feat(diagnostics): print source diagnostics in gcc file:line: layout

Lexer, parser, driver and codegen diagnostics now read
"<path>:<line>: error: <message>" instead of "error: <message> at line N
in <path>". The path is always printed as given on the command line.
Call sites pass the bare message to cps::sourceError/sourceWarning, which
carry printf format checking; reportError routes through the same helper.

The suite iii harness parses the new layout and matches the reason
against the message only. iii/template-class-pack-field passed only
because its reason regex matched the case path, so it is now
case-insensitive.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

3 files changed, 36 insertions, 34 deletions

diff --git a/include/cps/CodeGen.h b/include/cps/CodeGen.h
index 4730585..6477406 100644
--- a/include/cps/CodeGen.h
+++ b/include/cps/CodeGen.h
@@ -240,7 +240,7 @@ public:
     void print(llvm::raw_ostream &Output);
     const llvm::Module &getModule() const { return *TheModule; }
     bool hadError() const { return HadError; }
-    void reportError(const char *Fmt, ...);
+    __attribute__((format(printf, 2, 3))) void reportError(const char *Fmt, ...);
     static bool isBuiltinName(const std::string &Name);
     
     llvm::Value *emitExpr(ExprAST *Expr);
diff --git a/include/cps/Diagnostics.h b/include/cps/Diagnostics.h
index 6f26292..3492f46 100644
--- a/include/cps/Diagnostics.h
+++ b/include/cps/Diagnostics.h
@@ -2,43 +2,45 @@
 #include <cstdarg>
 #include <cstdio>
 #include <string>
-#include <vector>
 
 namespace cps {
-// Empty for single-source builds to preserve existing diagnostic text.
+enum class Severity { Error, Warning, Note, Fatal };
+
+// Path of the unit being diagnosed, as given on the command line.
 inline std::string DiagnosticSource;
-inline std::string lineSuffix(int Line) {
-    return " at line " + std::to_string(Line) +
-           (DiagnosticSource.empty() ? "" : " in " + DiagnosticSource);
+
+// gcc layout: "<locus>: <label>: "; an empty locus prints the label alone.
+inline std::string diagnosticPrefix(Severity S, const std::string &Locus = "cpsc") {
+    const char *Label = S == Severity::Error   ? "error: "
+                      : S == Severity::Warning ? "warning: "
+                      : S == Severity::Note    ? "note: "
+                                               : "fatal error: ";
+    return (Locus.empty() ? "" : Locus + ": ") + Label;
 }
-inline void sourceDiagnostic(int Line, const char *Format, ...) {
-    va_list Args, Copy;
+
+// Prints "<path>:<line>: <label>: <message>\n" (just "<path>:" when the line is
+// unknown, "cpsc:" when no source is set).
+inline void vsourceDiagnostic(Severity S, int Line, const char *Format, va_list Args) {
+    std::string Locus = DiagnosticSource.empty() ? "cpsc" : DiagnosticSource;
+    if (!DiagnosticSource.empty() && Line > 0) Locus += ":" + std::to_string(Line);
+    std::fputs(diagnosticPrefix(S, Locus).c_str(), stderr);
+    std::vfprintf(stderr, Format, Args);
+    std::fputc('\n', stderr);
+}
+
+__attribute__((format(printf, 2, 3)))
+inline void sourceError(int Line, const char *Format, ...) {
+    va_list Args;
+    va_start(Args, Format);
+    vsourceDiagnostic(Severity::Error, Line, Format, Args);
+    va_end(Args);
+}
+
+__attribute__((format(printf, 2, 3)))
+inline void sourceWarning(int Line, const char *Format, ...) {
+    va_list Args;
     va_start(Args, Format);
-    va_copy(Copy, Args);
-    int Size = std::vsnprintf(nullptr, 0, Format, Copy);
-    va_end(Copy);
-    if (Size < 0) { va_end(Args); return; }
-    std::vector<char> Buffer(Size + 1);
-    std::vsnprintf(Buffer.data(), Buffer.size(), Format, Args);
+    vsourceDiagnostic(Severity::Warning, Line, Format, Args);
     va_end(Args);
-    std::string Message(Buffer.data(), Size);
-    if (!DiagnosticSource.empty()) {
-        auto At = Message.find("at line ");
-        if (At != std::string::npos) {
-            auto End = Message.find_first_not_of("0123456789", At + 8);
-            Message.insert(End == std::string::npos ? Message.size() : End,
-                           " in " + DiagnosticSource);
-        } else {
-            auto Old = Message.find("(line ");
-            if (Old != std::string::npos) {
-                auto End = Message.find(')', Old);
-                Message.replace(Old, End - Old + 1, lineSuffix(Line).substr(1));
-            } else {
-                auto End = Message.find_last_not_of("\n");
-                Message.insert(End == std::string::npos ? 0 : End + 1, lineSuffix(Line));
-            }
-        }
-    }
-    std::fputs(Message.c_str(), stderr);
 }
 }
diff --git a/include/cps/FunctionGen.h b/include/cps/FunctionGen.h
index a50e70b..a176e84 100644
--- a/include/cps/FunctionGen.h
+++ b/include/cps/FunctionGen.h
@@ -72,7 +72,7 @@ private:
     IdentifierMap<SymbolInfo> &Symbols;
     bool &HadError;
     int &DiagnosticLine;
-    void reportError(const char *Fmt, ...);
+    __attribute__((format(printf, 2, 3))) void reportError(const char *Fmt, ...);
 
     IdentifierMap<std::deque<FuncSig>> Signatures;
     IdentifierMap<std::pair<std::string, size_t>> LinkIndex;