+
+class TestCase(TestBase):
+    def test(self):
+        self.build()
+        (_, process, _, _) = lldbutil.run_to_source_breakpoint(
+            self, "main", lldb.SBFileSpec("main.cpp")
+        )
+
+        self.expect(
+            "expression some_template_func<int, long>(5)",
+            error=True,
+            substrs=[
+                "does not name a template but is followed by template arguments",
+                "note: Naming template instantiation not yet supported.",
+                "Template functions can be invoked via their mangled name.",
+            ],
+        )
+
+        self.expect(
+            "expression some_template_func<int, long>(5) + some_template_func<int, long>(5)",
+            error=True,
+            substrs=[
+                "does not name a template but is followed by template arguments",
+                "does not name a template but is followed by template arguments",
+            ],
+        )
+
+        self.assertEqual(
+            self.res.GetError().count(
+                "note: Naming template instantiation not yet supported"
+            ),
+            1,
+        )
+
+        self.expect(
+            "expression Foo<int>::smethod()",
+            error=True,
+            substrs=[
+                "no template named 'Foo'",
+                "note: Naming template instantiation not yet supported.",
+                "Template functions can be invoked via their mangled name.",
+            ],
+        )
diff --git a/lldb/test/API/lang/cpp/template-diagnostic-hint/main.cpp b/lldb/test/API/lang/cpp/template-diagnostic-hint/main.cpp
new file mode 100644
index 000000000000..914d9b59484d
--- /dev/null
+++ b/lldb/test/API/lang/cpp/template-diagnostic-hint/main.cpp
@@ -0,0 +1,14 @@
+template <typename T, typename K> static K some_template_func(int x) {
+  return (K)x;
+}
+
+template <typename T> struct Foo {
+  template <typename K> T method(K k) { return (T)k; }
+  static T smethod() { return (T)10; }
+};
+
+int main() {
+  Foo<int> f;
+  return some_template_func<int, long>(5) + Foo<int>::smethod() +
+         f.method<long>(10);
+}